Strong gravitational magnification by massive galaxy clusters enable us to detect faint background sources resolve their detailed internal structures and in the most extreme cases identify and study individual stars in distant galaxies. Highly magnified individual stars allow for a wide range of applications including studies of stellar populations in distant galaxies and constraining smallscale dark matter structures. However these applications have been hampered by the small number of events observed as typically one or a few stars are identified from each distant galaxy. Here we report the discovery of 46 significant microlensed stars in a single stronglylensed highredshift galaxy behind the Abell 370 cluster at redshift of 0.725 when the Universe was half of its current age dubbed the Dragon arc based on two observations separated by one year with the James Webb Space Telescope it JWST. These events are mostly found near the expected lensing critical curves suggesting that these are magnified individual stars that appear as transients from intracluster stellar microlenses. Through multiwavelength photometry and colors we constrain stellar types and find that many of them are consistent with red giantssupergiants magnified by factors of thousands. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc and proves that it JWSTs timedomain observations open up the possibility of conducting statistical studies of highredshift stars and subgalactic scale perturbations in the lensing dark matter field.
['Astrophysics - Astrophysics of Galaxies', 'Astrophysics - Cosmology and Nongalactic Astrophysics']
JWST Discovery of 40 Microlensed Stars in a Magnified Galaxy the Dragon behind Abell 370
{"JWST Discovery of 40+ Microlensed Stars in a Magnified Galaxy, the ' Dragon ' behind Abell 370": 'Yoshinobu Fudamoto, 1 Fengwu Sun, 2 Jose M. Diego, 3 Liang Dai, 4 Masamune Oguri, 1, 5 Adi Zitrin, 6 Erik Zackrisson, 7, 8 Mathilde Jauzac, 9, 10, 11, 12 David J. Lagattuta, 9, 10 Eiichi Egami, 2 Edoardo Iani, 13 Rogier A. Windhorst, 14 Katsuya T. Abe, 1 Franz Erik Bauer, 15, 16, 17 Fuyan Bian, 18 Rachana Bhatawdekar, 19 Thomas J. Broadhurst, 20, 21, 22 Zheng Cai, 23 Chian-Chou Chen, 24 Wenlei Chen, 25 Seth H. Cohen, 26 Christopher J. Conselice, 27, 28 Daniel Espada, 27, 28 Nicholas Foo, 26 Brenda L. Frye, 29 Seiji Fujimoto, 30, ∗ Lukas J. Furtak, 6 Miriam Golubchik, 6 Tiger Yu-Yang Hsiao, 31 Jean-Baptiste Jolly, 32 Hiroki Kawai, 33, 1 Patrick L. Kelly, 34 Anton M. Koekemoer, 35 Kotaro Kohno, 36, 37 Vasily Kokorev, 38 Mingyu Li, 23 Zihao Li, 23 Xiaojing Lin, 23, 2 Georgios E. Magdis, 39, 40, 41 Ashish K. Meena, 6 Armin Nabizadeh, 7 Johan Richard, 42 Charles L. Steinhardt, 43, 44 Yunjing Wu, 23 Yongda Zhu, 2 and Siwei Zou 23', 'ABSTRACT': "Strong gravitational magnification by massive galaxy clusters enable us to detect faint background sources, resolve their detailed internal structures, and in the most extreme cases identify and study individual stars in distant galaxies. Highly magnified individual stars allow for a wide range of applications, including studies of stellar populations in distant galaxies and constraining small-scale dark matter structures. However, these applications have been hampered by the small number of events observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of 46 significant microlensed stars in a single strongly-lensed high-redshift galaxy behind the Abell 370 cluster at redshift of 0.725 when the Universe was half of its current age (dubbed the 'Dragon arc'), based on two observations separated by one year with the James Webb Space Telescope ( JWST ). These events are mostly found near the expected lensing critical curves, suggesting that these are magnified individual stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry and colors, we constrain stellar types and find that many of them are consistent with red giants/supergiants magnified by factors of thousands. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc, and proves that JWST 's time-domain observations open up the possibility of conducting statistical studies of high-redshift stars and subgalactic scale perturbations in the lensing dark matter field.", '1. INTRODUCTION': "Galaxy clusters are the most massive gravitationally bound objects in the Universe and produce strong gravitational lens events. The high magnification afforded by such massive clusters (Bartelmann 2010; Natarajan et al. 2024) allows us to identify faint sources to the highest redshifts (Atek et al. 2023; Fujimoto et al. 2023; Wang et al. 2023), study their detailed internal structure (Adamo et al. 2024; Fudamoto et al. 2024; Fujimoto et al. 2024), and in most extreme cases allows us to identify and study even