Active galactic nuclei AGNs are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade a growing number of sources have been observed to show dramatic flux and spectral changes in both the Xray and the opticalultraviolet regimes. Such events commonly described as changinglook AGNs can be divided into two welldefined classes. Changingobscuration objects show strong variability of the lineofsight column density mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changingstate AGNs are instead objects in which the continuum emission and broad emission lines appear or disappear and are typically triggered by strong changes in the accretion rate of the supermassive black hole. Here we review our current understanding of these two classes of changinglook AGNs and discuss open questions and future prospects.
['Astrophysics - Astrophysics of Galaxies', 'Astrophysics - Cosmology and Nongalactic Astrophysics', 'Astrophysics - High Energy Astrophysical Phenomena']
Changinglook active galactic nuclei
{'No Header': '<!-- image -->', 'Changing-look Active Galactic Nuclei': "Claudio Ricci 1 , 2 , 3 &Benny Trakhtenbrot 4  \n- 1 N'ucleo de Astronom'ıa de la Facultad de Ingenier'ıa, Universidad Diego Portales, Av. Ej'ercito Libertador 441, Santiago, Chile\n- 2 Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China\n- 3 George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA\n- 4 School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel  \nActive Galactic Nuclei (AGN) are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade, a growing number of sources have been observed to show much more dramatic flux and spectral changes, both in the X-rays and in the optical/UV. Such events, commonly described as 'changing-look AGN', can be divided into two well-defined classes. Changing-obscuration objects show strong variability of the line-ofsight column density, mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changing-state AGN are instead objects in which the continuum emission and broad emission lines appear or disappear, and are typically triggered by strong changes in the accretion rate of the supermassive black hole. Here we review our current understanding of these two classes of changing-look AGN, and discuss open questions and future prospects.", '1 Background': "Supermassive black holes (SMBHs) are ubiquitously found at the centers of most massive galaxies 1 , and grow predominantly through the accretion of material from their surroundings. The accretion process may produce highly luminous radiation across the entire electromagnetic spectrum. Such active galactic nuclei (AGN) are commonly classified based on their observed optical/UV or X-ray spectral properties. In the optical/UV, objects showing both broad ( /greaterorsimilar 1000 kms -1 ) and narrow ( /lessorsimilar 1000 km s -1 ) emission lines are classified as 'Type 1' AGN, while those displaying only narrow lines are referred to as 'Type 2' sources. Several studies use a finer classification scheme, based on increasingly fainter broad emission lines (i.e., Type 1.2, 1.5, 1.8 and 1.9). In the X-rays, objects showing lineof-sight column densities N H ≥ 10 22 cm -2 are commonly referred to as 'obscured', while those with N H < 10 22 cm -2 are 'unobscured'. A very good match has been found between these two classification schemes, with the vast majority of Type 1s (Type 2s) being unobscured (obscured) 2,3 . A good, first order explanation for these basic, observationally-motivated AGN sub-classes is provided by the AGN unification model 4-6 , according to which in Type 1 (unobscured) AGN the central engine is observed at low inclination angles (i.e., closer to face-on than edge-on), so that one can see both the broad and the narrow-line regions (BLR and NLR, respectively). On the other hand, Type 2 (obscured) AGN are observed at higher inclinations (i.e., edge on), through significant columns of circumnuclear dusty gas, distributed in an anisotropic, axisymmetric structure, possibly concentrated near the plane of the central accretion disk. This so-called dusty torus would obscure the X-ray and optical/UV continuum emission emerging from the central engine, as well as the broad emission lines emerging from the BLR (see Fig. 1 of  \nRef. 6). Over the past decades it has become clear that additional ingredients should be included in the unification model 6,7 . In particular, the covering factor of the obscuring material, which could play an important role in the probability of an AGN to be observed as Type 1 or Type 2 (Ref. 8), shows important trends with AGN luminosity 9 and/or Eddington ratio 10 ( λ Edd ).  \nAnother element that could significantly affect the spectral properties of AGN, and their classification into obscured/Type 2 or unobscured/Type 1, is variability. AGN variability has been observed across the entire electromagnetic spectrum 11,12 , and on timescales ranging from hours to years 13 . While usually this variability is stochastic and does not exceed 10s of per-cent, over the past twenty years an increasing number of accreting SMBHs have been observed as they undergo much more dramatic and coherent changes in their spectral properties, over a range of (exceedingly short) timescales. These changes defy the expectations from our basic understanding of AGN structure, and specifically the classical unification model. These events, commonly called Changing-look AGN to denote their varying appearance, have been discovered both in the X-rays and in the optical/UV regimes, and are caused by very different physical processes. In the X-ray band these transformations can be mostly ascribed to changes in the line-of-sight column density ( N H ), which are typically produced by gas located near the SMBH ( Changingobscuration AGN 14 , CO-AGN). On the other hand, in the optical/UV, these events are usually associated to rapid changes in the accretion-driven radiation field itself, which in turn lead to the suppression, enhancement, or indeed (dis-)appearance of the blue continuum and broad optical/UV lines typical of Type 1 AGN( Changing-state 15 AGN,CS-AGN).Figure 1 illustrates the basic spectral states of AGN, and thus the potential transitions between them, as viewed in the X-ray and in the optical regimes. With the advent of new facilities and surveys focused on the transient universe, including the Vera Rubin Observatory 16 , the Ein-  \nX-rays", 'UV/optical': 'Figure 1: Spectral transitions in different types of Changing-look AGN. a) Changes in the X-ray emission from an unobscured state (blue line) to a heavily obscured, Compton-thick state (red line). b) Spectral transition from a type 1 (blue line) to a type 2 (red line) AGN in the optical band. The term CL-AGN can be used to refer to all AGNs that show drastic spectral transitions, while CO-AGN and CS-AGN can be used whenever robust evidence for the corresponding physical mechanism is available.  \n<!-- image -->  \nSpectral transitions usually driven by:  \n<!-- image -->  \nChanging obscuration (CO-AGN)  \nChanging accretion state (CS-AGN)  \nstein Probe 17 , ULTRASAT 18 , and many others, a full understanding of these peculiar sources will be one of the main novel efforts within the study of AGN. In this Review we summarize our current understanding of changing-look AGN, discussing first changing-obscuration AGN (§2) and then changing-state AGN (§3). We stress that throughout this Review, we use the term CLAGN to refer to all AGNs that show drastic spectral transitions, regardless of the physical mechanisms driving these changes, and refer to CO-AGN and CS-AGN only whenever robust evidence for the corresponding physical mechanism is in hand.  \n1.1 The typical scales of key AGN physical elements We first briefly review the scales of the different AGN regions that are relevant for the changing-look AGN phenomena. Most of the X-ray emission in AGN is produced within ∼ 10r g = 10 GM BH / c 2 from the SMBH 22,23 by seed (optical/UV) photons, created in the accretion flow, and then Comptonized in a corona of hot electrons 24 . The size of the accretion disk has been inferred by micro-lensing 25 and reverberation 26 studies to be of order ∼ 10 -100 r g . Both the BLR and the dusty obscuring torus have sizes that are known to scale with luminosity. The typical emission-weighted size of the BLR ( R BLR ), and its dependence of the AGN luminosity, can be inferred through intensive BLR reverberation mapping studies. A useful example is the following relation, involving the X-ray luminosity 27 :  \nR BLR = 7 . 2 × 10 -3 ( L 2 -10 10 43 erg s -1 ) 0 . 532 pc , (1)  \nwhere L 2 -10 is the AGN luminosity in the 2-10 keV band. The scale typically associated with the inner edge of the torus is dictated by the sublimation of dust at sufficiently high temperatures ( T sub ). This can be estimated theoretically, for certain dust  \ntypes 28 :  \nR sub = a ( L Bol 10 46 erg s -1 ) 0 . 5 ( T 0 sub T sub ) 2 . 