The fourth catalog of active galactic nuclei AGNs detected by the Fermi Gammaray Space Telescope Large Area Telescope 4LAC between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes b gt 10. It includes 85 more sources than the previous 3LAC catalog based on 4 yr of data. AGNs represent at least 79 of the highlatitude sources in the fourth FermiLarge Area Telescope Source Catalog 4FGL which covers the energy range from 50 MeV to 1 TeV. In addition 344 gammaray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars 98 while the remainder are other types of AGNs. The blazar population consists of 24 Flat Spectrum Radio Quasars FSRQs 38 BL Lactype objects and 38 blazar candidates of unknown types BCUs. On average FSRQs display softer spectra and stronger variability in the gammaray band than BL Lacs do confirming previous findings. All AGNs detected by groundbased atmospheric Cerenkov telescopes are also found in the 4LAC.
['Active galactic nuclei', 'Blazars', 'Gamma-ray sources', 'Relativistic jets', 'Quasars', 'BL Lacertae objects', '16', '164', '633', '1390', '1319', '158', 'Astrophysics - High Energy Astrophysical Phenomena']
The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope
{'The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope': "- 30 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  \n- 44 School of Education, Health and Social Studies, Natural Science, Dalarna University, SE-791 88 Falun, Sweden\n- 45 Centre d' ' Etudes Nucl'eaires de Bordeaux Gradignan, Universit'e de Bordeaux, IN2P3/CNRS, 33175 Gradignan Cedex, France", 'ABSTRACT': 'The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gammaray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains 2863 objects located at high Galactic latitudes ( | b | > 10 · ). It includes 85% more sources than the previous 3LAC catalog based on 4 years of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi -Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, 344 gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects (BL Lacs), and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cherenkov telescopes are also found in the 4LAC.  \nSubject headings: gamma rays: galaxies - gamma rays: observations - galaxies: active - galaxies: jets - BL Lacertae objects: general - quasars: general', '1. Introduction': 'Thanks to its broad energy range, excellent sensitivity, and all-sky monitoring capabilities, the Fermi Gamma-ray Space Telescope Large Area Telescope (LAT) has revolutionized our view of the gamma-ray sky. The fourth Fermi -LAT source catalog (4FGL, The Fermi-LAT collaboration 2019), based on the first 8 years of data from the mission, contains 5064 sources in the energy range 50 MeV to 1 TeV.  \nThe fourth catalog of AGNs detected by the LAT (4LAC), presented here, is derived from the 4FGL catalog. At high Galactic latitudes, active galactic nuclei (AGNs) represent by far the dominant class of gamma-ray sources in the 4FGL. The vast majority of these AGNs are of the blazar type, which are characterized by having relativistic jets closely aligned with our line of sight. The two main classes of blazars are Flat Spectrum Radio Quasars (FSRQs) and BL Lac-type objects (BL Lacs), distinguished according to the strength of their optical emission lines. FSRQs have strong, broad emission lines, while BL Lacs have weak, narrow, or no such lines. In addition to the improvements of the 4FGL relative to previous gamma-ray catalogs, the 4LAC has benefited from updated methods of associating gamma-ray AGNs with those known at other wavelengths. The 4LAC supersedes the third catalog of AGNs detected by the LAT (3LAC, Ackermann et al. 2015), which was based on 4 years of data.  \nGamma-ray AGN catalogs constitute unique resources for a broad range of astrophysics research. Recent applications include: population studies probing the BL Lac-FSRQ dichotomy (e.g. Ghisellini et al. 2017; Nalewajko & Gupta 2017); works on individual sources investigating the connections between gamma-ray loudness and brightness/polarization at other observational bands (e.g. Angelakis et al. 2016; Lico et al. 2017; Massaro et al. 2017; Fan & Wu 2018; Zargaryan et al. 2018); timing correlations between activity in the gamma-ray band and other wavelengths (e.g. Fuhrmann et al. 2016; Itoh et al. 2016); and tests of the possible link between gamma-ray AGNs and sources of ultra high-energy cosmic rays (e.g. Kagaya et al. 2017) or high-energy neutrinos (e.g. Padovani et al. 2016; Aartsen et al. 2017; Garrappa et al. 2019). These catalogs also enable probes of the extragalactic background light (EBL, e.g., Abdollahi et al. 2018) and the intergalactic magnetic field (e.g., Broderick et al. 2018; Ackermann et al. 2018), along with a measurement of the AGN contribution to the extragalactic diffuse gamma-ray background (e.g., Fornasa et al. 2016; Di Mauro et al. 2018).  \nThe paper is organized as follows. Section 2 briefly describes the observations by the LAT and the analysis employed to produce the eight-year catalog. In § 3, we present the methods for associating gamma-ray sources with AGN counterparts and the different schemes for classifying them. Section 4 describes the contents of the 4LAC fits table and gives the statistics of the blazar and non-blazar populations. This section also includes a brief presentation of low-latitude ( | b | < 10 · ) AGNs, which do not formally belong to the 4LAC. Some of the basic properties of the catalog sources are given in § 5, along with a discussion of the overlap with the AGNs detected at very high energies (VHE; energies above 100 GeV) by atmospheric Cherenkov telescopes. Section  \n6 summarizes our findings.  \nIn the following, we use a ΛCDM cosmology with values from the Planck results (Planck Collaboration et al. 2013); in particular, we use h = 0 . 67, Ω m = 0 . 32, and Ω Λ = 0 . 68, where the Hubble constant H 0 = 100 h km s -1 Mpc -1 .', '2. Observations with the Large Area Telescope - Analysis Procedures': 'The 4LAC analysis was performed in the context of the 4FGL catalog, which is briefly summarized here. We refer the reader to the parent paper describing the 4FGL catalog for details (The Fermi-LAT collaboration 2019). The data were collected over the first 8 years of the mission, from 2008 August 4 (MJD 54682) to 2016 August 2 (MJD 57602). The reprocessed P8R3 SOURCE V2 event class (Bruel et al. 2018) data were used, with photon energies between 50 MeV and 1 TeV, broadening the energy interval with respect to the 100 MeV - 300 GeV range of 3FGL (Acero et al. 2015). The increase in acceptance relative to the P7REP class used in 3FGL is 20% at high energies, accompanied by a better point-spread function (PSF). These improvements are beneficial to the source detection and localization and hence to the counterpart association. A dedicated diffuse emission model was developed for analyses using the new event class. Weights penalizing photons with low energies and/or having directions close to the Galactic plane were introduced in the 4FGL likelihood to better account for systematic uncertainties. More details are available in Table 2 of the 4FGL paper. Different spectral models (power-law, log-parabola, power-law with super-exponential cutoff) were tested, and the results are systematically reported in 4FGL. Sources with a maximum likelihood Test Statistic (TS) greater than 25 were retained in 4FGL, corresponding to a significance just over 4 σ evaluated for the χ 2 distribution with 4 degrees of freedom. Variability was assessed via both 1-year and 2-month light curves.', '3. Source association and classification': 'The associations of 4FGL gamma-ray sources are based on positional coincidence with potential counterparts that display AGN-type spectral characteristics in the radio, infrared, optical, or X-ray bands. A conservative policy adopted early in the mission by the Fermi -LAT Collaboration is that firm identification is only claimed when correlated variability with a counterpart detected at lower energy has been reported. So far, only 78 AGNs have met this condition (see Table 7 of 4FGL). For the other sources, we use statistical approaches for finding associations between LAT sources and AGNs. The two approaches used here, the Bayesian method and the likelihood-ratio method, have been extensively described in previous catalogs (Abdo et al. 2010a; Ackermann et al. 2011, 2015) and are briefly summarized below.', '3.1.1. The Bayesian Association Method': 'The Bayesian method was adapted for the Fermi -LAT catalogs following the work of Mattox et al. (1997) developed for the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory . This method is described in Abdo et al. (2010a) and implemented with the gtsrcid tool 1 . The angular distance between a LAT source and a candidate counterpart corresponds to the position uncertainty in the case of a real association, while it is driven by the counterpart density in the case of a false (random) association. The prior distribution is fully characterized by a single number, which is the a-priori probability that a given source of a catalog is the true counterpart of a gamma-ray source. This probability (referred to as the prior in the following) is assumed to be constant for a given catalog and is calibrated via Monte Carlo simulations so that the number of false associations, N false , is equal to the sum of the association-probability complements. For a given counterpart catalog, the prior is found to be close to N assoc /N tot , where N assoc is the number of associations from this catalog and N tot is the number of catalog sources. A uniform threshold of 0.80 is applied to the posterior probability for the association to be retained.  \nThe list of catalogs used for the AGN associations with 4FGL sources is given in Table 1. With respect to 3LAC (Ackermann et al. 2015), updates of counterpart catalogs, e.g., BZCAT (Massaro et al. 2015), have been used when available. An important addition to the set of catalogs is the Radio Fundamental Catalog 2 (RFC, Petrov et al. 2019), with 2720 associations with 4FGL sources (representing 85% of all AGNs). The high-efficiency of association with VLBI catalogs that are sensitive to parsec-scale emission at 4-8 GHz is attributed to two factors: 1) the fact that γ -ray emission and parsec scale radio emission which originate from contemporary AGN activity are related, 2) the scarcity of radio sources with parsec scale emission at 4-8 GHz. The RFC includes many new entries that came from dedicated follow-up observations (Petrov et al. 2013; Schinzel et al. 2015, 2017) of unassociated gamma-ray sources, triggered by the publication of previous Fermi -LAT catalogs. Applying the Bayesian method to the whole catalog and retaining associations with P ≥ 0.80, the association probability attached to the recent additions (181 sources) are reported as NULL to distinguish them for the others.', '3.1.2. The Likelihood-Ratio Association Method': "The Likelihood Ratio (LR, e.g., Cash 1979; de Ruiter et al. 1977; Prestage & Peacock 1983; Sutherland & Saunders 1992; Lonsdale et al. 1998; Masci et al. 2001; Ackermann et al. 2011) method developed in the Fermi -LAT context makes use of large, relatively uniform surveys in the radio and  \nin X-ray bands. These surveys enable us to search for possible counterparts among the faint radio and X-ray sources. The LR method is similar in nature to the Bayesian method but the false association rate is computed from the density of objects brighter than the considered candidate, assessed from the survey log N-log S distribution. The method for computing the probability that a candidate is the 'true' counterpart (called the reliability in this context) is described in detail in § 3.2 of the 3LAC paper (Ackermann et al. 2015). A source is considered as a high-confidence counterpart of a given gamma-ray source if its reliability is greater than 0.80 for at least one survey.  \nFor the LR approach, we analyzed the NRAO VLA Sky Survey (NVSS; Condon et al. 1998), the Sydney University Molonglo Sky Survey (SUMSS; Mauch et al. 2003), the Australia Telescope 20 GHz radio source catalog (Murphy et al. 2010b), and the ROSAT All Sky Survey (RASS) Bright and Faint Source Catalogs (Voges et al. 1999, 2000). We also explored the second RASS catalog (Boller et al. 2016), but this attempt did not lead to further associations.", '3.2. Association Results': 'The threshold adopted for the association probability is 0.80 in either method. This value represents a compromise between association efficiency and purity. The fraction of sources associated by both methods is 73% (2082/2863), with 684 and 97 sources being solely associated with the Bayesian and LR methods, respectively. The overall false-positive rate is 1.6%, where N false is calculated as described in § 3.1.1. The estimated number of false positives among the 1353 sources not previously reported in 3LAC is 35 . 6.  \nAs in previous LAT AGN catalogs, we define a Clean Sample as those 4LAC sources that did not have any cautionary analysis flags, as described in § 3.7.3 of the 4FGL paper (The Fermi-LAT collaboration 2019). The most frequent flags are flag 5 (source close to a brighter neighbor), flag 3 (large flux variation when changing diffuse emission model) and flag 2 (large position shift when changing diffuse emission model). Table 2 compares the performance of the two methods in terms of total number of associations N assoc , estimated number of false associations N false , and number of sources associated solely via a given method, N S , for the full and Clean samples.', '3.3. Source Classification': 'The classification of a source as an AGN primarily relies on its optical spectrum. Other characteristics, like the radio loudness, the presence of a flat/steep radio spectrum between 1.4 GHz and 5 GHz, the broadband emission, the core compactness or the level of radio extended emission, the detection of variability and degree of polarization observed in different bands are used as ancillary information. If available, earlier classifications reported in the literature have been checked.', '3.3.1. Optical Classification': "The different resources used in the 4LAC for the optical classification are, in decreasing order of precedence:  \n- · optical spectra from recent intensive follow-up programs (e.g., Shaw et al. 2013a,b; ' Alvarez Crespo et al. 2016a,c,b; Landoni et al. 2018, 2015; Marchesi et al. 2018; Marchesini et al. 2019; Massaro et al. 2015b,c,a, 2014; Paggi et al. 2014; Paiano et al. 2019, 2017a,b,c; Pe˜na-Herazo et al. 2017; Ricci et al. 2015; Chiaro et al. 2016; de Menezes et al. 2019); these data are especially valuable for blazar candidates of previous LAT AGN catalogs that had never been observed.\n- · the optical classification published in the BZCAT list (which is a compilation of sources classified as blazars, Massaro et al. 2015)\n- · spectra available in the literature or from online databases, e.g., the Sloan Digital Sky Survey (SDSS; Ahn et al. 2012; Massaro et al. 2014), 6dF Galaxy Survey (Jones et al. 2009), when more recent than the latest version of BZCAT (Massaro et al. 2015). The latter information was used only if the spectrum was published.  \nThe relevant references are reported in the electronic table of the catalog. We did not use the blazar classes from the Simbad database 3 , since some of them correspond to predictions based on the WISE -strip approach (D'Abrusco et al. 2014) and were not obtained from spectral observations.  \nIn the 4LAC, we classify the AGN-like gamma-ray detected objects adopting the following terminology:", '· confirmed classifications:': '- -FSRQ, BL Lac, radio galaxy, steep-spectrum radio quasar (SSRQ), compact steep spectrum radio source (CSS), Seyfert galaxy, and Narrow-Line Seyfert 1 galaxy (NLSy1) these are sources with a well-established classification in the literature and/or an optical spectrum with clear evidence for or lack of emission lines.', '· tentative classifications:': "- -BCU, blazar candidates of uncertain type: these are considered candidate blazars because the association methods (see § 3.1.1 and 3.1.2) select a candidate counterpart that satisfies at least one of the following conditions:\n- ∗ a BZU object (blazars of uncertain/transitional type) in the BZCAT list;  \n- ∗ a source with multiwavelength data in one or more of the WISE , AT20G, RFC (Petrov et al. 2019), CRATES (Healey et al. 2007), PMN-CA (Griffith & Wright 1993), CRATES-Gaps (Healey et al. 2009), or CLASS (Caccianiga et al. 2002) lists that indicates a flat radio spectrum and shows a typical two-humped, blazar-like spectral energy distribution (SED);\n- ∗ a source included in radio and X-ray catalogs not listed above and for which we found a typical two-humped, blazar-like SED (see Bottcher 2007).  \nThe scheme followed in 3LAC whereby BCUs were further divided in three subclasses according to the quality or availability of their optical spectra has not been reconducted in 4LAC. The large number of new BCU sources would have made this task excessively manpower intensive.  \n- -AGN - for these candidate counterparts the existing data do not allow an unambiguous determination of the AGN type and do not meet any of the criteria to be classified as BCU. Their SEDs display properties typical of radio-loud compact core objects, but the literature information is either incomplete or conflicting for different epochs or wavelengths.  \nAt low Galactic latitudes, the surveys include a large number of Galactic sources; therefore the 4FGL class of | b | < 10 · sources associated solely via the LR-method has been set to the 'unknown' class as opposed to the 'BCU' class used by default for sources at larger latitudes. These sources are thus not considered here.", '3.3.2. Classification based on the broadband spectral energy distribution': 'Blazars and more generally radio-loud AGNs can also be classified according to the peak photon frequency ν s , peak of the synchrotron part of their broadband SEDs. As a large number of 4LAC sources do not have a measured redshift (see Section 5.2), the frequency in the observer frame was used. The SEDs of all 4LAC AGNs were generated using the SED data archive and SED(t)Builder interactive web-tool available at the Italian Space Agency (ASI) Space Science Data Center (SSDC) 4 . Inspection of the error ellipse and the position of the counterpart was first performed using the Sky Data Explorer at SSDC. Two different approaches were followed to enable a cross-check. The first was the parametric procedure used in Abdo et al. (2010c) and Ackermann et al. (2011), which is based on the broadband spectral indices α ro (between 5 GHz and 5000 ˚ A) and α ox (between 5000 ˚ A and 1 keV). The list of surveys and catalogs providing the broadband flux density data is given in Abdo et al. (2010c). The second method, already used in 3LAC and favored here, consisted of fitting the SED synchrotron hump with a third-degree polynomial fit in the log-log plane. This fit was carried out manually on a source-by-source basis after carefully discarding outlying data  \n(e.g., taken during flaring episodes) and those dominated by the thermal emission of the accretion disk or of the host galaxy. This methodology allowed us to assign a ν s , peak value to more objects, since a measured X-ray flux is not required, provided the SED curvature is sufficiently pronounced in the IR-optical band. This fit also provided the νF ν value at the peak position. Limitations arose from possible human errors, the use of non-simultaneous broadband synchrotron data, and remaining contamination of thermal emission. This contamination may result in an overestimation of the ν s , peak values for FSRQs, while the near-IR-optical contribution of the host galaxy may bias ν s , peak low in BL Lacs. Comparing the results of the two procedures indicated that the fitting method led to an average shift of -0.23 (rms: 0.53) and -0.22 (rms: 0.80) in log ν s , peak relative to the initial method for FSRQs and BL Lacs, respectively. We identify these shifts as systematic uncertainties. Relative to 3LAC, a more conservative approach was followed when little data were available, leading to a lower fraction of classified sources. The mean differences in log ν s , peak between 4LAC and 3LAC are -0.14 (rms: 0.37) and -0.11 (rms: 0.65) for FSRQs and BL Lacs, respectively.  \nFollowing Abdo et al. (2010c), the value of the observed ν s , peak was used to classify the source as either a low-synchrotron-peaked blazar (LSP, for sources with ν s , peak < 10 14 Hz), an intermediatesynchrotron-peaked blazar (ISP, for 10 14 Hz < ν s , peak < 10 15 Hz), or a high-synchrotron-peaked blazar (HSP, if ν s , peak > 10 15 Hz). To obtain the rest-frame value of ν s , peak , a correction by a (1+ z ) factor is needed, where z is the redshift.', '4. The Fourth LAT AGN Catalog (4LAC)': 'Figure 1 displays the loci of the 4LAC sources in Galactic and J2000 equatorial coordinates. As already noted in previous LAC catalogs, it is clear from this figure that sources of different classes are not uniformly distributed over the sky. This anisotropy is demonstrated in Figure 2 showing the Galactic-latitude distributions for all 4LAC sources as well as for FSRQs, BL Lacs, and BCUs separately. The anisotropy is most noticeable for BL Lacs, which are 42% more abundant in the northern Galactic hemisphere than in the southern one. BCUs show the opposite pattern and somewhat offset the overall anisotropy as seen in the total distribution of sources, which is close to being uniform. The observed anisotropies stem from the larger and better spectroscopic data available in the literature for the northern hemisphere relative to the southern one. Better spectroscopic data are required to assess the BL Lac nature of an object relative to a FSRQ, because of the weaker optical emission lines in the spectrum of a BL Lac object.  \nThe format of the 4LAC fits table 5 is described in Table 3. In addition to relevant parameters 6  \nfrom the 4FGL fits file 7 , we report the optical and SED-based classes, redshifts, observer-frame synchrotron-peak frequencies, and νF ν at the synchrotron-peak frequencies. When available, we also provide the VLBI and Gaia counterparts as given in the RFC. The median position accuracy of VLBI counterparts is 0.8 mas. Therefore, establishing association with VLBI immediately allows us to propagate the associations to the optical range using Gaia and IR using WISE. Following this route, we obtain Gaia associations with 2134 gamma-ray blazars (74%).', '4.1. Census': 'Table 4 summarizes the 4LAC statistics. The 4LAC includes 2863 sources, with 655 FSRQs, 1067 BL Lacs, 1077 BCUs, and 64 other AGNs. A total of 1353 sources were not reported in the previous 3LAC catalog, although some of these have been reported elsewhere, e.g., Arsioli & Chang (2017); Arsioli & Polenta (2018). The new sources include 204 FSRQs, 290 BL Lacs, 822 BCUs, and 36 non-blazar AGNs. The Clean Sample contains 2614 sources, with 591 FSRQs, 1027 BL Lacs, 941 BCUs, and 55 other AGNs. The figures shown in the following only include Clean-Sample sources, unless specified otherwise  \nThe SED-based scheme was able to classify 92% of the FSRQs and 85% of the BL Lacs in 4FGL. These values are lower than in 3LAC (99% and 97% for FSRQs and BL Lacs respectively) due to the more conservative classification procedure used here. The classified fraction decreases to 60% for BCUs, for which fewer broad-band archival data are available. Figure 3 shows the ν s , peak distributions for the different classes. FSRQs are overwhelmingly of the LSP class; therefore no distinction based on SED-based classes is made for them in figures and tallies. In addition to leaving more sources unclassified, the new procedure has produced a decrease in the the share of ISPs and HSPs among FSRQs, from 10% to 2%. Of the five HSP FSRQs, all located at z < 0 . 63, two are new: B3 0038+377 and PKS 1555 -140.  \nBL Lacs are fairly evenly distributed among the LSP, ISP and HSP subclasses. The 4LAC ν s , peak distribution differs substantially from the 3LAC one, where HSPs were the most abundant subclass. The new classification procedure caused 15% of the AGN previously classified as HSP to be reclassified as ISP.  \nAs a testimony to the substantial follow-up observational efforts already mentioned, 144 sources reported as BCUs in the 3LAC paper (either at high- or low-Galactic latitudes) are now classified as BL Lacs and 17 as FSRQs. Three BCUs have been reclassified as radio galaxies (IC 1531, TXS 0149+710, PKS 1304 -215). Eight sources have changed from a FSRQ to a BL Lac (RGB J0250+172, NVSS J040324 -242946, GB6 J0941+2721, 2MASS J11303636+1018245, PKS 1144 -379, 4C +15.54, TXS 1951 -115, PKS 2233 -173) and three more from a BL Lac to a FSRQ (PMN J0709 -0255, B2 2234+28A, TXS 2241+406).  \nThe 3LAC sources that are not present in 4LAC are listed in Table 5, along with the various reasons for this situation. Fifty-five 3LAC sources have not been detected in 4FGL. Nineteen 3LAC sources were duplicate associations for which the smaller 4FGL error boxes relative to 3FGL have enabled the association ambiguity to be lifted. Fifteen sources reported in 3LAC have lost their associations (becoming unassociated) while three others are now associated with non-AGN counterparts, all due to improved localizations.', '4.2. Non-blazar AGNs and misaligned AGNs': "Table 6 lists the 70 non-blazar AGNs included in the 4FGL, 64 of which belong to the 4LAC and 6 are part of the low-latitude sample. Non-blazar sources represent about 2% of the total number of AGNs in the 4FGL, a fraction that is basically identical to that found in the 3LAC. Non-blazar sources are further separated into six different classes: 41 radio galaxies 8 , 9 NLSy1s, 5 CSSs, 2 SSRQs, 1 Seyfert galaxy and 11 other AGNs.  \nA total of 36 new non-blazar AGNs are reported in the 4LAC, 22 of which are radio galaxies. The median 1.4 GHz radio luminosity of the newly detected radio galaxies is about 10 24 . 