individual stars in distant galaxies. In particular, Miralda-Escude (1991) suggested that highly magnified individual stars may be observed within strongly lensed galaxies near critical curves pro-  \nCorresponding author: Yoshinobu Fudamoto yoshinobu.fudamoto@gmail.com  \nduced by massive foreground clusters. Later, Kelly et al. (2018) indeed reported transients near the critical curve of the cluster MACS J1149.5+2223, which were interpreted as background stars magnified by the foreground cluster by a factor of thousands. Such phenomena are explained by microlensing events. Namely, the gravitational potential of galaxy clusters sets macrolensing magnifications of order ∼ 100 or more near critical curves, and on top of that, masses in the foreground intracluster medium act as microlenses resulting in total magnifications of ≳ 1000. Subsequent microlens observations and analyses using the Hubble Space Telescope ( HST ) and the James Webb Space Telescope ( JWST ) show that such lensed stars are ubiquitous and provide us with a window to study stellar physics at cosmological distances (e.g., Rodney et al. 2018; Kaurov et al. 2019; Chen et al. 2019; Welch et al. 2022; Kelly et al. 2022; Meena et al. 2023; Diego et al. 2023a,b; Furtak et al. 2024).  \nSuch highly magnified individual stars allow for a wide range of applications, including constraints on compact dark matter (Venumadhav et al. 2017; Oguri et al. 2018; Diego et al. 2018; Vall Muller & Miralda-Escud'e 2024) or self-gravitating micro-structures (Dai & Miralda-Escud'e 2020), the measurement of the abundance of dark matter subhalos (Dai et al. 2018, 2020; Williams et al. 2024), direct constraints on the stellar initial mass function (Kelly et al. 2018), direct observations of Population III stars (Windhorst et al. 2018; Schauer et al. 2022; Zackrisson et al. 2023) and stellar properties of individual magnified stars (Han & Dai 2024), and direct constraints on the Hubble constant ( H 0 ) when supernovae (SNe) are multiply imaged (Kelly et al. 2023; Frye et al. 2024; Pascale et al. 2024). However, some of these applications have been hampered by the small number of events observed so far, as typically one or a few stars are identified from each galaxy. With its superb lightcollecting power and excellent spatial resolution, JWST has been expected to significantly increase the number of detected events and open up these exciting applications.  \nHere, we report JWST identifications of 46 microlensing events in a single z = 0 . 725 galaxy behind Abell 370 ( z = 0 . 375), which has been magnified into an arc known as the 'Dragon'. Kelly et al. (2022) reported the discovery of several high-significance microlensing events in the Dragon with deep HST observations, which already indicated that the microlensing rate in the Dragon is particularly high due to its relatively low redshift and the high surface brightness. Our finding with JWST confirms a very high occurrence of microlensing events in the Dragon and opens up the possibility of conducting statistical studies of highly magnified stars with a large number of events. Multi-wavelength high-resolution observations with JWST 's NIRCam up to λ obs ∼ 5 µ m allow us to constrain stellar types of some of the brightest microlensed stars.  \nThis paper is organized as follows: in § 2 we describe JWST observations used in this study. In § 3, we present data analyses and measurements. In § 4, we present the results of the study and discuss our findings. Finally, we conclude with the summary in § 5. Throughout this paper, we assume a cosmology with (Ω m , Ω Λ , h ) = (0 . 3 , 0 . 7 , 0 . 7).", '2. OBSERVATIONS': "We used JWST NIRCam imaging (Rieke et al. 2023b) observations from two programs: Cycle-1 GTO1208 (the CAnadian NIRISS Unbiased Cluster Survey, CANUCS, PI: C. Willot; Willott et al. 2022) and Cycle2 GO-3538 (PI: E. Iani) which are publicly available.  \nThese two programs targeted the Abell 370 cluster at z = 0 . 375 using multiple NIRCam filters. Observations were executed in December 2022 and December 2023, respectively, i.e., separated in a time period of ∼ 1 year. The Abell 370 field includes a giant arc of a star-forming galaxy at z = 0 . 725 also known as the 'Dragon' (Soucail et al. 1987a,b, 1988; Richard et al. 2010; Dai et al. 2018; Patr'ıcio et al. 2018; Kelly et al. 2022), which is the target of this study (Upper panel of Figure 1).  \nGTO-1208 CANUCS obtained images in filters of F090W, F115W, F150W, F200W, F277W, F356W, F410M, F444W, each with integration times of ∼ 6400 s. GO-3538 performed NIRCam wide-field slitless spectroscopy and direct imaging in filters of F300M, F335M, F410M, and F460M, accompanied with shortwavelength imaging using F182M ( ∼ 19400 s) and F210M ( ∼ 19090 s) filters. Direct imaging for long wavelength filters were also taken with integration time of 2770-3092 s.  \nAll observations are calibrated using the standard JWST pipeline 1 (Bushouse et al. 2023) version v1.11.2, with reference file jwst 1188.pmap . We also included customized routines to remove well-known artifacts, including the subtraction of 1/f noise stripes in both row and column directions, template-based wisp subtraction in NIRCam short-wavelength detectors, hot-pixel masking in long-wavelength detectors, and manual masking of the persistence from bright objects (e.g., Rigby et al. 2023; Rieke et al. 2023b,a). The astrometry of each image was carefully corrected using a combined catalog of HST sources detected with HFF/BUFFALO images (Steinhardt et al. 2020; primary) and DESI Legacy Imaging Survey catalog (Dey et al. 2019; secondary), both registered to Gaia (Gaia Collaboration et al. 2018). As a result, each image is well aligned, with an absolute astrometric error of < 0 . '' 03, and an internal RMS astrometric error of ≈ 0 . '' 004 for all > 5 σ sources. The final mosaicked images were drizzled with pix frac=1.0 and a pixel size of 0 . '' 03.", '3.1. Identifications of microlensing events': "We first investigated the 'Dragon' by directly comparing the F200W (2022) and F182M (2023) images. where we find that several bright sources are only seen in one of the epochs, representing potential transient events across the Dragon arc. Then, to more precisely identify fainter transient events, we subtracted a combined F182M + F210M image (2023) from the F200W  \n<!-- image -->  \n<!-- image -->  \nFigure 1. Upper panel : A false-color image of the entire 'Dragon' arc behind the Abell 370 cluster (Soucail et al. 1987a), using JWST filters F090W, F150W, and F200W. North is up, East to the left, and a reference angular scale of 1 '' is shown by the solid horizontal bar at the bottom right corner The dashed white rectangle shows the region of interest further analyzed in Figure 2. Lower panels : F200W zoom in on a part of the Dragon in the 2022 image (left panel) and in the effective F200W in 2023 (right panel). The effective F200W image in 2023 was made using F182M and F210M images (see § 3.1). Examples of the apparently bright microlensing events are indicated, where dashed half-crosses show bright sources seen only in 2022 data and solid half-crosses show sources only in 2023. Many additional microlensing events exist, but are only visible in the differential image in Figure 2. Horizontal bars in the lower right corners show 1 '' scales.  \n<!-- image -->  \nimage (2022). We use λ obs = 2 µ m images to identify transients instead of the repeated F410M filter because the 2 µ m data are deeper by ∼ 1 mag and higher spatial resolution. To make the 2023 epoch F182M + F210M image (i.e., 'effective' F200W image), we first performed point spread function (PSF) homogenization for the F182M and F200W images to match their PSFs to that of F210M. The PSF for each image was made using WebbPSF (Perrin et al. 2014) applying the wavefront measurements for the closest date of each observation. Convolution kernels were produced using pypher (Boucaud et al. 2016) by applying a regularization factor of 0 . 0001. The kernels were convolved using the convolve2d function of the scipy.signal submodule (Virtanen et al. 2020). Using the PSF-homogenized F182M and F210M images, we created an effective F200W image by linearly interpolating fluxes at the pivot wavelength of each filter. The differential image was then created by subtracting the effective F200W  \nimage (2023) from the PSF-homogenized F200W image (2022).  \nFinally, we subtracted the global background of the differential image. This is to remove any large-scale background in the differential image, mostly because of the slight difference in background (sky and intracluster light) subtraction during the production of mosaicked images. As a result, a clean differential image was created with the pixel distribution of ∼ 0 . 0 ± 0 . 8 nJy. The differential image clearly shows significant positive and negative peaks across the Dragon arc, indicating a large number of transient events observed over the two epochs (Figure 2).", '3.2. Transient Detection': "We used DAOFIND (Stetson 1987) incorporated in the photutils.detection submodule and SExtractor (Bertin & Arnouts 1996) to detect microlensing transients from the differential image. These two source-  \nFigure 2. Top Panel : The observed ∼ 2 µ m differential image between the 2022 and 2023 epochs. Positive signals (red) show objects that appear only in 2023, while negative signals (blue) show sources only seen in 2022. Contours show critical curves for z = 0 . 