6 pc , (2)  \nwhere L Bol is the bolometric luminosity of the central engine, a is 0.5 (1.3) for graphite (silicate) grains, while T 0 sub corresponds to the temperature at which grains sublimate, i.e., 1800 K and 1500 K for graphite and silicate grains, respectively. Observational approaches to quantify the location of the dusty clouds in AGN, for a given luminosity, focus either on the hottest dust emission in the inner torus, observed through K -band reverberation mapping 29 and direct imaging 30 , or on the cooler dust observed through 12 µ m interferometry 31 . Note that the torus is generally thought to reside just outside of the BLR, and indeed many studies consider the clouds in the latter to be the dust-free extension of the former.', '2 Changing-obscuration AGN (CO-AGN)': 'Obscuration by neutral intervening gas can leave a clear imprint on the X-ray emission observed in AGN: as N H increases the photoelectric cutoff is shifted towards higher energies. At N H /greaterorsimilar 10 24 cm -2 the effect of Compton scattering becomes important, and the primary X-ray continuum becomes increasingly suppressed and more difficult to detect. Most of the AGN in the nearby Universe are obscured (i.e., ∼ 70% with N H ≥ 10 22 cm -2 ; Ref. 32), and ∼ 20 -30% of all AGN are Comptonthick (CT, i.e. with N H ≥ 10 24 cm -2 ; Ref. 32). As the column density increases, reprocessed X-ray radiation produced by the circumnuclear material (i.e., a Fe K α line and a Compton hump at ∼ 20 -30 keV), emerges above the suppressed primary', 'Changing-look AGN - Basics': 'Spectral classes identified in:  \nFigure 2: X-ray spectra and N H variations of changing-obscuration events. a) Evolution of the X-ray spectrum of the Type 1 AGN ESO 323 -G77 19 during a CO event. The spectra show the progression of the event, going from the least obscured ( Chandra ; green) to the most obscured ( Suzaku , blue) phase. As the column density increases the X-ray flux decreases, and the Fe K α line at 6.4 keV becomes more prominent. b) Variations of the column density as a function of time for NGC 1068, obtained from a dedicated XMM-Newton / NuSTAR campaign 20,21 , which allowed to probe changes of N H /greaterorsimilar 3 × 10 24 cm -2 .  \n<!-- image -->  \nX-ray continuum. Some of the most obscured CT AGN, with N H /greaterorsimilar 10 24 . 5 cm -2 , can show a reflection-dominated spectrum, in which most of the emission can be attributed to reprocessed X-ray radiation (left panel of Fig. 2). Thus, the broad-range Xray spectra of AGN offer clear and robust measures of line-ofsight obscuration.  \nSome AGN have been discovered to show temporal variations in the line-of-sight column density 33 . These changingobscuration AGN (CO-AGN) can, in the most extreme cases, transition from/to being obscured by Compton-thin ( N H ≤ 10 24 cm -2 ) material to/from CT [ ∆log( N H / cm -2 ) ∼ 1 -2 dex]. Such transitions are typically observed in the X-rays: given its small size, the X-ray corona is considerably easier to obscure than e.g. the significantly more extended BLR. Variability in N H has provided clear indication that the material around the SMBH is clumpy, and offers unique insights into the its dynamics. This dynamic obscuring material could be located either inside the dust sublimation radius of the AGN, and then be associated with the BLR; or further out, beyond the sublimation radius, and then be associated with the putative torus. Interestingly, at least in some cases, these changes in the line-of-sight obscuration could explain 34 the large deviation from the Galactic N H /A V value observed in numerous AGN 35 .  \n2.1 Early works One of the first known cases of clear column density variability was discovered in the nearby broadline AGN NGC4151. X-ray observations of this source carried out in December 1976 36 found a N H that is ∼ 4 times higher than that determined from data taken in January of the same year 37 . Such a spectral change was confirmed by a subsequent study 38 , which argued that the X-ray absorbing medium could be partially ionized by the UV/X-ray continuum. It was later proposed 39 that the X-ray absorption in this source is related to clouds in the BLR, with the variability being caused by some of these clouds moving in or out of the line-of-sight. The timescale expected from clouds moving at 10 , 000 kms -1 ( ∼ 12 days) agreed rather well 40 with the most stringent constraints obtained  \nfor NGC4151 ( ∼ 1 week 38 ). A similar variation was detected in the AGN ESO103 -G35, which was found to show a change in N H of a factor ∼ 2 over a period of 90 days 41 . The discovery of yet more extreme N H variations came a few years later, from observations of NGC 1365. This AGN was observed by ASCA in 1994 and found to be in a reflection-dominated state, with N H /greaterorsimilar 1 . 5 × 10 24 cm -2 (Ref. 42). Three years later, BeppoSAX observations found the source to be in a Compton-thin state 43 , implying a change in column density of ∆ N H ∼ 10 24 cm -2 . Since then this source has been observed repeatedly by all major X-ray telescopes, which confirmed further strong N H variability on a wide range of timescales and column densities 22,23,44 .  \nIn the years following these first discoveries, several other nearby AGN were observed to undergo large-amplitude variations in N H . From the analysis of XMM-Newton and ASCA observations, the nearby AGN UGC 4203 was found to transition from a reflection dominated state, to a Compton-thin state in less than six years 45 . The presence of N H variability was later confirmed by Suzaku observations 46 . Another nearby AGN, NGC6300 was found to be CT by RXTE 47 in 1997, while observations carried out 2.5 years later by BeppoSAX found the source in a Compton-thin state 48 . More recent observations confirmed the presence of obscuration variability, finding a variation in N H of a factor 2 between 2007 and 2016 49 . We note that CO events were detected on very short timescales, including days and even hours, when higher cadence observations were available (see NGC 4151 above and §2.2.1 below).  \nThe first large study of a CO-AGN sample was carried out in the late 1990s: by studying ∼ 50 AGN with multiple X-ray observations, equally split between obscured and unobscured, it was found that ∼ 70% of the objects show variable N H on timescales of months to years 40 . A dedicated study of 25 narrow line (Type 2) AGN 33 , collecting X-ray observations spanning timescales from months to several years, found that 22 sources (i.e. nearly 90%) showed large amplitude ( ∼ 20% -100% ) variations in their N H . For a sub-sample of 11 objects with  \nat least five X-ray observations, the typical variation timescales were found to be shorter than several months, with the obscuring material varying by ∆ N H ∼ 10 22 -10 23 cm -2 . These studies confirmed the importance and high occurrence of CO events in nearby AGN.  \n2.2 The origin of CO events While the most widely accepted explanation for CO events is related to intrinsic column-density variability, due to clouds moving in and out the line-of-sight (§2.2.1), several additional explanations have been proposed over the years. One such scenario is that the observed N H changes are driven by changes in the ionization state of the material (§2.2.2), associated with an increase or decrease of the intrinsic AGN luminosity. In some other cases, CO events have been instead attributed to powerful AGN outflows (§2.2.3). The most extreme CO events, where the AGN goes from/to a transmission-dominated to/from a reflection-dominated state in the X-rays (i.e., ∆ N H /greaterorsimilar 10 24 cm -2 ), could be associated to a switch off/on of the AGN (see §2.2.4). We briefly discuss each of these possible scenarios.', '2.2.1 Eclipses': 'Eclipses of the X-ray source are likely one of the most common causes for CO events. This was initially suggested based on the detection of rapid (i.e., a few hours timescale 50 ) variations of column density in nearby AGN, and then confirmed by the observation of covering and uncovering events. A Chandra monitoring campaign of NGC 1365, with six observations carried out over 10 days, found that the source was first in a Comptonthin state, then showing a reflection-dominated X-ray spectrum two days later 22 . Subsequent observations found that the source was back to its initial, Compton-thin, state. A continuous 5-day XMM-Newton monitoring campaign of NGC 1365 recorded the uncovering of the central X-ray source 23 . The source was in a reflection-dominated state for the first 1.5 days of the campaign, then showing a ∼ 10 hours long increase in flux, followed by a rather symmetric decrease. The X-ray spectral analysis showed that the increase in flux was due to a decrease of the column density, with the nuclear source being less obscured for several hours. Another, albeit incomplete, transit was detected in a previous, shorter XMM-Newton observation 51 . The frequency of these events strongly argues in favour of the clumpy nature of the obscurer in NGC 1365. Long X-ray observations of other objects (e.g., SWIFT J2127.4+5654 by Ref. 52, Mrk 766 by Ref. 53 and NGC6814 by Ref. 54) allowed to detect the transit of clouds with N H ∼ 10 22 -10 23 cm -2 on time-scales of a few hours.  \nThe aforementioned X-ray facilities, operating at E < 10 keV, cannot observe and identify N H transitions of ∼ 3 × 10 24 -10 25 cm -2 . The advent of NuSTAR , with its novel focusing capabilities in the hard X-ray band ( > 10 keV), has opened a new window to study obscured AGN, allowing to detect and characterize the absorption properties of a growing number of AGN, and to discover new CO-AGN 55 . A joint XMM-Newton and NuSTAR campaign of the reflectiondominated AGN NGC1068 20 carried out in 2014 and 2015 found a transient excess in the hard X-ray band. This event was associated to temporary decrease of the obscuring material, from N H /similarequal 10 25 to /similarequal 6 . 7 × 10 24 cm -2 . This allowed, for the first time, to observe the primary X-ray continuum from the AGN in  \nthis object. Mid-IR observations carried out before and after the X-ray event showed that the intrinsic, accretion-driven radiation from the source did not change during the transient event 56 , thus confirming that it was due to a change in the line-of-sight column density. A follow-up NuSTAR campaign identified two additional CO events 21 : one unveiling [ ∆ N H ∼ 1 . 