4 WHz -1 , with the distribution ranging over more than 4 decades (from below 10 22 WHz -1 , for NGC 2892, to above 10 26 WHz -1 , for PKS 2324 -02). The detection of the FR I radio galaxy 3C 120 in gamma rays was first reported by Abdo et al. (2010b) using 15 months of LAT data, but it is reported in a LAT catalog for the first time. Its absence from previous catalogs can be attributed to periods of flaring interspersed with long periods of low activity (Tanaka et al. 2015). TXS 1303+114 and TXS 1516+064 are members of the FRICAT (Capetti et al. 2017a), although they were earlier proposed as candidate low-power BL Lacs in Capetti & Raiteri (2015) based on their mid-infrared and optical emission. Among the sources already present in previous LAT catalogs, Fornax A stands out because it is for the first time detected as an extended source (see Ackermann et al. 2016). Three 3FGL sources have changed class from BCU to radio galaxy (IC 1531, TXS 0149+710, and PKS 1304 -215). We associate 4FGL J1346.3 -6026 with Cen B, although its location is not coincident with that of the radio-galaxy core but points to the southern radio jet. Similarly, we associate 4FGL J1516.5+0015 with the radio galaxy 4C +00.56 (PKS 1514+00, z = 0 . 0525) while the gamma-ray position is closer to the lobes than to the core of the radio galaxy.  \nThe only new SSRQ is 3C 212, for which X-ray emission associated with both lobes was detected by Chandra (Aldcroft et al. 2003). Three new CSSs are reported in the 4LAC: 3C 138, 3C 216, and 3C 309.1. These new CSSs are hosted in quasars. At the pc scale they show a core-jet structure with apparent superluminal motion, indicating Doppler effects and small viewing angles (Paragi et al. 2000; Shen et al. 2001; Lister et al. 2019). While the CSSs all have very high radio luminosity, at the opposite end of the radio luminosity distribution are two other new LAT sources,  \nNGC 3894 and NGC 6328. Based on the small extent of their radio emission, two-sided parsec scale morphology (Taylor et al. 1998; Tingay & de Kool 2003), and low radio luminosity, these sources are excellent candidates for being young radio galaxies (see also Migliori et al. 2016; Principe et al. 2019).  \nThe classification of a source as a NLSy1 relies on three criteria, as reported in Osterbrock & Pogge (1985), Goodrich (1989), and Pogge (2000): (i) a full width half maximum (FWHM) of the H β line < 2000 km s -1 ; (ii) a [OIII] λ 5007/H β ratio < 3; and (iii) unusually strong FeII lines. Nine NLSy1 are reported in the 4LAC. Four of them are new with respect to the 3LAC: 1ERS B1303+515, B3 1441+476, MG2 J164443+2618, and TXS 2116 -077. B3 1441+476, MG2 J164443+2618, and TXS 2116 -077 were previously reported by D'Ammando et al. (2015a) and Paliya et al. (2018), respectively. The Circinus galaxy (Hayashida et al. 2013) remains the only radio-quiet Seyfert galaxy detected by the LAT.  \nAmong other AGNs, there are two remarkable cases. One is PKS 0521 -36, previously classified as BCU in the 3LAC, which shows a knotty VLBA radio structure similar to misaligned AGN. Based on the broad emission lines in the optical and ultraviolet bands and the steep radio spectrum, a possible classification as an intermediate object between broad-line radio galaxies and SSRQ has been suggested by D'Ammando et al. (2015b). The new source PKS 2331 -240 was the subject of a multiwavelength study revealing features of a giant radio galaxy restarted as a blazar (Hern'andez-Garc'ıa et al. 2017).  \nFinally, 10 non-blazar AGNs reported in the 3LAC are not confirmed in the 4LAC. Five of them had a double association in the 3LAC and are now firmly associated with the other counterpart; two have changed associations (formerly with 3C 221 and 3C 275.1); one has been reclassified as a BCU (GB 1310+487); while two 3FGL non-blazar AGNs are missing in 4FGL (TXS 0348+013, PKS 1617 -251).", '4.3. Low-Latitude AGNs': 'In addition to high-latitude ( | b | > 10 · ) 4LAC sources, we present a low-latitude sample 9 . This sample is less complete than the 4LAC because the LAT detection flux limit is higher in this region (by factors of a few) and the counterpart catalogs suffer from Galactic extinction. Because a large contamination of Galactic sources is present in the radio and X-ray surveys used in the LR association method, the classes of the resulting associations are highly uncertain. Consequently, these associations were not considered here. The census of the low-latitude sample is given in the last column of Table 4. The fraction of BCUs (62%) is overwhelming, as expected from the observational hindrance mentioned above.', '5.1. Flux and Spectral Properties': "Although many 4FGL/4LAC sources show significant spectral curvature in the gamma-ray band (The Fermi-LAT collaboration 2019), the gamma-ray power-law photon index, Γ, represents a convenient way to compare the spectral hardness of different sources across various classes and flux values. This index is plotted vs. the 8-year average energy flux above 100 MeV, S 25 , in Figure 4. The flux detection limit ranges from (1-4) × 10 -12 erg cm -2 s -1 (i.e., close to 1 eV cm -2 s -1 ), with a slight dependence on the spectral hardness. This dependence is stronger than in 3LAC due to the introduction of weights in the 4FGL likelihood to better account for systematic uncertainties. BCUs are rare among the brightest sources (i.e., with S 25 > 10 -11 erg cm -2 s -1 ), while they are dominant close to the flux limit.  \nFigure 5 displays the photon index distributions for the different optical blazar classes, for 4LAC sources already present in 3LAC and the new ones. Newly detected gamma-ray FSRQs, i.e., not reported in previous LAT AGN catalogs, have somewhat softer spectra (difference in median Γ /similarequal 0.08) than the previously reported ones, possibly indicating the emergence of sources with SED peaking at lower energy. The difference between the FSRQ and BL Lac distributions is striking. The Γ medians and rms are 2.44 ± 0.20 and 2.02 ± 0.21 for FSRQs and BL Lacs respectively. The relative separation in gamma-ray spectral hardness between FSRQs and BL Lacs already reported in previous LAT catalogs is confirmed: 89% of FSRQs and 86% of BL Lacs have Γ greater and lower than 2.25 respectively. This feature by itself carries significant discrimination power between the two classes. The photon index varies among the BL Lac subclasses, with medians and rms in Γ of 2.17 ± 0.16, 2.05 ± 0.19, and 1.88 ± 0.14 for LSPs, ISPs, and HSPs respectively. The BCU index distribution straddles that of the two classes and extends to Γ=3. The expectation that both the BCU photon-index and ν s , peak distributions correspond to linear combinations of the observed FSRQ and BL Lac distributions is tested in Figure 6. Composite distributions were built assuming the same fractions of FSRQs and BL Lacs as in the observed sample for the photon index distributions. For the ν s , peak distribution, a slight correction (a factor of 0.92) in the normalization was introduced in disfavor of FSRQ to account for the difference in efficiency for fitting the SEDs successfully, due to better broad-band data for FSRQs (see section 4.1). The reasonable agreement between the composite and BCU distributions for both photon index and ν s , peak seen in Figure 6 supports the idea that the sample of unclassified blazars (i.e., BCUs) is of similar composition as the classified sample in 4LAC.  \nFigure 7 displays the photon index as a function of the observed synchrotron peak frequency. Even though ν s , peak should be corrected by a factor 1+z to obtain the rest frame value and make the correlation more physical, the fairly strong correlation already noted in previous catalogs is clearly visible. The correlation obtained in 3LAC was reproduced theoretically by Dermer et al. (2015) using an equipartition blazar model with a log-parabola description of the electron energy distribution. In the region of ν s , peak where BL Lac LSPs and FSRQs overlap, their photon index  \ndistributions are very similar. This is the expected region for objects that might be transitioning between being FSRQs and BL Lacs. Ruan et al. (2014) found 6 of such transitioning objects. Five of them, all of the LSP subclass, are present in 4LAC: OJ 451 (4FGL J0833.9+4223, FSRQ, Γ = 2 . 44 ± 0 . 07), TXS 1013+054 (4FGL J1016.0+0512, FSRQ, Γ = 2 . 18 ± 0 . 04), PKS 1247+025 (4FGL J1250.6+0217, BLL, Γ = 2 . 00 ± 0 . 10), 5C 12.291 (4FGL J1308.5+3547, FSRQ, Γ = 2 . 29 ± 0 . 06), and PMN J2206 -0031 (4FGL J2206.8 -0032, BLL, Γ = 2 . 25 ± 0 . 05). Three of these sources have Γ very close to the Γ=2.25 limit outlined above. The four HSP FSRQs in the Clean Sample all have Γ < 2 . 02, as expected from the class of 'HFSRQs', which is not fitting with the 'blazar sequence' (see e.g., Padovani 2007).  \nSome blazars have undergone statistically significant spectral changes since 3LAC: PKS 1349 -439 (BL Lac, ∆Γ ≡ Γ 4 FGL -Γ 3 FGL = -0 . 37 ± 0 . 12), RX J1415.5+4830 (BL Lac, ∆Γ = -0 . 70 ± 0 . 16), PKS 1532+01 (FSRQ, ∆Γ = -0 . 41 ± 0 . 12) and S4 1800+44 (FSRQ, ∆Γ = -0 . 33 ± 0 . 09). The changes in photon index are all such that Γ 4 FGL is closer to the median of the class than was Γ 3 FGL . Inspection of the light curves reveals that all four sources show enhanced activity in the last 4 years of the 4LAC period relative to that of 3LAC , which may be correlated with the observed spectral hardening. Fourteen other sources have experienced spectral variations greater than 3 σ but of lower amplitudes than these four.  \nAs noted above, many FSRQs and BL Lacs show significant spectral curvature. The comparison of the TS distributions of sources with significantly curved spectra to those of the whole sample of FSRQs and BL Lacs (Figure 8) demonstrates that essentially all bright blazars have curved spectra in the LAT energy range. A total of 212 FSRQs, 172 BL Lacs, and 70 BCUs have significantly curved spectra. To enable comparison, if we apply the more stringent threshold on the curvature significance (i.e., approximately 4 σ instead of 3 σ ) used in 3LAC, these numbers become 146 FSRQs, 112 BL Lacs, and 26 BCUs. In 3LAC, the comparable numbers were 57, 32 and 8, respectively. It is therefore likely that fainter blazars have curved spectra as well, but the current data do not allow their curvature to be established with high confidence.", '5.2. Redshifts': 'We conducted a literature search for spectroscopic redshifts. Well established redshifts (999) came from BZCAT or the optical campaigns mentioned in Section 3.3.1. For the other sources (656), remaining contamination from photometric values or from bad S/N optical spectra cannot be excluded. We found redshifts for all the FSRQs but were unable to find those for 36% of the BL Lacs in our sample (compared to 50% of the BL Lacs without redshifts in 3LAC). This clear improvement has been primarily achieved thanks to follow-up observations of 3LAC blazars (see Section 3.3.1 for references). The fraction without redshifts is similar for the three BL Lac subclasses (41%, 42% and 28% for LSPs, ISPs, and HSPs, respectively).  \nThe redshift distributions are displayed in Figure 9 for FSRQs and BL Lacs. The FSRQ dis-  \ntribution shows a broad peak around z=1. This trend confirms the conclusion that the number density of FSRQs grows dramatically up to redshift /similarequal 0.5-2.0 and declines thereafter (Ajello et al. 2012). For BL Lacs, the overall peak lies at z /similarequal 0.3. For the sake of comparison, the distributions for previously and newly reported AGNs are plotted separately. The redshifts of the 3LAC and newly detected blazars are similar, with medians and widths of 1.14 ± 0.62 and 1.09 ± 0.68 respectively for FSRQs and 0.34 ± 0.42 and 0.36 ± 0.34 respectively for BL Lacs. The median redshifts decrease between BL Lac LSPs, ISPs and HSPs from 0.47 to 0.36 to 0.25, respectively. While the maximum redshift for a FSRQ was 3.1 in earlier LAC catalogs, five counterparts to 4LAC sources have higher redshifts: GB 1508+5714 (z=4.31), PKS 1351 -018 (z=3.72), PKS 0335 -122 (z=3.44), MG3 J163554+3629 (z=3.65), and PMN J0833 -0454 (z=3.45). GB 1508+5714 and PKS 0335 -122 are not present in the Clean Sample, however, due to analysis flags. The detections of three of these sources (PKS 1351 -018, GB 1508+5714, MG3 J163554+3629 were reported earlier in Ackermann et al. (2017). Two other high-z sources (NVSS J064632+445116 and NVSS J212912153841), whose detections were also announced in Ackermann et al. (2017), are absent in the 4FGL/4LAC, possibly due to variability effects.  \nFigure 10 displays the photon index vs. the gamma-ray luminosity for the different blazar classes. The trend of softer spectra with higher luminosity observed in earlier catalogs is confirmed. We reiterate the word of caution expressed in 3LAC: this trend is only significant when considering the whole sample of 4LAC blazars. It is not significant when considering the different blazar classes/subclasses individually. Figure 11 shows the corresponding plot for the non-blazar sources. Radio galaxies show a large scatter in photon index, while sources of the other classes have fairly soft spectra akin to those of FSRQs.', '5.3. Variability': "Variability is a key property of blazars and is known to depend on the energy band considered (e.g., Aleksi'c et al. 2015). This feature can be naturally explained as emitting electrons (assuming a leptonic scenario) of different energies and thus different acceleration/cooling times contribute preferentially to the distinct bands. The assessment of variability in 4FGL does not only depend on intrinsic variability but also on the overall significance of the source detection. Two different values of the variability index 10 are provided in 4FGL, derived from the sets of 1-year and 2-month light curves. The 1-year light curves allow the variability of fainter sources to be established compared to the monthly light curves used in early FGL catalogs. Since the variability indices are distributed as χ 2 functions with N dof degrees of freedom, a source is defined as variable if at least one of the variability indices is greater than the 99% confidence limit of 18.48 and 72.44 for the 1-year ( N dof =7) and 2-month ( N dof =47) light curves, respectively.  \nFigure 12 displays the TS distributions of variable sources compared to those of the whole sets of FSRQs and BL Lacs. All bright blazars are found to be variable. The fraction of variable sources goes down from 79% (464/591) for FSRQs to 35% for BL Lacs (362/1027). A monotonic trend is observed for the BL Lac subclasses, with fractions of 49% (140/288), 36% (98/270) and 31% (98/316) for LSPs, ISPs, and HSPs, respectively. Only 17% (157/941) of the BCUs show variability, as expected from the fact that these sources tend to be fainter ( § 5.1). The median fractional variability amplitude is 0.63 for FSRQs and 0.27 for BL Lacs. More extended studies about variability of the LAT-detected blazars can be found in the 3LAC paper.  \nAmong the radio galaxies, IC 310, NGC 1275, 3C 120, NCG 1218, 3C 111, NGC 2892, and IC 4516 are found to be variable, hinting at a blazar-like behavior for these sources. All NLSy1s show significant variability except for IERS B1303+515, B3 1441+476, TXS 2116 -077. Three out of five CSS sources are variable (3C 138, 3C 309.1, 3C 380) as well as one SSRQ (3C 207). The three variable sources of the AGN class are: PKS 0521 -36, PMN J1118 -0413, and CGCG 050 -083.", '5.4. Potential transiently detected AGNs missing in 4LAC': "Some Fermi -LAT sources show blazar-like flaring activity during limited time intervals but do not meet the detection significance criterion to be included in the 4FGL, which is based on a summed 8-year data set. We present here information about some of these transient sources spatially consistent with AGNs.  \nWhen undergoing periods of enhanced activity in 6-hour and 1-day time intervals, some sources can be caught in near-real time by the Fermi -LAT Flare Advocate Gamma-ray Sky Watcher (FAGSW) service (Ciprini & Fermi-LAT Collaboration 2012; Thompson et al. 2015, and references therein). The brightest sources are often reported to the community in Astronomer's Telegrams (ATels). A total of 371 ATels (plus 3 errata) were posted on behalf of the LAT Collaboration in the 8-year period considered in the 4FGL/4LAC (2008 August 4 to 2016 August 2), and a total of 472 ATels were published from 2008 July 24 (ATel 1628) to 2019 August 15 (ATel 13032). At the time of writing, announced transient sources that were positionally consistent with blazars or other AGNs include: NVSS J104516+275136 (ATel 12906); S5 0532+82 (ATel 12902; PKS 2247 -131 (ATels 9285, 9620, 11141); PKS 1915 -458 ( z = 2 . 47, ATels 2666, 2679 and already reported as missing in the 3FGL/3LAC catalogs); TXS 0135+291 (ATel 12888); 2MASX J15441967 -0649156 ( z /similarequal 0 . 04, ATel 10482); PKS 1251 -71 (ATel 8215), PMN J0508 -5628 (ATel 6658); PMN J2010 -2524 ( z = 0 . 825, ATel 6553); TXS 1731+152A (ATel 6395, 6410); PKS 2136 -642 (ATel 5695); and PKS 1510 -319 ( z = 1 . 71, ATel 2528).  \nSources detected on a 1-week time scale were reported in the Second Fermi -LAT All-sky Variability Analysis (FAVA) Catalog (2FAV, Abdollahi et al. 2017), based on the first 7.4 years  \nof Fermi -LAT mission data. In the FAVA catalog 11 the analysis was run in 100-800 MeV and 0.8-300 GeV energy bands, leading to the identification of 518 flaring gamma-ray sources. Among these sources, 13 were associated with established blazars or blazar candidates that are included in neither 4LAC or 3LAC: PMN J0231 -4746, 2MASS J06164292 -4021527, TXS 0723+220, 4C +38.28 (B2 0913+39), PKS 1200 -051, PMN J1322 -8419, PKS 1354 -17, RX J1410.5+6100, PKS 1510 -319, TXS 1534+378, TXS 1731+152A, PKS 1824 -582, 1RXS J235018.0 -055928.  \nFor longer timescales, the first catalog of gamma-ray transient sources (1FLT, The Fermi LAT Collaboration in preparation) reports detections on monthly time intervals during the first 96 months of FermiLAT operation. This catalog contains 64 new gamma-ray sources not present in 4LAC/4FGL or earlier LAT catalogs. Their mean photon spectral index Γ of 2.6 indicates softer spectra than exhibited by the 4LAC sources (mean Γ ∼ 2 . 