725 estimated from several programs: the solid black critical curve is derived with the WSLAP+ code (Diego et al. 2007), the dashed green lines are from the lenstool software (Jullo & Kneib 2009) analyzed in Niemiec et al. (2023), and the dash-dotted purple lines are from the Light-Traces-Mass (LTM) method described by Zitrin et al. (2009). Star symbols indicate locations of previous HST detections of microlensing events (Kelly et al. 2022). Lower Panels : Zoom-ins of crowded transient regions indicated with A, B, C, and D letters in the upper panel. Circles ( r = 0 . '' 12) show detected microlensed events. Solid circles show lensed stars significantly detected in 2023 epoch, while dashed circles indicate stars detected events in 2022 epoch. In total, 46 microlensed stars are significantly detected.  \n<!-- image -->  \nfinding algorithms are complementary in a way that DAOFIND efficiently detects circular Gaussian sources in crowded regions while Sextractor can detect less circular sources that can also be found in the crowded regions in the differential image.  \nFor the Sextractor run, to detect compact sources, we applied DETECT MINAREA = 5 and DETECT MINTHRESH = 2 . 68 (i.e., > 6 σ detection). To deblend crowded sources, we applied a relatively low contrast parameter of DEBLEND MINCONT = 0 . 0005. With these settings, we run Sextractor for positive and negative instances of the differential image, corresponding to transient events in 2023 and 2022, respectively. For DAOFIND run, we used a 4 σ threshold of 0 . 03 MJy / sr (0 . 6 nJy pix -1 ) and a kernel full width at half maximum of 2 . 5 pixels. Similarly, DAOFIND was used for positive and negative instances of the differential image.  \nFor both of the source detections, we masked regions where bright sources (e.g., bright cluster member galaxies and bulges of the Dragon arc) exist in the original F200W image. This is to avoid the contamination from their relatively strong residuals caused by minor PSF differences between 2022 and 2023.  \nTo assess the impact of noise signal on detection, we ran the same source-finding routine over the same F200W differential image of several types of galaxies (e.g., clumpy or smooth morphology with different luminosities). Except for genuine transients such as supernovae that can be also detected in other bands, we do not detect any significant bad pixels that only appear in the single band with similar significance. Therefore, we conclude that the vast majority of transients detected on the Dragon arc are not data-quality artifacts but real transient events.  \nWe note that strong emission lines from H ii regions still have some probability to contaminate as transients due to the imperfect match between F182M+F210M and F200W filter transmission curves. At z = 0 . 725, He i λ 1.083 µ m and Paschen γ can elevate the flux density from F182M photometry, and Paschen β may boost the F200W photometry if they are strong enough. In the 'head' of the Dragon arc (the least magnified multiple image of the Dragon arc seen in the most eastern part of Figure 1), three bright clumps can be tentatively seen as faint F200W (2022) excess sources with peak brightness ∼ 1 nJy pix -1 in the difference image. However, counter images of these clumps are not selected in our sample, as they are much fainter than our selected transients. Additionally, there are no underlying compact star-forming clumps seen in continuum emission. For these reasons, we conclude that the emission line contamination in our transient sample is negligible.  \nUsing the PSFphotometry routine of the photutils.psf module, we assessed the sizes of the individually detected transient events by subtracting scaled PSFs at the positions of detected transients. The residuals appear consistent with background noise, leading us to conclude that the transients are consistent with point sources.", '3.3. Photometry': 'To measure the F200W fluxes of detected transients, we performed forced aperture photometry on the difference images at the detected positions. We used a fixed aperture photometry with an aperture radius of 0 . 06 arcsec. Photometric errors are estimated by placing random apertures with the same size on the transientfree regions of differential images. To take into account realistic Poisson noise from the host galaxy, we only place random apertures where the surface brightness in F200W image is 0.4-50 nJy pixel -1 , i.e., similar to the surface brightness of the background Dragon arc around the transient positions. We performed aperture correction based on the PSF models obtained using WebbPSF (Perrin et al. 2014). The 5 σ detection limit is therefore determined as 28.75 AB mag in the difference image.  \nWe also performed photometry of transients using F410M images because F410M is the only repeated filter used in both epochs. During this long wavelength photometry, we limited our sample only to F410M bright sources, namely F410M detection of > 4 . 5 σ . Because of the larger PSF sizes and higher sky background, longer wavelength images only have limited detection capability. Especially, within the bright region of the Dragon arc, large correlated noise buries weak transient signals. Therefore, we only focused on F410M photome-  \nry and analysis for the brightest microlensing events. For these bright sources, we similarly performed forced aperture photometry with a fixed aperture radius of 0 . 1 arcsec. The F410M detection limit of transient is ∼ 27 . 7 ABmag.', '4.1. Summary of transient events in 2022 and 2023': "From two independent source detections, we found 44 and 33 sources from Sextractor and DAOFIND , respectively. 31 sources are detected with both software. We merged sources identified in Sextractor and DAOFIND to make the final transient catalog. In total, we detected 46 microlensing events from the Dragon; 28 in the 2022 observation and 18 in the 2023 observation (Table 1). Compared with previous identifications of microlensing events and magnified single star observations (e.g., 8 in Dragon arc identified with HST Flashlights program down to 3 . 3 σ significance; Kelly et al. 2022), our finding of 46 microlensing events in a single galaxy far surpasses previous records, which shows the extremely high occurrence rate of microlensing events in the Dragon arc captured by JWST . This is because the dragon arc has a lower redshift than other galaxies that host microlensing events (e.g., Kelly et al. 2018; Welch et al. 2022; Diego et al. 2023a), and therefore more stars are visible to the JWST when highly magnified.  \nWe also note that in Figure 2, many point-like residuals in the dragon arc are currently not selected as transients because of our conservative 5 σ detection limit. These surface brightness fluctuations are also likely caused by fainter microlensed events, or events off from the peak phase. As the magnitudes get fainter, we expect it increasingly probable that multiple, independently micro-lensed stars blended within the PSF collectively contribute to the observed flux variability. Such signals have been discussed in the contexts of pixel variability (Tuntsov et al. 2004) and collective variability of unresolved stellar associations (Dai 2021). Future deep multi-epoch JWST /NIRCam imaging observations will be able to examine this interpretation.  \nThe detection of microlensing events requires extremely high magnification ( µ ≳ 10 3 ). Therefore, they occur frequently around the critical curves of the largescale gravitational lens powered by major dark matter halos of the galaxy cluster (macrolens). Our detections of microlensing events are generally consistent with macrolens critical curves for z = 0 . 725 objects previously estimated in different studies and with different software and methods (see Figure 2). The estimated magnifications from macrolens based on existing cluster mass models are typically at µ ≳ 10 2 despite large  \nTable 1. F200W-based detections of microlensing events in 2022 and 2023  \nNote -Noteworthy events include † : event α , ∗ : event β ' and ∗∗ : event β (see discussions in § 4.3.1). Lensing magnifications are from a: Kawamata et al. (2018); b: Diego et al. (2007); c: Zitrin et al. (2009); d: Niemiec et al. (2023); e: Lagattuta et al. (2022). The absolute astrometric error is < 0 . '' 03, and we estimate the relative astrometric error as 0 . '' 06 / (S / N) per each of RA and DEC direction, where S/N is the signal-to-noise ratio of each event.  \nFigure 3. Color-magnitude diagram of F410M-detected transients. Square symbols show events in 2022 and circular symbols show events in 2023. Filled space shows the F410M detection limit. Except for event α , β , 0 . 4 mag of dust attenuation corrections are applied for all F200W - F410M colors estimated from measurements of Patr'ıcio et al. (2018). Solid line in the upper panel shows typical F200W - F410M color of different temperature giant stars redshifted at z = 0 . 725 (Lejeune et al. 1997). Right panel shows a histogram of detected F200W magnitude and absolute magnitude at the rest-frame J-band. The observed J-band absolute magnitude of these sources show that these stars are strongly magnified by µ ≳ 1000 -10000 assuming they are red giants or asymptotic giant stars (e.g., Madore et al. 2022).  \n<!-- image -->  \nuncertainties as indicated by the model-to-model scattering in Table 1.  \nAlthough the event positions are generally consistent with the macrolens critical curves, we note that existing lens models of the Abell 370 cluster do not simultaneously explain all the microlensing events. In particular, several transient events have large angular offsets from the expected positions of macrolens critical curves (e.g., region C in Figure 2). These large offsets between the macrolens critical curve and microlensing events may indicate the complex structure of the dark matter sub-halo structure of the lensing cluster (e.g., Meneghetti et al. 2017; Dai et al. 2018, 2020). In-depth analyses of detailed mass models with complex sub-halo distributions will be presented in future works.  \nFurthermore, we identified an event which has a large angular offset ( ∼ 1 . 8 '' ) from the Dragon arc. We call this event as event α (see Figure 2), and detailed analysis and discussion is presented in Section 4.3.1.", '4.2. Color magnitude diagram': "The color magnitude diagram of the detected sources shows that most of the lensed sources have red colors in F200W - F410M (roughly corresponding to J -K in the rest frame; Figure 3). Previous Herschel obser-  \nvations at 100-500 µ m have shown that the Dragon arc is a dusty star-forming galaxy (Rawle et al. 2016), and it exhibits significant interstellar dust attenuation of up to E(B -V) = 0 . 4 mag (Patr'ıcio et al. 2018). This implies a necessity of color corrections between F200W and F410M of up to 0 . 4 mag to derive intrinsic F200W F410M colors of each source, assuming a uniform dust screen. We simply applied a color correction of 0 . 4 mag to demonstrate possible stellar type classification for detected sources, except for the off-arc event alpha and beta, but we caution the potential variation of dust attenuation of observed stars, which cannot be corrected with the data taken so far.  \nSources detected in microlensing events have F200W - F410M colors of -0 . 5 ∼ +0 . 5 mag (Figure 3). By comparing with theoretical stellar atmosphere spectra of giant stars (lowest surface gravity available at each effective temperature in the set of Lejeune et al. 1997, under the assumption of [M / H] = 0), these colors are consistent with typical ranges of stellar spectra with temperatures of T ≲ 5000 K. Due to their low temperature and high luminosity, most of these sources are likely to be red giants/supergiants (and potentially binaries) magnified by factors of µ > 1000, and thus reaching absolute J-band magnitude of -14 ∼ -16 AB mag (see also § 4.3 below). These color distributions are particularly biased toward red sources because we performed source detection in F200W filters and because of the shallowness of F410M observations. Detecting higher temperature stars at this redshift (e.g., ≳ 8000 K) requires multiepoch observations at λ obs < 1 µ m (e.g., Kelly et al. 2022).", '4.3. SEDs analysis of bright microlensed sources': "For three particularly bright events, event α , event β and event β ' , we study their SEDs using multiwavelength photometry. As these events occurred in regions that are away from the bright arc, relatively accurate background subtraction is possible using singleepoch data. We conducted photometry of these events using background subtracted images of observations in 2022 and 2023. Aperture photometry was performed with aperture radii of 0 . 06 arcsec and 0 . 10 arcsec for short wavelength filter images and long wavelength filter images, respectively. Annuli with a width of 0 . 10 arcsec were used to measure local backgrounds, and flux density uncertainties were measured from random aperture experiments.", '4.3.1. Event α': "From our analysis of the difference images between the 2022 and 2023 observation, Event α has the largest angular separation from the arc ( ∼ 1 . 8 '' ; Figure 2). The  \n<!-- image -->  \nFigure 4. JWST single-epoch SEDs of event α (left) and event β , β ' (right). For event α , we consider three possible scenarios, including (i) microlensed red supergiant star in the halo of Dragon arc (model from Lejeune et al. 1997; top pannel); (ii) nebularphase SN Ia in the foreground Abell 370 cluster (spectrum from Kwok et al. 2023; middle panel); (iii) lensed SN Ia at z = 1 . 32 with no obvious host detection (129-day post-peak, Hsiao et al. 2007; bottom panel). All three templates fit the observed SED with similar goodness. For event β / β ' , although they are found at almost identical positions with similar F200W fluxes, their SEDs are largely different, showing that event β and event β ' are different objects experiencing different microlensing events.  \n<!