8 × 10 24 cm -2 ] and one eclipsing [ ∆ N H /greaterorsimilar 2 . 4 × 10 24 cm -2 ] the X-ray source, thus confirming that the obscuring medium in NGC 1068 is rather dynamic (right panel of Fig. 2).  \nEclipses of the X-ray source can be used to put constraints on the location of the clouds producing the variable obscuration (see §2.4), and to infer the size of the X-ray corona. This can be done assuming that the clouds are moving in a Keplerian orbit around the SMBH, and that the size of the obscuring cloud is larger than (or consistent with) that of the X-ray source. In the case of NGC 1365, it was found that the size of the X-ray source is < 10 13 cm , corresponding to a few gravitational radii 22,23 . Similar scales were also derived for Mrk 766 (Ref. 53) and SWIFT J2127.4+5654 (Ref. 52). Interestingly, the X-ray coronae sizes obtained through this approach are consistent with the of micro-lensing studies 57 .', '2.2.2 Changes in the ionization state of the obscuring gas': 'An increase of the AGN luminosity could lead to an enhancement in the ionization state of the obscuring material, making it more transparent to X-ray radiation, which would in turn lead to an apparent decrease of the line-of-sight column density. This was one of the first explanations proposed for the N H variations observed in NGC 4151 (Ref. 38). For NGC 1365, it was argued that a change of two orders of magnitude in intrinsic AGN luminosity was required to explain the N H variations between some of the observations 44 . However, such a strong luminosity variation was not observed. In general, this explanation does not appear to be viable for most CO-AGN events, as they usually do not display large variations of their bolometric luminosity associated with the CO events.', '2.2.3 Outflows': 'While most objects discussed so far showed variations of neutral absorption, in some cases, and particularly in unobscured/Type 1 AGN, it has been found that the X-ray spectrum is strongly affected by variable ionized absorbing gas along the line of sight. Such absorbers are associated with outflows, which are common in AGN, and have been routinely detected though absorption troughs both in the X-ray and optical/UV regimes 59 . An intensive multi-wavelength monitoring campaign of the Type 1 AGN NGC5548 carried out in 2013 showed a clear CO event. A detailed study of the X-ray spectra revealed two obscuring components 60 : one with a low ionization level and a column density of ∼ 10 22 cm -2 covering 86% of the X-ray source, and another one almost neutral, with a higher column density ( ∼ 10 23 cm -2 ) and smaller covering factor ( ∼ 30% ). The simultaneous UV spectroscopy allowed to observe broad absorption lines due to the low-ionization component, which showed that the material was outflowing with velocities of up to ∼ 5000 kms -1 . This event, which lasted several years, was ascribed to a wind from the accretion disk extending beyond the BLR. A similar obscuration event was later found in  \nFigure 3: The cometary-shaped absorbers in NGC 1365. Geometry and physical properties of the two clouds obscuring the X-ray source in NGC 1365, as inferred by a long Suzaku observation 58 . The clouds are thought to have a cometary shape, with a dense core and an elongated, lower-density, tail.  \n<!-- image -->  \nNGC3783 (Ref. 61). This event lasted about one month, with material partially obscuring the central source with a column density of ∼ 10 23 cm -2 , and outflowing with a velocity of a few 1000 km s -1 . An analysis of previous X-ray observations of this source identified several additional CO events 62 . Dedicated campaigns have recently found similar CO events in other nearby AGN, such as NGC 3227 (Ref. 63) and Mrk 817 (Ref. 64). Some of these campaigns have also been able to observe the outflows obscuring the accretion disk 64 , and might be able in the future to shed light on the interplay between the accretion disk and the outflows. It should be noted that low signal-to-noise ratio and spectral resolution X-ray observations could sometimes associate spectral changes due to outflowing ionized absorbers to eclipses from neutral material.', '2.2.4 Switch-off/switch-on of the central engine': 'In addition to the scenarios discussed above, at least in some cases the dramatic changes observed in the X-ray spectral appearance of AGN could be directly related to changes in the (intrinsic) X-ray luminosity 65 . If the accretion power of an AGN decreases rapidly, the primary X-ray continuum (i.e. the powerlaw component) would also decrease. On the other hand, the reprocessed X-ray radiation, produced by material on scales of /greaterorsimilar 0 . 1 -1 pc, would react more slowly to changes in the irradiating X-ray flux, and could dominate the X-ray emission when the primary flux is at a low level. Therefore, for a certain time after a significant drop in the intrinsic X-ray radiation, the source could be observed as a reflection-dominated AGN, and could be confused for a CT AGN. A re-increase of the accretionpowered radiation would then lead the source to again appear as a continuum-dominated X-ray AGN. This switch-off/switch-on process could (mistakenly) lead to identifying the source as a CO-AGN.  \nTwo cases in which this mechanism could explain the spectral transitions observed in AGN are those of NGC 2992 (Ref. 66) and the highly variable AGN NGC 4051 (Ref. 67). Both sources are switch-off AGN that appeared reflectiondominated in several epochs of observation. Multiple X-ray observations of NGC 2992, starting from 1978, suggested that the X-ray emission faded over ∼ 15 -20 years, and then went back to its originally observed flux level in 1998 66 . The X-ray spectrum of the Type 1 AGN NGC 4051 was found to be reflectiondominated in a 1998 observation 67 , which was associated to a prolonged low-flux state 67,68 . On the other hand, UGC 4203 was  \ninitially classified as a possible switch-off AGN 45 , while subsequent studies showed that the appearance of this AGN is likely affected by line-of-sight eclipses 46 . In order to clearly differentiate between switch-on/switch-off sources and sources with varying N H , it is fundamental to carefully analyze the long-term X-ray light curves and have a simultaneous broad spectral coverage, to best decipher the rich spectral information available in the X-rays.  \n2.3 Recent studies Over the past two decades, repeated observations of nearby AGN carried out by XMM-Newton , NuSTAR Chandra , RXTE , Swift and Suzaku , found a growing number of CO transitions, showing that this phenomenon is rather common in AGN. These variations have been found either by repeated or very long observations. The best studied CO-AGN, NGC1365, has been the subject of several dedicated monitoring campaigns. These studies found variability of N H on timescales down to ∼ 10 hours, with a variable column density going from ∼ 10 23 to ∼ 10 24 cm -2 . Similarly, rapid variations of N H ∼ 10 22 -10 23 cm -2 down to timescales of ∼ 4 hours and ∼ 10 -30 hours were observed in NGC 4388 (Ref. 50) and in NGC 4151 (Ref. 69). While typically these events are detected in AGN showing obscuration signatures in the optical regime (Type 1.8 -2 sources), in a few instances they have also been observed in narrow-line Seyfert 1 (e.g., Mrk 766, Ref. 53 and SWIFTJ2127.4+5654, Ref. 52) and Type 1 (e.g., ESO323 -G77, Ref. 19) sources, although their occurrence rate appear to be lower. Polarimetry studies 70 of ESO323 -G77 suggested it is observed at an intermediate inclination angle ( ∼ 45 · ) with respect to the normal to the disk. This might imply that the detection of CO events in unobscured AGN could be associated with lines of sight grazing the edge of the torus.  \nSeveral other studies have focused on searching for CO events in large samples of AGN, to better understand their occurrence rates and links to other AGN and SMBH properties. A careful analysis of RXTE observations of ∼ 40 nearby Type 1 and Compton-thin Type 2 AGN uncovered 12 eclipse events in eight of the sources 71 , with typical column densities of ∼ 10 22 -10 23 cm -2 . Analyzing hardness ratio light-curves of 40 nearby AGN with long XMM-Newton and Suzaku observations, it was found that spectral variability that could be ascribed to eclipses by BLR-like clouds was detected in all the cases where the length of the observations was sufficiently long 72 . More recently, N H variability was detected in seven out of 20 objects observed by various X-ray satellites 73 , with variations in the range  \n∼ 10 21 -10 23 cm -2 . The CO-AGN phenomena, as probed by X-ray observations, is thus rather common.  \n2.4 The location of the dynamic absorber The distance of an eclipsing cloud from the central X-ray source ( R C ) can be estimated by considering that the clouds are moving in Keplerian orbits around the SMBH, and by making some further simple assumptions 22,74 , specifically that the size of the cloud ( D c ) is similar to that of the X-ray source ( D s ), and that the transverse velocity ( v k ) is given by the ratio between the size of the X-ray source and that of the crossing time ( v k = D C / ∆ t cr ). The Xray source is known to be compact (§1.1) and, setting the radius of the cloud to be D C /similarequal D s = 10 r g , one would then obtain:  \nR C = GM BH v 2 k = GM BH ∆ t 2 cr D 2 s = 6 . 