2). These new sources include 24 BCUs (e.g., PKS 1649 -031, TXS 0209+168, TXS 1601+160, PKS 2108 -326 and others), 20 FSRQs (e.g., PKS 1524 -13, TXS 1226+046, PKS 1200 -051, PMN J2010 -2524, PKS 1706+006 and others), one BL Lac object (1RXS J112100.6+014515), one NLSy1 (Mkn 1501), and two radio galaxies (S5 1733+71 and PKS 2236 -364).", '5.5. Highest-Energy Photons': "The highest-energy photons detected for each source were selected within the purest, i.e., with the lowest instrumental background, class (P8R3 ULTRACLEANVETO V2). Based on the energy-dependent PSF, we required a probability 12 greater than 0.95 for the photons to belong to the source being considered. Blazar gamma-ray spectra provide insight into the density of the EBL via the effect of photon-photon absorption, often defined in terms of an optical depth τ =1, where a photon has only a 1/e probability of reaching the observer (e.g., Abdollahi et al. 2018). A comparison between the energy of the highest-energy photon measured for a given source with the energy computed for τ =1 at that redshift by theoretical models of the EBL provides a simple and direct test of these models. Figure 13 compares the 4LAC highest-energy photons with the optical depth predictions of Finke et al. (2010), Dom'ınguez et al. (2011), and Gilmore et al. (2012). Only a few photons exceed the τ =1 mark, thus remaining compatible with the predictions of these models.", '5.6. Gamma-Ray Detected versus Non-Detected Blazars': 'The blazars detected in gamma-rays after 8 years of LAT operation represent a sizable fraction of the whole population of known blazars as listed in BZCAT. This catalog represents an exhaustive list of all sources ever classified as blazars but is by no means complete. Although a comparison between the gamma-ray-detected and non-detected blazars within that sample has no strong statistical meaning in terms of relative weights, it is nevertheless useful to look for general trends.  \nIn total, the 4LAC includes 29% (562/1909), 66% (764/1151) and 38% (88/227) of the BZCAT (Massaro et al. 2015) FSRQs, BL Lacs and BCUs respectively. Out of the 1353 new 4FGL sources, 405 are present in BZCAT. Figure 14 compares the distributions of radio flux at 1.4 GHz, optical R-band magnitude and X-ray (0.1-2.4 keV) fluxes for the gamma-ray-detected and undetected BZCAT blazars, as well as the fraction of gamma-ray-detected blazars relative to the total as a function of the different fluxes. The gamma-ray-detected blazars are somewhat brighter on average in all bands, confirming previous findings (Lister et al. 2011; Bock et al. 2016; Paliya et al. 2017). The fraction of 4LAC blazars in the total population of BZCAT blazars remains non-negligible even at the faint ends of the radio, optical and X-ray flux distributions, in particular for BL Lacs. This observation is a clue that even the faintest known blazars could eventually shine in gamma rays at LAT-detection levels.', '5.7. Sources Detected at Very High Energies': 'Table 7 shows the list of 78 AGNs detected by ground-based Cherenkov telescopes, as listed in TeVCat 13 . All are present in 4LAC, and the table shows their optical and SED-based classes, redshifts and 4LAC photon indices. The 3LAC catalog included 55 of the 56 VHE AGNs with detections published or announced at that time. Only HESS J1943+213 was missing. The overall mean of the 4LAC photon indices for these VHE AGNs is 1.91 ± 0.20. For the most numerous subclass, the HSP BL Lacs, we find the mean index to be 1.81 ± 0.08, i.e., a very hard gamma-ray spectrum. Of the 78, 56 4LAC AGNs are variable at a significance greater than 99%.', '5.8. Miscellaneous notes about individual sources': "In the course of the 4LAC analysis, we found a number of individual sources that have changed classification, or have unclear associations. We include this information about them for completeness. In case of conflicting associations, we only retained the most probable one in the 4LAC catalog.  \n- · 4FGL J0140.6 -0758 is associated with the BL Lac RX J0140.7 -0758. Another object, SDSS J014040.63 -075857.2, is located 9' away (i.e., 0.04r 95 , where r 95 is the 95% confidence radius), at a redshift of 2.674 coming from a broad-line SDSS optical spectrum. This source has no reported radio emission.\n- · 4FGL J0242.3+5216 has two possible high-confidence associations: with GB6 J0242+5209 and TXS 0239+520, which have similar brightness in the radio band.\n- · 4FGL J0337.8 -1157 is associated with PKS 0335 -122, a distant FSRQ at z = 3 . 442 that has a damped Lyα absorption system at z = 3 . 180 along the line of sight (Kanekar & Chengalur 2003).\n- · 4FGL J0618.1+7819 is associated with the starburst galaxy NGC 2146 ( z = 0 . 002975), but the FSRQ 6C 060948+781625 ( z = 1 . 43) has a similar association probability.\n- · 4FGL J0647.7 -4418 is associated with the high-mass X-ray binary RX J0648.0 -4418 in 4FGL, but the blazar candidate SUMSS J064744 -441946 has a just barely lower association probability (0.80 vs. 0.85).\n- · 4FGL J0720.0 -6237 is associated with the FSRQ PMN J0716 -6240, but another FSRQ, PMN J0719 -6218, lies just outside the 95% confidence region and might contribute to the gamma-ray emission.\n- · 4FGL J0814.4+2941 is associated with the BL Lac EXO 0811.2+2949, but a broad-line quasar, SDSS J081425.89+294115.6, also lies within the 95% confidence region.\n- · 4FGL J0941.9+2724 may have a double association with the BL Lac 5BZBJ0941+2722 and the FSRQ MG2 J094148+2728, which was an association in 3FGL.\n- · 4FGL J1300.4+1416 (3FGL J1300.2+1416) is associated with OW 197 (PKS 1257+145, z = 1 . 1085), as it was in 3LAC. At about 5' distance from OW 197, a blazar candidate NVSS J130041+141728 lies in both the 3FGL and 4FGL 95% confidence ellipses, despite a reduction in size of the ellipse by a factor of 4. The 4FGL source position is about midway between OW 197 and NVSS J130041+141728.\n- · 4FGL J1625.7+4134 may have a double association, with 4C +41.32 and B3 1624+414.", '6. Summary': "The 4LAC, derived from the 4FGL catalog, based on 8 years of Fermi -LAT data, includes 1353 (85%) more AGNs than the 3LAC. At high-Galactic latitudes, AGNs represent at least 79% of the 4FGL sources. Unassociated sources lying in this sky region share common spectral features with BCUs (see Figure 22 of the 4FGL paper), suggesting that most of them are AGNs as well.  \nBL Lacs and FSRQs represent 38% and 24% of the blazar population respectively. The increase of the fraction of BCUs in the sample from 29% in 3LAC to 38% in 4LAC emphasizes the value of the spectroscopic endeavor carried out by several groups. From their photon index and ν s , peak distributions, BCUs probably contain similar fractions of (still unclassified) FSRQs and BL Lacs as observed in the classified population.  \nFits of the synchrotron-peak positions have been performed manually for all 4LAC sources, leading to a SED-based classification for 75% of the 4LAC blazars. The number of non-blazar AGNs has almost doubled relative to 3LAC, from 32 to 64, including 22 new radio galaxies. The overall properties of the 4LAC AGNs are similar to those found in 3LAC. A fairly clear separation in spectral hardness between BL Lacs and FSRQs is observed, with a transition around Γ=2.25. Five HSP FSRQs, which are not fitting with the 'blazar sequence' picture, are present in 4LAC. They all show spectra harder than average for the FSRQ class. Significant spectral curvature is observed for essentially all the brightest (TS > 3000) blazars. The redshift distributions of newly detected blazars resemble those found for 3LAC. Five new FSRQs have redshifts greater than the highest 3LAC redshift (z=3.1), reaching z=4.31. The correlation between photon index and gamma-ray luminosity is strong overall for blazars, but much weaker if the different classes are taken separately. Analysis of 1-year and 2-month light curves shows that 79% of the FSRQs and 35% of the BL Lacs are variable, along with 7 radio galaxies and 16 other AGNs. The highest-energy photons show compatibility with the EBL γ -γ attenuation predicted by some recent models. About 30% of the new blazars are present in BZCAT. Although the 4LAC blazars are predominantly associated with higher-than-average radio, optical and X-ray fluxes in BZCAT, they remain non-negligible even at the faint ends of these flux distributions, in particular for BL Lacs. All 78 known VHE blazars are detected by the LAT, with 56 of them being variable in the GeV range.", '7. Acknowledgments': "The Fermi LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat 'a l'Energie Atomique and the Centre National de la Recherche Scientifique / Institut National de Physique Nucl'eaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d' ' Etudes Spatiales in France. This work performed in part under DOE", 'Contract DE- AC02-76SF00515.': "This research has made use of data obtained from the high-energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA's Goddard Space Flight Center; the SIMBAD database operated at CDS, Strasbourg, France; the NASA/IPAC Extragalactic Database (NED) operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of data archives, catalogs and software tools from the ASDC, a facility managed by the Italian Space Agency (ASI). Part of this work is based on the NVSS (NRAO VLA Sky Survey). The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.", 'REFERENCES': "```\nAartsen, M. G., et al. 2017, ApJ, 835, 45 Abdo, A. A., Ackermann, M., Ajello, M., et al. 2010a, ApJS, 188, 405, (1FGL)\n```  \nAbdo, A. 