3 M 8 R -2 10 ∆ t 2 10 pc , (3)  \nwhere ∆ t 10 is the crossing time in units of 10 days, M 8 is the black hole mass in units of 10 8 M /circledot , and R 10 = D s / 10 r g .  \nIf the obscuring material is ionized, its distance from the SMBH can be simply inferred, under the assumption that the cloud is in photoionization equilibrium with the radiation field, by the definition of the ionization parameter ξ :  \nR C = ( L ion ξn H ) 0 . 5 , (4)  \nwhere L ion is the ionizing radiation, commonly integrated over 1-1000 Ryd, and n H is the density of the obscurer. Since constraining n H is difficult, Equation 4 can be rewritten, considering that the depth of the absorber l C is smaller than its distance from the central source ( l C /similarequal N H /n H /lessorsimilar R C ):  \nR C /lessorsimilar L ion ξN H . (5)  \nWhile for many objects only very loose constraints on the timescales of the CO event (i.e., /lessorsimilar months-years) could be found, for a few sources very stringent constraints were obtained. NGC1365 showed CO transitions on time-scales of weeks 44 , days 22 , and down to ∼ 10 hours 23 . The distance of the varying CT absorber in this object was constrained to be 3 , 000 -10 , 000 r g , i.e. consistent with what is expected for the BLR. The CO-AGN NGC7582 (Ref. 75) showed column density variations on timescales shorter than a day 76 ( /lessorsimilar 20 hours), and associated with clouds in the BLR, or just outside it. Similarly, the varying Compton-thin and CT absorbers were associated to the BLR in several other objects, such as NGC 4388 (Ref. 50), SWIFTJ2127.4+5654 (Ref. 52), Mrk 766 (Ref. 53) and NGC4151 (Ref. 69). In the case of ESO323 -G77, two components were distinguished 19 . The less variable one, with N H ∼ 2 -6 × 10 22 cm -2 , a density of ≤ 1 . 7 × 10 8 cm -2 , varying on timescales longer than a few months and shorter than 3.6 years, was associated to clouds in the clumpy torus. The larger amplitude variations ( N H ∼ 0 . 3 -3 × 10 24 cm -2 ) detected on shorter timescales were instead linked to clouds in the BLR. Assuming a size of the obscuring cloud comparable to that of the X-ray source, for NGC 3783 a distance of ∼ 10 light days from the SMBH was inferred 61 for the outflowing gas, which is also consistent with that of the BLR.  \nA large study of dozens of nearby AGN found that the transient obscuring clouds are located at R C ∼ 0 . 3 -140 × 10 4 r g  \nfrom the SMBH, which is comparable to the dust sublimation radius 71 , with the mean of the individual values indeed indicating R C /similarequal R sub . The object in which clouds are located farthest away from the sublimation radius is Cen A, with R C /R sub ∼ 2 -7 . For the three objects in the sample for which IR interferometry or optical-to-near-IR reverberation mapping was performed, R C was found to be consistent with the radial distance of the IR-emitting material. It should be noted however that this study might preferentially detect eclipses lasting tens of days 71 and, similarly, long continuous observations might be biased towards detecting CO events on timescales of hours to days. Interestingly, these longer-timescales eclipses have typically lower column densities ( /similarequal 10 22 -10 23 cm -2 ) than those observed on shorter timescales, which could imply that a significant fraction of the obscuration is caused by material within the sublimation radius.  \n2.5 The structure of the BLR from X-ray eclipses Studies of some of the most extreme CO events have shown that the density of the obscuring clouds tends to be high. For example, in the case of NGC 1365 (Ref. 23) and ESO 323 -G77 (Ref. 19) the clouds were found to have a density of ∼ 10 11 cm -3 and 0 . 1 -8 × 10 9 cm -3 , respectively. Similarly high densities were found for objects in which only a lower limit could be obtained, i.e. > 3 × 10 8 cm -3 and ≥ 1 . 5 × 10 9 cm -3 for NGC7582 (Ref. 76) and SWIFTJ2127.4+5654 (Ref. 52), respectively. These values are consistent with the typical density of BLR gas ( /greaterorsimilar 10 9 . 5 cm -3 , Ref:77), and higher than what is expected for the dusty torus. Combined with the aforementioned BLR-like distances of the transient obscurers, this strongly supports the idea that these CO-causing clouds are associated with those producing broad optical/UV lines.  \nSince a significant fraction of CO events could be caused by BLR clouds, X-ray spectroscopy can provide an important tool to shed light on this region. A crucial step forward in our understanding of the relation between BLR clouds and CO events came from a 300 ks Suzaku observation of NGC 1365 58 , which found that the rapidly-moving clouds in that system appear to have a cometary shape, with a high-density ( ∼ 10 11 cm -3 ) core and an elongated ( ∼ 10 13 cm ), lower density, structure (Fig. 3). The opening angle of the tail was estimated to be small, i.e., only a few degrees. Similarly, in Mrk 766, besides dense ( ∼ 10 10 -10 11 cm -3 ) neutral clouds, a tail of lower density, highly ionized gas, causing blueshifted iron lines (Fe XXV and Fe XXVI with ∆ v /similarequal 3 , 000 -15 , 000 kms -1 ), was also detected 53 . As the mass loss rate of the cloud core material into the tail would imply their destruction within a few months, it was speculated that the clouds should be continuously replenished, possibly through outflows from the accretion disk 58 . This would be in agreement with some recent models of the formation of BLR clouds 78 .', '3 Changing-state AGN (CS-AGN)': 'The accretion phase onto SMBH is expected to last ∼ 10 5 -10 9 years 79,80 , and during this period AGN are well known to show variability, with the typical amplitude of optical continuum variations being tens of percents 11 . A growing population of AGN has been recently found to show much stronger flux variability, over relatively short timescales, in both the optical 81-84  \nand X-ray 85 regimes. In some cases, these strong flux variations have been associated to CS transitions, with the (dis-)appearance of broad optical/UV emission lines being closely linked to the (dis-)appearance of the accretion-powered continuum emission (Fig. 4).  \n3.1 Early works One of the first observed CS transition was found in 1975 from repeated observations of NGC 7603, which showed that the broad H β line weakened by a factor ∼ 3 within one year 86 . The interacting galaxy NGC 2992, one of the first candidate CO-AGN (see also §2.2.4), was also found to undergo a CS event. Early optical spectra published in the 1980s 87 showed the presence of a weak broad H α component but no detectable broad H β , marking the source a Type 1.9 AGN. In 1994 the source appeared to also lose its broad H α line 88 , transitioning to a Type 2 class. The broad H α line then reappeared 5 years later 66 . Interestingly, the variations were showed to be associated with the X-ray flux: in the 1980s the source was in a high X-ray flux state 89 , which declined over time, reaching its minimum in 1994, before going back to the originally observed flux level in 1999 66 . Other systems found to show disappearing broad line emission include 3C 390.3 90 , Mrk 372 91 and Mrk 993 92 . In several other nearby sources (e.g., NGC 7582 93 and NGC3065 94 ), the opposite process was observed, with the broad line emission appearing .  \nA small fraction of CS-AGN have been found to undergo a full cycle, transitioning twice. Mrk1018 went from Type 1.9 to Type 1 over a time span of < 5 years 95 , and then transitioned back to Type 1.9 after 30 years 96 . NGC1566, which was recently found to have transitioned from Type 1.9-1.8 to Type 1.2 97 , shows recurrent activity 98 : between 1970 and 1985, four separate periods of activity of ∼ 1300 days each were identified 98 , with the flux of both the continuum and the broad lines increasing and then decreasing, leading to CS transitions. NGC4151, which was originally found to be a Type 1.5 AGN 99 , first lost its broad lines (with the exception of weak and possibly asymmetric wings) during the 1980s 90 , and then regained them 100 . Similar transitions were also observed for Mrk 590 101 and NGC3516 102 . Importantly, optical 103 and X-ray 104 continuum emission studies showed that these transitions are usually not associated with CO events (see §3.4).  \n3.2 The advent of large surveys and statistical studies Over the past few years, with the advent of large imaging and spectroscopic surveys, there has been a sharp increase in the number of CS-AGN identifications, greatly expanding the scope of early studies in terms of redshift and key SMBH properties. One prominent example is the first changing-look quasar 103 (at z = 0 . 31 ), SDSSJ0159+0033, which was observed in two different programs within the Sloan Digital Sky Survey (SDSS 108 ). The original SDSS spectrum, taken in 2000 (and reported in the first SDSS data release 109 ) showed broad emission lines, which however disappeared by 2010, when another spectrum was taken as part of SDSS-III. This transition was confirmed by further spectroscopy obtained during 2014 (Ref. 103). Several additional studies pursued a similar approach, systematically searching the (repeated) spectroscopy of SDSS programs and/or other spectroscopic surveys, and revealing a growing number of CSAGN 105,110-113 .  \nIn most such studies, the identification of CS events is  \nbased on drastic changes in the H α and H β lines in spectra that are taken several years apart. At higher redshifts ( z /greaterorsimilar 1 . 5 ) CS-AGN were also identified based on the MgII λ 2798 , CIV λ 1549 , and other (rest-frame) UV broad emission lines 114,115 . These emission lines probe somewhat different regimes of the variable ionizing AGN continuum and/or the BLR (see e.g., Table 1 in Ref. 115), and may thus provide invaluable, complementary insights regarding the nature of extreme variability, and indeed CS transitions, in AGN 84 . For example, several works studied the somewhat peculiar variability properties of the broad Mg II λ 2798 emission line 116 , including its persistence in optical spectra that show weak (or no) other AGN indicators 117 . Such systems can be interpreted as recently turned-off AGN 118 , where the Mg II line is not as responsive to the CS event as the Balmer lines 119,120 , in what seems like an extreme version of some reverberation mapping results 121,122 . Another examples of this behaviour is 1ES 1927+654 (Ref. 107), for which the appearance of a blue continuum was followed by the appearance of broad Balmer emission lines, while neither MgII nor C IV were detected.  \nThe samples obtained with such spectroscopically-focused searches for CS-AGN can already provide some insights regarding their occurrence rates and the typical key properties for AGN that experience CS transitions. Re-observing 102 local ( 0 . 02 ≤ z ≤ 0 . 1 ) broad-line AGN with previous SDSS spectroscopy, it was found 123 that ∼ 38% of the sources had changed spectral (sub-)classification, and that in ∼ 3% of the sources the broad H β disappeared completely, within ∼ 3 -9 years. A spectroscopic effort to re-observe SDSS quasars that exhibited extreme variability in multi-year optical photometry [ ∆( g ) > 1 mag] identified 17 robust CS-AGN among 130 carefullyselected candidates 112 . Another search, within SDSS-IV repeated spectroscopy (and no photometric pre-selection) 113 resulted in a total of 19 (15 newly identified) CS-AGN, out of a parent sample of ∼ 64,000 broad-line quasars. This translates to a significantly lower occurrence rate of ∼ 0.03%. The discrepancy in occurrence rate may be attributed not only to the different search methods (see also below), but also to the wellknown anti-correlation between AGN luminosity and variability amplitude 11,124 . Some of these relatively large, spectroscopyled samples of CS-AGN, for which some form of comparison samples can be constructed, suggest that CS-AGN have preferentially relatively low Eddington ratios 112,113 . This, however, should be treated with caution, as such studies may be subject to various observational biases (see below).  \nSince CS events are typically associated with drastic changes in both the optical and X-ray fluxes (§3.3.1), timedomain imaging surveys can be a powerful tool to identify candidate highly variable AGN which can then be confirmed as CS-AGN through spectral observations. In particular, several wide-field optical imaging surveys, including the Catalina Real-time Transient Survey (CRTS 125 ), the Palomar Transient Factory (PTF 126 ), Panoramic Survey Telescope and Rapid Response System (Pan-STARRS 127 ), the All-Sky Automated Survey for SuperNovae (ASAS-SN 128 ), the Zwicky Transient Facility (ZTF 129 ), have greatly increased the number of known CS-AGN. One of the first CS-AGN events to be identified by these facilities was NGC 2617, for which ASAS-SN identified a significant flux increase in 2013. Spectroscopic follow up  \n<!-- image -->  \n<!-- image -->  \nFigure 4: Light curves and spectra of CS-AGN showing an emergent BLR. a) SDSS (black diamonds), Pan-STARRS (blue triangles), and CRTS (red dots) light curves in the g band 105 . b) SDSS (red) and BOSS (black) spectra, corresponding to the epochs indicated by the vertical dotted lines in the left panel 105 . The lower sub-panels show the flux difference ( | ∆ f λ | ) between the two spectra, together with the best-fit power-law f ν ∝ ν β , with the red-dashed curve showing a power-law with β = 1 / 3 , as expected for a standard thin disk 106 . c) Transition from a Type-2 AGN (bottom) to a Type-1 AGN in 1ES 1927+654, with an intermediate stage of a blue-continuum-dominated emission 107 .  \n<!-- image -->  \nshowed that the source, which was originally classified as a Type 1.8 AGN in 2003, had transitioned to Type 1 128 . Some of these surveys have discovered extremely peculiar objects, such as 1ES 1927+654 107 , the first source in which the CS transition was clearly temporarily resolved. This source, a known nearby AGN which historically showed no broad emission lines and no X-ray obscuration 130 , underwent an outburst at the end of 2017, which was detected by ASAS-SN. Intensive spectroscopic follow up showed the source first developing a blue, quasarlike continuum, then showing the appearance of broad Balmer lines 107 (right panel of Fig. 4). Photometric surveys have also found cases in which the AGN rapidly turned on, transitioning from LINER to broad-lined AGN 131,132 .  \nThe study of infrared variability has proven to be an effective approach to detect rapidly varying AGN, particularly thanks to the repeated all-sky imaging of the WISE mission. Among the > 4 million AGN identified with WISE 133 , about 700 (0.015%) showed strong variability, and some of those were indeed associated with CS transitions 133-135 . In one exemplary WISE -identified source, the broad H α emission has disappeared within /lessorsimilar 10 years 133 . By using the various WISE -based datasets, another CS-AGN, in which the broad H β disappeared on a timescale of /lessorsimilar 11 years, was identified 134 . The flux of this source has faded by a factor of /greaterorsimilar 2 (at 3 . 4 and 4 . 6 µ m) between 2010 and 2014. Follow-up observations showed that the X-ray flux of the source had decreased by a factor of ≈ 10 since 1995. Recently, by combining CRTS and WISE photometry with spectroscopic observations, 73 additional CS-AGN were identified 15 ,  \n36 (37) of which with fading (increasing) H β , greatly increasing the number of known CS-AGN. These systems were found to be typically more luminous and at a slightly higher redshift than previously known CS-AGN.  \nThe growing samples of CS-AGN, from all observational approaches, would allow to probe population properties and thus be very insightful for understanding the mechanisms behind such extreme events. Perhaps the most basic property to consider is the occurrence rate of CS events in AGN, with some preliminary findings mentioned above. All efforts to understand the (statistical) population properties of CS events are naturally limited by the diversity of identification methods, as well as the associated observational challenges related to (irregular) cadence and/or availability of follow-up data. Moreover, there are some biases that are intrinsic to the search methodology of choice. For example, (spectroscopic) monitoring of previously-known luminous AGN (quasars) is expected to be biased towards turn-off events (and against turn-on events), given the historically high accretion rates. In contrast, some imaging time-domain surveys are geared to alert on rapid (and/or extreme) brightening sources, thus potentially providing a biased view regarding turnon events, and neglecting turn-off events. These important limitations require further analysis 136 if observations of samples are to be used to constrain models, and specifically those that have distinct predictions in terms of occurrence rates, turn-on or turnoff behavior, and/or relevant SMBH properties (as discussed in §3.6).  \nFigure 5: X-ray spectra and light curve of CS-AGN a) Optical, UV, and X-ray spectra of Mrk 1018 in 2008 August (black), 2008 June (red), 2013 (green), and 2016 (blue) 137 . b) Best-fit model of the different optical/UV and X-ray observations, showing an increasing soft X-ray component for increasing luminosities 137 . The grey points show the spectrum of an S0-type host galaxy. c) Spectral evolution of 1ES 1927+654 104,138 showing the disappearance and reappearance of the power-law component after the CS event (2018/2019), compared with pre-outburst observations 130 . d) 0.5-10 keV emission of 1ES 1927+654, compared to the historical value (green horizontal dot-dashed line) 104,138 , showing extreme X-ray variability (a factor ∼ 10 , 000 and ∼ 100 on timescales of ∼ 100 days and a few hours, respectively).  \n<!-- image -->  \nc)  \nb)  \nd)', '3.3.1 X-rays': "When observed in the X-rays, most CS-AGN show a change in flux correlated with that observed in the optical regime. This has been observed for several nearby objects such as Mrk 1018 139 and NGC1566 140,141 , with the X-ray flux varying up to a factor ≈ 30 . At higher redshifts, SDSS J0159+0033 showed a dimming by a factor of ∼ 10 in the X-rays between 2000 and 2005, similar to that observed in the optical 103 . In a few cases the change in X-ray flux was found to be associated to significant X-ray spectral variability 137 , with a strong soft excess appearing during the brightest intervals, when the broad optical lines are observed, and disappearing in the lowest flux periods, when the lines fade (top panels of Fig.5). While no significant changes in the line-of-sight (neutral) N H were observed during or after CS events, in NGC 1566, XMM-Newton /RGS shows the presence of absorption lines from an outflow moving at ≈ 500 kms -1 ,  \nwhich could be associated with the drastic increase in flux and in radiation pressure traced in this source 140 .  \nOne very clear exception to the generally 'synchronized' behaviour discussed was observed in the system 1ES19267+654 104,138 . After the optical/UV outburst that led to the CS event, the X-ray spectral shape of the source changed drastically, with the power-law component almost completely disappearing (i.e. dropping by a factor of at least 100), and the emission becoming dominated by a soft blackbody component, with a temperature kT /similarequal 80 -120 eV (bottom left panel of Fig. 5). This was the first time that the power-law emission component, produced in the hot X-ray corona and ubiquitously observed in AGN, was observed to vanish. The source was monitored for over 450 days, and showed further drastic variability on a broad range of timescales, varying by up to ∼ 4 dex on timescales of ∼ 100 days, and by ∼ 2 dex within as little as ∼ 8 hours (bottom right panel of Fig. 5). Following the strong dip in the X-ray flux that followed the UV/optical CS event, the X-ray emission increased rapidly, with the power-law compo-  \n)  \n-1  \nV  \nke  \n-1  \ns  \n-2  \nm  \nc  \nons  \nhot  \n(P  \n2  \nV  \nke  \n0.01  \n-3  \n10  \n-4  \n10  \n0.001  \n0.01  \n10  \n100  \n0.1  \n1  \nEnergy (keV)  \nnent recovering its dominance over the X-ray regime ∼ 300 days after the identification of the CS event 104,138 . Another unexpected feature of this AGN is the clear harder-when-brighter behaviour, with both the temperature of the blackbody component and the relative strength of the power-law component increasing with source luminosity 138,142 .", '3.3.2 Infrared': 'Infrared observations have been extremely useful to pinpoint the origin of CS-AGN, and to exclude a significant contribution of reddening events associated to transient obscuration 111 . In general, the properties of CS-AGN in the infrared appear to be similar to those of the general AGN population, specifically showing evidence for (1) the mid-infrared emission echoing the variability observed in the optical band, with time lags that are consistent with what would be expected for reprocessing by torus dust 143 ; and (2) a redder-when-brighter behaviour, likely associated to hotter dust for increasing luminosities 111 . Infrared observations of CS-AGN can also be used, exploiting their rapid and large-amplitude variability, to infer the size of the dusty torus through reverberation mapping measurements 144 . A study of the dust around Mrk 509 was also able to infer that the replenishment time-scale of the dust in the innermost regions of the system is very rapid 144 ( ∼ 4 years).', '3.3.3 Radio': "Due to the current scarcity of radio studies of CS events in (unbeamed) AGN, it is still unclear whether the radio emission varies at all following a CS event and, if so, whether it is tightly correlated to the optical and X-ray flux in such events, or what are the typical lags. In principle, understanding the radio behaviour could provide unprecedented insights regarding disc/corona-related radio (or sub-mm) emission mechanisms 145 , jet launching and/or quenching. For wellstudied nearby sources, it was found that NGC 2617 showed no significant radio core emission variability after the 2013 CS outburst 146 , while Mrk 590 showed correlated radio and optical/UV/X-ray variability 147 , besides a weak radio jet that could be the byproduct of the recurring AGN activity 148 . In Mrk1018 the 5 GHz radio flux decreased by ∼ 20% during the Type 1.9 dimming phase (2016 -2017) 149 . Radio observations of the UV/optical-flaring AGN 1ES 1927+654 have instead revealed a dimming, by a factor ∼ 4 , of the core radio component during the CS event, followed by a re-brightening 150 . In this case the radio-to-X-ray flux ratio continuously decreased during the CS event. A study of a sample of CS-AGN at z < 0 . 83 found that these objects unlikely to be more active in the radio than normal AGN 146 .  \n3.4 CS events: generally accretion, not obscuration The idea that (most) CS events can be attributed to changes in the level of obscuration of the BLR has been generally discarded over the past few years by numerous lines of evidence, which have thus promoted the view that most CS events are driven by changes in the intrinsic AGN luminosity. The main arguments relate to the observed transition timescales, unobscured spectral appearances, BLR lags, and other multi-wavelength properties of CS-AGN, as we detail next.  \nSpecifically, these challenges include i) The long timescales ( /greaterorsimilar 10 years) that would be needed for a dusty cloud to efficiently cover the BLR 103 , assuming reasonable cloud velocities. ii) The absence of clear signatures of reddening in the optical spectra after the broad lines have weakened 103,105 . iii) The observation of the optical broad lines appearing in 1ES 1927+654 ∼ 2 -3 months after the UV/optical continuum flare, and following the development of a blue continuum 107 . iv) The lack of obscuration in the X-rays for sources in which the broad lines are not clearly observed 151 . 1ES 1927+654 showed a similar level of X-ray obscuration before and after the CS event 104,130,138 , while Mrk590 did not show strong X-ray obscuration after the nuclear luminosity dropped by two orders of magnitude and the broad optical lines disappeared 101 . v) The lack of a clear difference in E ( B -V ) /N H between CS-AGN and 'normal' AGN 152 . vi) The detection of rapid infrared variability (§3.3.2), specifically the identification of IR flux drops in several sources that exhibited H α or H β disappearance 133,134,153 . The infrared-emitting region is too large to be obscured on timescales of /lessorsimilar 10 years (similarly to the BLR mentioned above), and-importantly-the radiation at these mid-IR wavelengths is much less affected by dust obscuration (compared with the optical/UV regime). vii) The correlation between radio and optical/UV (and/or X-ray) variability observed in several objects 147 (§3.3.1,§3.3.3). viii) The low levels of optical polarization found in CS-AGN 154 . ix) The relation between optical/IR colours and flux in CS-AGN being similar to that of normal AGN 111 . x) The broad line widths varying in a similar fashion as normal AGN 110 , with the line being broader (narrower) when the source is dimmer (brighter). This is consistent with a picture where, as the continuum flux increases (decreases), the line is produced from parts within the BLR that are further (closer) to the SMBH. Quantitatively, for a virialized BLR the expected trend follows FWHM ∼ L -1 / 4 UV / opt , which is indeed supported by several reverberation mapping studies 155 .  \nWhile some sources, such as NGC 7582 and NGC 4151, have been found to show both CO and CS transitions, these phenomena are not correlated, and appear on very different timescales, suggesting that the two processes are generally independent. One sort of exception are CO-AGN in which the apparent change in N H is caused by a switch-off/switch-on of the central engine (see §2.2.4), such as seen in NGC 2992. Another possible exception is if a transient (dusty) cloud obscures a part of the BLR (but not the whole of it). This scenario may be more likely to occur in systems that are observed with linesof-sight that 'graze' the torus, and was invoked to explain the spectral transitions related to some Type 1.8/1.9 AGN 156 . A recent study of a CL-AGN identified in the new SDSS-V survey provides another example of how a transient obscuration may explain spectral transitions in the optical regime 157 . It is possible that future high cadence spectroscopic studies will reveal more events of this type. Otherwise, it appears that the vast majority of CS events detected so far are driven by changes in the accretion flow itself.  \nDepending on the origin of the BLR, a change in the accretion flow (or rate) could either dramatically transform the BLR or change its ionization properties. First, the line emission from a pre-existing BLR is expected to respond to any (strong) ionizing continuum variability, as unambiguously seen in reverbera-  \ntion mapping studies. CS events could thus be regarded as the most extreme cases of such BLR response, where it essentially turns on/off (with the associated light-travel lags). Second, it has been proposed that the BLR, and indeed even the torus, could be created by outflows produced in the accretion disk 158 . In this framework, the disappearance of broad optical/UV lines in CSAGN would arise straightforwardly from a strong decrease in the accretion rate 159 , which would lead to a decrease in the mass outflow rate of the disk wind. In particular, the disappearance of the BLR would be expected to happen at bolometric luminosities of L bol /lessorsimilar 2 . 3 × 10 40 erg s -1 M 2 / 3 8 erg s -1 (Ref. 160). Therefore, as L bol increases, the optical classification of AGN would transition following the sequence Type 2.0 → 1.9/1.8 → 1.5/1.2 → 1.0 (Ref. 161).  \n3.5 Accretion disks and variability timescales To contextualize the accretion-driven (dramatic) variability seen in CSAGN, we rely on the geometrically-thin, optically-thin accretion disk (aka ' α -disk) model 106 , which is commonly applied to AGN, and specifically invoked in many studies that (try to) interpret CS-AGN. The thin nature of the disk implies an aspect ratio (at a given radius R ) of H/R /lessmuch 1 , where the height of the disk is given by H = c s / Ω , with c s being the sound speed and Ω the Keplerian orbital angular frequency, Ω K = √ GM BH /R 3 . In this model the angular momentum transport is driven by (pseudo-)viscosity, which is likely associated to magneto-rotational instability 162 . Despite its many successes, observational evidence gathered over the past 30 years has shown that this model fails to explain several important aspects of the AGN emission, such as their UV luminosity and optical variability, suggesting that accretion disks are not in a quasi-steady state equilibrium 163,164 .  \nThe observed timescales of CS events, typically ∼ 1 -10 years or even shorter 107,131,137 , are generally much shorter than the timescales over which the accretion flow in an AGN α -disk is expected to vary drastically 165 . Depending on the physical process responsible for the changes in the AGN accretiondriven radiation, different timescales would be at play. In the following we review the most important timescales associated to thin disks in accreting SMBHs, in line with many CS-AGN studies 77,103,134 .  \nWhen dealing with casually-connected varying zones, for a region of size R the shortest timescale to consider is the light crossing timescale :  \nt lc ≡ R c /similarequal 0 . 875 ( R 150 r g ) M 8 days , (6)  \nwhere c is the speed of light, and M 8 is (again) the black hole mass in units of 10 8 M /circledot . This timescale is typically associated with radiation reprocessing, and is significantly shorter than that of CS events.  \nGas motion within the disk can occur over the dynamical timescale ( t dyn ) or the closely related orbital timescale ( t orb = 2 πt dyn ). Standard relations for Keplerian (circular) motion yield:  \nt dyn ≡ R v /similarequal ( GM BH R 3 ) -0 . 5 /similarequal 10 ( R 150 r g ) 3 / 2 M 8 days  \n(7)  \nwhere G is the Gravitational constant. The timescale at which sound waves would propagate radially through the disk would be longer than t dyn by a factor ( R/H ): t snd /similarequal t dyn ( H/R ) -1 , which for a postulated ( H/R ) < 0 . 1 yields timescales of order several months. Both these timescales are thus typically too short to explain most CS events.  \nThe timescale associated with (significant parts of) the disk heating up or cooling down, i.e. the thermal timescale , can be estimated as the ratio between t dyn and the viscosity parameter 106 ( α ):  \nt th /similarequal t dyn α /similarequal ( α 0 . 03 ) -1 ( R 150 r g ) 3 / 2 M 8 yrs . (8)  \nThis timescale is suggested as the origin of stochastic optical variability observed on ∼ months timescales in AGN 166 , and it has been argued that this timescale could agree with some CSAGN 134 .  \nCooling and heating fronts can propagate radially through the accretion disk at v front /similarequal c s α , implying timescales of t front /similarequal t th ( H/R ) -1 , or:  \nt front /similarequal 20 ( H/R 0 . 05 ) -1 ( α 0 . 03 ) -1 ( R 150 r g ) 3 / 2 M 8 yrs (9)  \nFinally, changes in the global accretion rate through the disk are associated with the radial inflow timescale , or so-called viscous timescale :  \nt vis /similarequal 400 ( H/R 0 . 05 ) -2 ( α 0 . 03 ) -1 ( R 150 r g ) 3 / 2 M 8 yrs . (10)  \nWe note that since all these timescales depend on the location within the disk (namely, R/r g ), and since in such disk the continuum emission at any given wavelength is preferentially emitted from certain disk regions, one may want to consider a more detailed derivation of wavelength-dependent timescales, which in principle further depend on M BH , λ Edd and even BH spin. The R /similarequal 150 r g scaling used above is generally appropriate for the near-UV-to-optical continuum.  \nThe large-amplitude and coherent nature of the radiation changes seen in CS-AGN are generally thought to be associated with larger-scale processes, which would act on timescales of either t front or t vis . However, both these timescales are longer than what is typically observed (see §3.1 and §3.2). For example, in the nearby system Mrk 1018 a drop in the luminosity of a factor of ∼ 13 -15 was observed over a timescale of ∼ 8 years 137 . Assuming the nominal values of Eqs.9-10, for this object the cooling/heating front and viscous timescales would be expected to be ∼ 15 and ∼ 280 years, respectively. Likewise, the CSAGN transitions in many other systems are constrained to occur within /lessorsimilar 10 years 66,86,131 . Perhaps the most extreme case reported to date is that of 1ES 1927+654, where the UV/optical luminosity increased by more than a factor of ∼ 50 within a few months 107 .  \nSeveral possible mechanisms have been proposed to overcome this inconsistency, typically by considering thicker disks, since increasing the aspect ratio ( H/R ) would lead to shorter timescales. Among the mechanisms explored in the literature, thickening the disk was suggested to be possible through: i) an enhanced total opacity in the disk, due to line opacity from heavy  \nelements, which could alter the thermal stability of the disk and increase its scale height 167 ; ii) magnetic torques in the innermost regions of the disk, which would cause the inner disk to heat up and inflate 168 ; iii) magnetic pressure dominating in the upper atmosphere of the disk, so that the magnetic field could support the disk vertically, increasing its height 169 . Magnetically-driven disk-winds 170 , carrying away most of the angular momentum of the gas, would also considerably shorten the viscous timescales of the accretion disk.  \nMany recent studies present more elaborate calculations and/or simulations of globally stable accretion flows, which however present large-amplitude variability on exceedingly short timescales, reminiscent of what is seen in CSAGN 137,171,172 We discuss some of these models in more detail in what follows.  \n3.6 The origin of CS transitions Several different scenarios have been proposed to explain CS events, which can be roughly divided into two main categories: instabilities in the (globally stable) accretion disk (§3.6.1), which could lead to changes in the emission properties of the accretion flow; and major accretion disk perturbations (§3.6.2), such as stellar tidal disruption events (TDEs). As discussed below, it is possible that these two different types of mechanisms could be at work in different CS events.", '3.6.1 Disk instabilities': 'Disk instabilities can be triggered by various effects, and can thus occur over a range of timescales. One possibly relevant mechanism for CS-AGN is the thermal instability 173 , associated with changes in the surface density of the disk, driven by changes in opacity, which could lead to significant, though localized, variations in temperature. The magnetorotational instability 162 could also lead to changes in the local heating on thermal timescales. It has been shown 137 that, in the case of Mrk1018, the disappearance/appearance of the broad optical lines is directly related to the absence/presence of a strong soft X-ray excess at /lessorsimilar 1 -2 keV (top panels of Fig. 5). Such a component, possibly associated to a warm Comptonizing material, could extend to the UV, and be responsible for most of the ionizing photons that create the broad emission lines. In Mrk 1018 the Eddington ratio goes from λ Edd ≈ 8 × 10 -2 to ≈ 6 × 10 -3 during the event that leads to the disappearance of the broad lines and, while the /greaterorsimilar 2 keV emission drops only of a factor ∼ 7 , the soft X-ray excess decreases of a factor of /greaterorsimilar 60 . Qualitatively similar results were obtained for other CS-AGN 174 . This spectral hardening could be similar to the soft-to-hard state transitions observed in black hole binaries, associated to the hydrogen ionisation disk instability, where the inner disk puffs up and transitions into an advection dominated accretion flow (ADAF) at λ Edd /lessorsimilar 0 . 02 . These state transitions would therefore produce large variations of the amount of ionizing radiation emitted by the AGN. A consistent behaviour was also found 175 by studying the relation between the optical-to-X-ray spectral index ( α OX ) and λ Edd , which showed similarities with what is observed in black hole binaries, including an inversion of the α OX -λ Edd correlation at λ Edd /similarequal 10 -2 . This scenario would imply that CS events typically happen at an Eddington ratio of a few percent,  \nwhich appears to be in agreement with some recent studies 15,112 . The timescale of these transitions would be the viscous timescale (Eq. 10), which is considerably longer than the timescale of CS events. However, since radiation pressure in AGN is more important than in binaries 137 , the sound speed in the disk would be higher, leading to a decrease in t vis . Including magnetic pressure would further increase the sound speed and decrease the timescales. A similar effect would be produced by thicker disks, as outlined in §3.5.  \nAnother viable mechanism for CS events is the propagation of a cooling front in an inflated accretion disk, associated to a sudden change in the magnetic torques in the innermost regions of the accretion flow, close to the innermost stable circular orbit 134,135 . Magneto-hydrodynamical instability could cause the torque to decrease, leading to an outwards-propagating cold front, which would decrease the optical/UV luminosity of the AGN. At some point the cooling front would stop, and a heating front is expected to propagate inward, leading to a re-brightening of the AGN 135 . The inflated inner disk would have a larger aspect ratio ( H/R /similarequal 0 . 