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These names have been checked to be recognized by the strict 'name resolver' in the NASA-IPAC Extragalactic Database (and therefore by many other web databases like HEASARC derived from it).  \nRadio galaxies, quasars, blazars and other AGNs were first discovered as optical non-starlike nebulae objects (i.e. galaxies) listed in the C. Messier catalog in year 1791, NGC (J.L.E. Dreyer) and IC catalogs published between the years 1781 and 1905. Examples of sources in 4LAC are the radio galaxies M 87, NGC 315, NGC 1275 (also known as Per A or 3C 84), and the starburst galaxies M 82, NGC 253, and NGC 1068. Other AGNs were initially considered as optical variable stars (Argelander, e.g., BL Lac, W Com, AP Lib and BW Tau, known as 3C 120). Some blazars and AGNs were discovered as unusual optically blue starlike objects (Ton, PHL, Mkn catalogs, about 1957-1974, for example Ton 599, Mkn 180, Mkn 421, Mkn 501, PHL 1389) or in optical galaxy catalogs (CGCG, MCG, CGPG, ARP, UGC, Ark, Zw/I-V, Tol, during the period 1961-1976 (for example CGCG 050-083, UGC 773, I Zw 187, V Zw 326). Subsequent optically-selected objects and quasar catalogs provide some names of 4LAC associations (for example, PG, PB, US, SBS, PGC, LEDA, HS, SDSS).  \nIn parallel the largest fraction of radio galaxies and AGNs were discovered during the early era of radio astronomy, with objects like Vir A, Cen A, Cen B, Per A, appearing already in the first half of the 1950s, and the well-known point radio source catalogs 3C, CTA, PKS, 4C, O[+letter], VRO, NRAO, AO, DA, B2, GC, S1/S2/S3, all published between about 1959 and 1974. The PKS (Parkes Radio Catalog, Australia) is the source name preferred for southern celestial radio AGNs, while northern radio AGNs were likely first reported in Cambridge catalogs (especially 3C, 4C) and the Ohio State University Radio Survey Catalog (Ohio Big Ear radio-antenna, O(x) catalog prefix), or in the CTA, NRAO, DA, B2, TXS, S1-5, or MG1-4 catalogs. Examples in the 4LAC are Cen A (already known as NGC 5128, but better known with its original radio name), Per A, Vir A (M 87). Other radio catalogs published between about 1974 and the mid 1980s are: TXS, 5C, S4/S5, MRC, B3, while from the end of the 1980s until the end of 1990s we have MG1/MG2/MG4, 87GB, 6C/7C, JVAS, PMN, EF, CJ2, FIRST, Cul, GB6, FBQS, WN, NVSS, CLASS, IERS, SUMSS, CRATES. Some catalogs at IR or UV frequencies are also considered here (KUV, EUVE, 2MASSi, 2MASS). Further blazars and other AGNs, fainter in the radio band, were discovered directly thanks to the observations made by the first X-ray satellites (2A, 4U, XRS, EXO, H/1H, MS, 1E, 1ES, 2E, RX catalogs published from about 1978 to the mid '90s). Later came the ROSAT survey catalogs from re-analysis and cross-matches such as RGB, RBS, RHS, 1RXS, XSS.  \nTable A1 reports catalog and survey prefixes in an approximate chronological order, adopted for the associations names of the 4LAC catalog. The order is only approximate due to the lack of precise information for each catalog and by the need to follow, in some cases, the criterion of the most-used name in the literature and published papers for a source, even if this latter choice is rather arbitrary and subject to opinion.  \nThe most frequent source association names in 4LAC come from the 3C, 4C, PKS, O[+letter], B2, S2/S3/S5, TXS, MG1/MG2, PMN, GB6, SDSS, 1ES, RX, RBS, 1RXS, NVSS and 2MASS catalogs. Some 4LAC counterpart proper names may be somewhat inadequate because of the radio extension of the AGN/radio galaxy. In some cases the gamma-ray position may relate to the radio emission of the jet or lobe of an AGN, while the name refers to the radio core, which could be offset by a few arcsec. This is manifest in the two different gamma-ray point-source components of Cen A (designed as Cen A core and Cen A lobe in the 4FGL). Another example is 4FGL J1758.7 -1621 associated with AT20G J175841 -161703 (also known as NVSS J175841 -161705). This steep-spectrum AT20G radio source has a brighter neighbor (23.9 Jy at 160 MHz), PMN J1758 -1616 (not included in the 3C catalog because it lies 4 degrees off the Galactic plane) at about 10 arcsec offset. The radio structure map reveals that PMN J1758 -1616 is a FR-II radio galaxy, while AT20G J175841 -161703 corresponds to its radio lobe.  \nTable A1. Historical catalogs reference for naming of 4LAC associations  \nTable A1-Continued  \nTable A1-Continued  \nTable 1. Catalogs Used for the Bayesian Association Method  \nTable A1-Continued  \nTable 2: Comparison of association methods in terms of total number of associations, N assoc , estimated number of false associations, N false , and number of sources associated only via a given method, N S .  \nTable 3. 4LAC FITS Format  \nTable 4. Census of sources  \na Sources with single counterparts and without analysis flags. See Section 3.2 for the definitions of this sample.  \nC  \nA  \nL  \n4  \nn  \ni  \nt  \nn  \ne  \ns  \ne  \nr  \np  \nt  \no  \nn  \ns  \ne  \nc  \nr  \nu  \no  \ns  \nC  \nA  \nL  \n3  \n.  \n5  \ne  \nl  \nb  \na  \nT  \n†  \nL  \nG  \nF  \n4  \ns  \ns  \na  \nl  \nC  \nt  \nr  \na  \np  \nr  \ne  \nt  \nn  \nu  \no  \nC  \ne  \nm  \na  \nN  \nL  \nG  \nF  \n3  \nM  \nq  \nr  \ns  \nf  \n5  \n0  \n.  \n5  \n2  \n+  \nC  \n4  \n1  \n5  \n5  \n2  \n+  \n9  \n.  \n7  \n2  \n1  \n0  \nJ  \nM  \nu  \nc  \nb  \n1  \n9  \n6  \n+  \n0  \n3  \n1  \n0  \nS  \nX  \nT  \n7  \n2  \n9  \n6  \n+  \n0  \n.  \n5  \n3  \n1  \n0  \nJ  \nM  \nu  \nc  \nb  \n8  \n3  \n5  \n+  \n7  \n0  \n2  \n0  \nS  \nX  \nT  \n2  \n0  \n4  \n5  \n+  \n7  \n.  \n1  \n1  \n2  \n0  \nJ  \n- 37 -  \nM  \nq  \nr  \ns  \nf  \nA  \n8  \n4  \n7  \n2  \n+  \n5  \n0  \n9  \n0  \nJ  \n6  \nB  \nG  \n9  \n3  \n7  \n2  \n+  \n9  \n.  \n4  \n0  \n9  \n0  \nJ  \nM  \nu  \nc  \nb  \n4  \n0  \n3  \n7  \n+  \n9  \n2  \n9  \n0  \nJ  \n6  \nB  \nG  \n0  \n0  \n3  \n7  \n+  \n7  \n.  \n8  \n2  \n9  \n0  \nJ  \nM  \nu  \nc  \nb  \n8  \n2  \n9  \n3  \n4  \n6  \n-  \n2  \n1  \n6  \n5  \n9  \n0  \nJ  \nG  \n0  \n2  \nT  \nA  \n1  \n4  \n4  \n6  \n-  \n7  \n.  \n6  \n5  \n9  \n0  \nJ  \nM  \nu  \nc  \nb  \n8  \n1  \n0  \n5  \n-  \n6  \n0  \n0  \n1  \nJ  \nN  \nM  \nP  \n9  \n5  \n9  \n4  \n-  \n0  \n.  \n5  \n0  \n0  \n1  \nJ  \nM  \nu  \nc  \nb  \n4  \n2  \n6  \n3  \n6  \n0  \n-  \n6  \n2  \n6  \n1  \n0  \n1  \nJ  \nS  \nS  \nV  \nN  \n5  \n3  \n6  \n0  \n-  \n0  \n.  \n6  \n1  \n0  \n1  \nJ  \nM  \nu  \nc  \nb  \n6  \n5  \n0  \n0  \n+  \n4  \n2  \n0  \n1  \nJ  \nN  \nM  \nP  \n5  \n0  \n1  \n0  \n+  \n8  \n.  \n4  \n2  \n0  \n1  \nJ  \nM  \nq  \nr  \ns  \nf  \n4  \n7  \n3  \n-  \n4  \n3  \n0  \n1  \nS  \nK  \nP  \n2  \n4  \n7  \n3  \n-  \n4  \n.  \n7  \n3  \n0  \n1  \nJ  \nM  \nu  \nc  \nb  \n5  \n3  \n7  \n1  \n4  \n6  \n-  \n9  \n1  \n3  \n2  \n1  \n1  \nJ  \nG  \n0  \n2  \nT  \nA  \n5  \n1  \n4  \n6  \n-  \n2  \n.  \n3  \n2  \n1  \n1  \nJ  \nM  \nq  \nr  \ns  \nf  \n8  \n0  \n4  \n0  \n+  \n8  \n4  \n4  \n0  \n2  \n1  \nJ  \n1  \nG  \nM  \n2  \n1  \n4  \n0  \n+  \n4  \n.  \n5  \n0  \n2  \n1  \nJ  \nM  \nu  \nc  \nb  \n2  \n2  \n5  \n0  \n9  \n6  \n+  \n4  \n4  \n0  \n2  \n2  \n1  \nJ  \nS  \nS  \nV  \nN  \n2  \n1  \n9  \n6  \n+  \n5  \n.  \n8  \n1  \n2  \n1  \nJ  \nd  \ne  \nu  \nn  \ni  \nt  \nn  \no  \nC  \n-  \n5  \ne  \nl  \nb  \na  \nT  \n†  \nL  \nG  \nF  \n4  \ns  \ns  \na  \nl  \nC  \nt  \nr  \na  \np  \nr  \ne  \nt  \nn  \nu  \no  \nC  \ne  \nm  \na  \nN  \nL  \nG  \nF  \n3  \nM  \nl  \nl  \nb  \n8  \n9  \n2  \n+  \n3  \n2  \n3  \n1  \nS  \nX  \nT  \n1  \n3  \n9  \n2  \n+  \n1  \n.  \n6  \n2  \n3  \n1  \nJ  \nM  \nq  \nr  \ns  \nf  \n9  \n7  \n3  \n-  \n6  \n2  \n3  \n1  \nl  \no  \nT  \n8  \n1  \n8  \n3  \n-  \n0  \n.  \n0  \n3  \n3  \n1  \nJ  \nM  \nu  \nc  \nb  \n0  \n2  \n8  \n2  \n0  \n4  \n-  \n5  \n2  \n6  \n5  \n3  \n1  \nJ  \nS  \nS  \nM  \nU  \nS  \n9  \n2  \n0  \n4  \n-  \n3  \n.  \n6  \n5  \n3  \n1  \nJ  \n- 38 -  \nM  \nq  \nr  \ns  \nf  \n6  \n9  \n0  \n-  \n2  \n1  \n4  \n1  \nB  \nS  \nK  \nP  \n1  \n0  \n0  \n1  \n-  \n0  \n.  \n5  \n1  \n4  \n1  \nJ  \nM  \nu  \nc  \nb  \n7  \n2  \n8  \n4  \n-  \n7  \n0  \n1  \n2  \nJ  \nN  \nM  \nP  \n2  \n2  \n8  \n4  \n-  \n7  \n.  \n7  \n0  \n1  \n2  \nJ  \nM  \nq  \nr  \ns  \nf  \n2  \n4  \n7  \n2  \n-  \n1  \n5  \n1  \n2  \nJ  \nN  \nM  \nP  \n4  \n4  \n7  \n2  \n-  \n6  \n.  \n1  \n5  \n1  \n2  \nJ  \nM  \nq  \nr  \ns  \nf  \n3  \n6  \n.  \n1  \n3  \n+  \nC  \n4  \n3  \n4  \n1  \n3  \n+  \n7  \n.  \n3  \n0  \n2  \n2  \nJ  \nM  \nu  \nc  \nb  \n1  \n2  \n5  \n5  \n0  \n5  \n-  \n5  \n0  \n6  \n3  \n2  \n2  \nJ  \nS  \nS  \nM  \nU  \nS  \n9  \n4  \n0  \n5  \n-  \n2  \n.  \n6  \n3  \n2  \n2  \nJ  \nM  \nu  \nc  \nb  \n7  \n3  \n4  \n4  \n5  \n1  \n+  \n4  \n0  \n6  \n4  \n2  \n2  \nJ  \nS  \nS  \nV  \nN  \n7  \n4  \n5  \n1  \n+  \n2  \n.  \n6  \n4  \n2  \n2  \nJ  \nM  \nu  \nc  \nb  \n9  \n5  \n8  \n1  \n2  \n4  \n-  \n2  \n1  \n5  \n0  \n3  \n2  \nJ  \nS  \nS  \nM  \nU  \nS  \n9  \n1  \n2  \n4  \n-  \n3  \n.  \n5  \n0  \n3  \n2  \nJ  \nM  \nq  \nr  \ns  \nf  \n2  \n4  \n5  \n1  \n+  \n2  \n4  \n3  \n4  \n3  \n2  \nJ  \n1  \nG  \nM  \n1  \n5  \n5  \n1  \n+  \n6  \n.  \n3  \n4  \n3  \n2  \nJ  \nD  \nu  \nc  \nb  \n1  \n2  \n1  \n-  \n1  \n0  \n0  \n0  \nS  \nK  \nP  \n1  \n5  \n1  \n1  \n-  \n8  \n.  \n3  \n0  \n0  \n0  \nJ  \nD  \nl  \nl  \nb  \n5  \n2  \n6  \n0  \n+  \n9  \n0  \n0  \n0  \nJ  \n6  \nB  \nG  \n0  \n3  \n6  \n0  \n+  \n1  \n.  \n9  \n0  \n0  \n0  \nJ  \nL  \nu  \nc  \nb  \n2  \n7  \n5  \n-  \n6  \n5  \n0  \n0  \nS  \nK  \nP  \n1  \n0  \n7  \n5  \n-  \n1  \n.  \n9  \n5  \n0  \n0  \nJ  \nD  \nq  \nr  \ns  \nf  \n0  \n3  \n+  \n0  \n0  \n2  \n0  \n2  \nB  \n3  \n4  \n0  \n3  \n+  \n6  \n.  \n3  \n0  \n2  \n0  \nJ  \nd  \ne  \nu  \nn  \ni  \nt  \nn  \no  \nC  \n-  \n5  \ne  \nl  \nb  \na  \nT  \n†  \nL  \nG  \nF  \n4  \ns  \ns  \na  \nl  \nC  \nt  \nr  \na  \np  \nr  \ne  \nt  \nn  \nu  \no  \nC  \ne  \nm  \na  \nN  \nL  \nG  \nF  \n3  \nL  \nu  \nc  \nb  \n5  \n1  \n.  \n4  \n0  \n+  \nC  \n4  \n9  \n5  \n4  \n0  \n+  \n6  \n.  \n6  \n2  \n4  \n0  \nJ  \nD  \nl  \nl  \nb  \n3  \n2  \n8  \n1  \n0  \n0  \n-  \n8  \n.  \n9  \n2  \n2  \n4  \n4  \n0  \nJ  \nS  \nX  \nR  \n1  \n7  \n1  \n0  \n0  \n-  \n6  \n.  \n2  \n4  \n4  \n0  \nJ  \nL  \nq  \nr  \ns  \nf  \n2  \n1  \n2  \n-  \n6  \n4  \n4  \n0  \nS  \nK  \nP  \n9  \n1  \n1  \n2  \n-  \n8  \n.  \n7  \n4  \n4  \n0  \nJ  \n- 39 -  \nL  \nu  \nc  \nb  \n1  \n3  \n5  \n4  \n-  \n7  \n0  \n2  \n1  \nJ  \nN  \nM  \nP  \n7  \n3  \n5  \n4  \n-  \n6  \n.  \n7  \n0  \n2  \n1  \nJ  \nD  \nq  \nr  \ns  \ns  \n1  \n.  \n5  \n7  \n2  \nC  \n3  \n5  \n1  \n6  \n1  \n+  \n1  \n.  \n4  \n4  \n2  \n1  \nJ  \nL  \nu  \nc  \nb  \n8  \n3  \n5  \n+  \n4  \n5  \n2  \n1  \nS  \nX  \nT  \n8  \n2  \n3  \n5  \n+  \n7  \n.  \n6  \n5  \n2  \n1  \nJ  \nD  \nu  \nc  \nb  \n8  \n2  \n7  \n1  \n4  \n1  \n+  \n1  \n4  \n0  \n0  \n3  \n1  \nJ  \nS  \nS  \nV  \nN  \n6  \n1  \n4  \n1  \n+  \n2  \n.  \n0  \n0  \n3  \n1  \nJ  \nD  \nu  \nc  \nb  \n3  \n4  \n9  \n0  \n-  \n3  \n2  \n3  \n1  \nJ  \nN  \nM  \nP  \n8  \n3  \n9  \n0  \n-  \n8  \n.  \n2  \n2  \n3  \n1  \nJ  \nD  \nu  \nc  \nb  \n1  \n4  \n6  \n+  \n0  \n5  \n4  \n1  \nS  \nX  \nT  \n5  \n5  \n3  \n6  \n+  \n2  \n.  \n1  \n5  \n4  \n1  \nJ  \nL  \nq  \nr  \ns  \nf  \n3  \n0  \n0  \n5  \n4  \n4  \n+  \n6  \n3  \n4  \n1  \n5  \n1  \nJ  \nS  \nS  \nV  \nN  \n6  \n4  \n4  \n4  \n+  \n8  \n.  \n4  \n1  \n5  \n1  \nJ  \nL  \nl  \nl  \nb  \n3  \n0  \n2  \n+  \n2  \n5  \n5  \n1  \nS  \nE  \n1  \n0  \n1  \n0  \n2  \n+  \n4  \n.  \n4  \n5  \n5  \n1  \nJ  \nD  \nn  \ng  \na  \n1  \n5  \n2  \n-  \n3  \n1  \n6  \n1  \nS  \nX  \nT  \n9  \n1  \n5  \n2  \n-  \n3  \n.  \n7  \n1  \n6  \n1  \nJ  \nD  \nq  \nr  \ns  \nf  \n2  \n3  \n.  \n1  \n4  \n+  \nC  \n4  \n5  \n2  \n1  \n4  \n+  \n9  \n.  \n5  \n2  \n6  \n1  \nJ  \nd  \ne  \nu  \nn  \ni  \nt  \nn  \no  \nC  \n-  \n5  \ne  \nl  \nb  \na  \nT  \n†  \nL  \nG  \nF  \n4  \ns  \ns  \na  \nl  \nC  \nt  \nr  \na  \np  \nr  \ne  \nt  \nn  \nu  \no  \nC  \ne  \nm  \na  \nN  \nL  \nG  \nF  \n3  \nL  \nu  \nc  \nb  \n2  \n4  \n6  \n2  \n1  \n0  \n-  \n6  \n3  \n8  \n0  \n9  \n1  \nJ  \nS  \nS  \nV  \nN  \n0  \n3  \n1  \n0  \n-  \n8  \n.  \n8  \n0  \n9  \n1  \nJ  \nL  \nq  \nr  \ns  \nf  \n0  \n3  \n8  \n2  \n-  \n6  \n3  \n0  \n2  \nJ  \nN  \nM  \nP  \n0  \n3  \n8  \n2  \n-  \n8  \n.  \n6  \n3  \n0  \n2  \nJ  \nL  \nu  \nc  \nb  \nA  \n5  \n3  \n+  \n7  \n0  \n1  \n2  \n2  \nB  \n0  \n4  \n5  \n3  \n+  \n3  \n.  \n0  \n1  \n1  \n2  \nJ  \n- 40 -  \nL  \nG  \ne  \ne  \nc  \nr  \n-  \nr  \ne  \n.  \nC  \nb c u L i n 4 F G L . L : 3 F e d . C : 3 F G L s o u n o n -A G N c o u n t w m i s s i n g i n 4 L A  \n1  \n1  \n3  \n0  \n1  \n5  \n-  \n2  \n5  \n8  \n4  \n3  \n2  \nJ  \nS  \nS  \nM  \nU  \nS  \n0  \n0  \n1  \n5  \n-  \n4  \n.  \n8  \n4  \n3  \n2  \nJ  \nc  \nr  \nu  \no  \ns  \ny  \na  \nr  \n-  \na  \nm  \nm  \na  \ng  \ng  \nn  \ni  \ns  \ns  \ni  \nm  \n:  \nM  \n†  \n-  \n.  \ne  \nt  \no  \nN  \nt  \na  \ni  \nc  \no  \ns  \ns  \na  \nn  \nu  \nw  \no  \nn  \nt  \nu  \nb  \nL  \nG  \nF  \n4  \nn  \ni  \nt  \nn  \ne  \ns  \ne  \nr  \np  \ne  \nc  \nr  \nu  \no  \ns  \na  \nh  \nt  \ni  \nw  \nd  \ne  \nt  \na  \ni  \nc  \no  \ns  \ns  \na  \nw  \no  \nn  \nt  \nu  \nb  \nL  \nG  \nF  \n4  \nn  \ni  \nt  \nn  \ne  \ns  \ne  \nr  \np  \no  \nn  \nC  \nA  \nL  \n3  \nn  \ni  \nt  \nr  \na  \np  \nr  \ne  \nt  \nn  \nu  \no  \nc  \ne  \nt  \na  \nc  \ni  \nl  \np  \nu  \nd  \n:  \nD  \n.  \nt  \nr  \na  \np  \nTable 6. Non-blazar objects and misaligned AGNs  \nTable 6-Continued  \nNote. - The table includes the non-blazar objects and MAGNs at high latitudes (top) and low latitudes (bottom) associated with 4FGL sources (Cen A Core and Cen A Lobes constitute a single object). ∗ indicates that the value is the distance in Mpc.  \nTable 7. Properties of the 4LAC VHE AGNs  \nTable 7-Continued  \nNote. - * refers to low-latitude sources (not in 4LAC).  \n<!-- image -->  \nFig. 1.- Locations of the sources in the Clean Sample in Galactic (top) and J2000 equatorial (bottom) coordinates and Hammer-Aitoff projection.  \n<!-- image -->  \n1  \n1  \n1  \n1  \nFig. 2.- Distributions of Galactic latitudes for the different blazar classes.  \n<!-- image -->  \nFig. 3.- Distributions of the synchrotron peak frequency ν s , peak for the different blazar classes.  \n<!-- image -->  \nFig. 4.- Photon index as a function of energy flux above 100 MeV. The curve represents the approximate detection limit. Error bars have been omitted for clarity. The mean photon-index uncertainties are 0.08, 0.10, and 0.14 for FSRQs, BL Lacs and BCUs respectively.  \n<!-- image -->  \nFig. 5.- Photon index distributions for the different blazar classes and subclasses. In the top, second from top and bottom panels, the solid histograms represent the 4LAC sources already present in 3LAC and the dashed histograms the new 4LAC sources.  \n<!-- image -->  \nFig. 6.- Comparison between the ν s , peak (top) and photon index (bottom) distributions of BCUs (green) and the (normalized) distributions obtained by adding up the FSRQ and BL Lac distributions (black). See text for details.  \n<!-- image -->  \nFig. 7.- Photon index vs. frequency of the synchrotron peak ν s , peak in the observer frame. Error bars have been omitted for clarity. The mean photon-index uncertainties are 0.08 and 0.10 for FSRQs and BL Lacs respectively. The black cross depicts the FSRQ median photon index, while the gray crosses depict those for the three BL Lac subclasses.  \n<!-- image -->  \nFig. 8.- TS distributions of FSRQs (top) and BL Lacs (bottom) for curved-spectra sources and the whole sample.  \n<!-- image -->  \nFig. 9.- Redshift distributions (solid: 4LAC sources also in 3LAC, dashed: new 4LAC sources) for FSRQs (top) and BL Lacs (bottom)  \n<!-- image -->  \nRedshift  \nFig. 10.- Photon index vs. gamma-ray luminosity for the different blazar classes and subclasses. Error bars have been omitted for clarity. The mean photon-index uncertainties are 0.08 and 0.10 for FSRQs and BL Lacs respectively.  \n<!-- image -->  \nFig. 11.- Photon index vs. gamma-ray luminosity for the different non-blazar classes. Blazars are included for comparison and depicted in gray regardless of their classes. Error bars have been omitted for clarity. The mean photon-index uncertainty is 0.11 for the non-blazar AGNs.  \n<!-- image -->  \nFig. 12.- TS distributions of FSRQs (top) and BL Lacs (bottom) for variable sources and the whole sample.  \n<!-- image -->  \nFig. 13.- Energy of the highest-energy photon vs. redshift. The curves display the energies corresponding to an optical depth of 1 as predicted by different models.  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \nFig. 14.From top to bottom: radio flux density at 1.4 GHz, optical R magnitude, X-ray flux (0.1-2.4 keV) distributions for 4LAC (red) and non-4LAC (black) BZCAT sources. The insets show the fraction of 4LAC sources relative to the total for a given flux. Error bars have been omitted for clarity.  \n<!-- image -->"}