2 ) than typically assumed for standard accretion disk 134 , which would lead to shorter timescales (see Eq. 9), consistent with those observed in some CS events. This model was shown to explain well the fading and re-brightening of SDSSJ1100 -0053, which was associated to the disappearance and reappearance of the UV continuum and broad hydrogen emission lines 135 . There are other suggestions for how advection and radiation pressure regions in the inner disk may drive significant instabilities, relevant for driving CS transitions, even in sources accreting at Eddington rates of a few percent. Specifically, instabilities are expected to develop in a narrow region that links the inner ADAF-like part of the accretion flow and the standard, outer disk, and could explain the recurrent CS events observed in objects such as, e.g. NGC1566 and NGC4151 (Ref. 172). A sudden change of polarity of the magnetic flux advected onto the SMBH could also lead to a CS event, triggering an increase of UV/optical flux while at the same time destroying the X-ray corona 176 , as observed in 1ES 1927+654. For a magnetically supported and highly viscous ( α /similarequal 1 ) disk this would lead to changes on short timescales ( < 1 year), consistent with those observed in 1ES 1927+654.', '3.6.2 Major disk perturbations': "Some CS-AGN could be powered by the tidal disruption of a star passing near a SMBH. This explanation was first proposed for the changing look quasar SDSS J0159+0033 177 which, after the outburst that caused the CS event, showed a decline in flux consistent with the t -5 / 3 trend expected for TDEs (and supported by some observations; Ref. 178). The estimated virial black hole mass of this object ( ∼ 1 . 7 × 10 8 M /circledot , Ref:103) was however found to be considerably higher than what is observed in, and expected for, typical TDEs. This could pose a problem, since the tidal radius ( r tid ), i.e. the radius at which a star of mass M ∗ and radius R ∗ would likely be disrupted is inversely proportional to M BH :  \nr tid /similarequal 2 . 2 r g ( M ∗ M /circledot ) -1 / 3 ( R ∗ R /circledot ) M -2 / 3 8 . (11)  \nTherefore, if the star is compact and/or the black hole is massive, then the tidal disruption would only take place inside the event  \nTable 1: Probing different regions of accreting SMBHs with CL-AGN.  \nhorizon, and it would not be detected. For SDSSJ0159+0033 the tidal radius would be expected to be at only a few r g . It was however argued that the virial mass could be significantly overestimated in this objects, possibly due to the contribution of the stellar debris to the H α line 179 , since it would lie significantly above the M BH -σ relation. Another explanation may involve a BH with a non-zero spin, which would lead to a smaller ergosphere and allow for (observable) TDEs with smaller r tid (Ref. 180).  \nTDEs that occur in an already-active SMBH, and that interact with a pre-existing accretion disk, were also argued to explain the disappearance and reappearance of the Xray corona 104,138 , the strong blackbody component 104,138 , and the peculiar behaviour of the broad optical lines 181 seen in 1ES1927+654. Some models suggest that the occurrence rate of TDEs would be higher in (persistently accreting) AGN 182 , and that TDEs-in-AGN could be associated with the presence of stars within the accretion flow itself 183 . Recent simulations 184 demonstrated how the debris stream of the tidally disrupted star would hit an accretion disk, disrupting the gas in the disk and further depleting a large fraction of the angular momentum of the disk gas (including through shocks). This, in turn, would rapidly increase the accretion rate of material through the inner regions of the disk, significantly affecting the disk UV/optical emission 185 , and may lead to the disappearance of the X-ray corona, which is linked to the innermost regions of the accretion flow. As the inner disk is replenished the corona would be 'reignited'. If the gas pressure falls sharply from the outer disk to the inner disk, then magnetic loops could pull the gas, allowing it to spiral inward in tens of disk orbits (Eq. 7), roughly in agreement with the observed timescales. We note that detailed numerical studies of such processes are still scarce 184 and large parts of the parameter space are yet to be explored.  \nOther mechanisms proposed to explain CS events include the tidal interaction between disks in binary SMBHs 186 , and the interaction between the disk and a recoiling SMBH 187 . The latter explanation has been proposed for Mrk 1018, with the recoiling SMBHhaving a very eccentric orbit and perturbing the accretion flow with a period of 29 years. Finally, stellar-mass point perturbers in the disk (i.e., stars or stellar mass BHs) could cause extreme mass ratio inspiral events or changes in the accretion flow 134 , triggering (potentially recurring) CS events.", '4 Open questions and future prospects': 'While CO and CS events are very different in nature, both types of transitions allow us to probe the close vicinity of active SMBHs (Table 1). In the following we discuss some of the key open questions that could be addressed to better understand changing-look AGN in the coming years, and to improve our overall understanding of the AGN phenomenon.', '4.1 CO-AGN': '- · Is there a relation between the host galaxy properties and the fraction of AGN showing CO events?\n- AGN in the final stages of a major galaxy merger are typically heavily obscured 188 , likely due to the efficient depletion of the angular momentum of galactic gas caused by tidal forces. This could make the nuclear environment of AGN in mergers particularly dynamic, and could enhance the fraction of CO events 189 . While several 189 CO-AGN have been found in mergers, but it is not yet clear whether the rate of CO events changes with the host galaxy proper-\n- ties.\n- · What can we learn about properties of the circumnuclear material from CO events?\n- High-resolution X-ray spectroscopy can shed light on some fundamental properties (e.g., ionization state, velocity, structure) of the BLR and torus clouds. This will be possible in the coming years with the new generation of X-ray calorimeters, such as the one on-board XRISM 190 .\n- · Do lowλ Edd AGN show eclipses?  \nSeveral theoretical studies have predicted that at λ Edd /lessorsimilar 10 -3 the AGN would be too weak to produce outflows that refill both the BLR and the torus 159 . Therefore one would expect that in these objects eclipses are absent, or extremely rare. Large systematic studies of nearby AGN would allow us to better understand the relation between the likelihood of observing CO events and the physical properties of accreting SMBHs.  \nOne fundamental issue with constraining the properties of the obscuring clouds in CO-AGN has been the non-continuous monitoring in the X-ray band. Thanks to the advent of surveys such as those carried out by eROSITA 191 and in the future by the Einstein Probe 17 , it will be possible to significantly increase the  \nsample of CO-AGN, and to better understand their occurrence rates and typical timescales.', '4.2 CS-AGN': '- · How common are CS transitions in AGN, and what are their typical timescales?\n- Large samples of CS-AGN, together with high-cadence monitoring, will allow us to understand their occurrence rates and typical timescales. The on-going SDSS-V survey 192 is expected to identify a large number of CSAGN in the next few years, over timescales ranging daysyears. The Vera Rubin Observatory 16 , which will provide deep multi-band optical photometry with several-day cadence over the entire southern sky, will largely increase the number of known extreme AGN variability events. The identification of CS-AGN will be possible thanks to machine-learning techniques 193 and spectroscopic followups carried out with the next generation of highly multiplexed spectrographs, such as 4MOST 194 .\n- · Are CS transitions related to certain physical properties of the AGN?  \nIt is still unclear whether there is a particular set of physical parameters of the accreting SMBH (e.g., M BH , λ Edd ) that increase the likelihood of a CS transition, as it would be expected by some models 137 . Future large and wellunderstood samples of CS-AGN will allow us to identify and/or test such links, which would be a fundamental step towards understanding the origin of CS transitions.  \n- · What drives extreme AGN variability? Is there a unique\n- physical mechanism driving CS transitions? The large samples that will be available in the near future will allow to refine our understanding of the various physical mechanisms that drive extreme AGN variability. The various types of mechanisms, associated with e.g. disk instabilities (§3.6.1) or perturbations (§3.6.2), can produce CS events with markedly different properties. For example, the CS event in 1ES 1927+654 107 is clearly very different from other nearby CS-AGNs, showing very unexpected properties in the X-ray band 104,138,142 . Other events seem to necessitate a mechanism that can produce recurring CS transitions 96,98,100,102 . The combination of optical and Xray surveys will therefore be extremely important to fully characterise CS events.  \nWith the advent of large photometric (Vera Rubin Observatory 16 ) and spectroscopic (SDSS-V 192 , 4MOST 194 ) optical surveys, as well as with that of wide-field surveys in the X-rays (i.e., with eROSITA 191 and the Einstein Probe 17 ) and the UV (e.g., with ULTRASAT 18 ), in the next few years a very large number of CS-AGN will be identified and characterised, allowing us to discover new extreme variability and transient phenomena in AGN, and to understand what physical mechanisms are at play in these fascinating objects.', 'Acknowledgements': "C.R. acknowledges support from the Fondecyt Iniciacion (grant 11190831) and ANID BASAL (project FB210003). B.T. acknowledge support from the European Research Council  \n(ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 950533) and from the Israel Science Foundation, (grant 1849/19). 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