-- image -->  \nSED of event α is shown in the top-left panel of Figure 4, and we find that a T = 2200K template among the examples in the stellar spectral library of Lejeune et al. (1997) provides the best match to the observed SED. Therefore, if event α is a microlensing event of a single star, it should be a cool star in the galaxy halo of the Dragon arc. We also examine the F182M and F210M light curve of event α over epoch-2 (3.4-day span in the observer frame). Despite the data being noisy per each telescope visit, we detect no evidence of significant brightness change ( < 0 . 7 mag) within this time frame.  \nHowever, most of the existing cluster mass models do not explain the required high magnification of event α needed to be observed as a microlensing event (typical macrolens µ ∼ 10-100 in Table 1). Assuming a macro+micro lensing magnification of µ = 1000 at z s = 0 . 725 and no dust extinction, event α should have absolute J -band magnitude of -7 . 2 AB mag in the rest frame. Given the expected low number density of very luminous, short-living stars in galaxy halos, the likelihood of event α being a microlensing supergiant star in Dragon arc is very low. Although one can assume a significantly larger lensing magnification to accommodate a lower intrinsic luminosity of the source, this would require a much smaller impact parameter (and thus a rarer chance) for the alignment of the microlens with the background star. In addition, the separation of α  \nfrom the host galaxy is too large for an evolved star to be found at this distance, making the interpretation of α as a microlensing event even less likely. Therefore, we also consider the following physical explanations of event α and explore their likelihood:  \n- Type Ia Supernova in the Abell 370: Possible . Given its proximity to one cluster member galaxy of Abell 370, event α is likely a SN at the cluster redshift. Because most of galaxies in Abell 370 cluster are quiescent and although some cluster galaxies are known to show corecollapse supernovae (e.g., Graham et al. 2012; Golubchik et al. 2023), we consider event α a likely SN Ia. At the cluster redshift z = 0 . 375, the observed F200W magnitude implies an absolute magnitude of -13 . 2 AB mag at 1.5 µ m. With this brightness, it can only be a nebular phase SN Ia that is ∼ 200 days post-peak in the rest frame (e.g., Graur et al. 2020). We note that the two epochs of JWST observations have a separation of 260 days in the z = 0 . 375 rest frame, placing a stringent upper limit of the age of event α if it is a cluster SN. The middle-left panel of Figure 4 compares the SED of event α with the JWST spectrum of the nearby Type Ia supernova SN 2021aefx at +255 days post peak (i.e., nebular phase; Kwok et al. 2023). We find that the observed SED of event α can be explained by this spectral template with similar χ 2 ( ∼ 8) to that of the aforementioned cool star template, and the residual is mostly  \ndominated by the F460M non detection. Therefore, we conclude that event α could be a SN Ia at the redshift of the foreground cluster.  \n- Kilonova in the Abell 370: Unlikely . At z = 0 . 375, the observed F200W magnitude could match that of a kilonova like AT 2017gfo (the electromagnetic counterpart of GW170817; Abbott et al. 2017) at ∼ 10 days post merger (Villar et al. 2017). At this epoch, AT 2017gfo has already shown a red rest-frame H -K color of 1 AB mag. However, this is not observed for event α in F210M - F300M. Therefore, we conclude that event α is unlikely a kilonova at the cluster redshift. However, our knowledge on kilonovae's light curves and SEDs could still be very limited, which could show a large diversity because of different ejecta properties (Kawaguchi et al. 2020).  \n- Luminous red nova in Abell 370 or Dragon arc: Unlikely . Massive star merger can trigger luminous red novae reaching absolute V-band magnitude of ∼ -15 mag (e.g., Smith et al. 2016). They can also show similar rest-frame near-infrared color as that of event α . However, we argue that the likelihood of having a massive stellar progenitor in either the halo of Dragon or Abell 370 quiescent member galaxies is considerably low.  \n- 'Hostless' SN at z ≃ 1 -2: Possible . Event α could also be a lensed SN hosted by a low-mass galaxy that was not detected even by JWST. This is also known as 'hostless' supernova. If event α is a SN at z ≳ 2, based on glafic lens model (Oguri 2010; Kawamata et al. 2018), we expect the detection of a counter image of the SN with a short time delay to event α ( ≲ 20 days) but potentially larger magnification. If event α is a SN at z ≲ 0 . 8, then with the large magnification ( µ ∼ 10) and limited evolution time ( < 1 yr in observed frame), event α should have been as bright as ≲ 27 AB mag at 2 µ m, i.e., 1 mag brighter than the actual observed value. Therefore, if event α is a hostless SN, it can only be at z ≃ 1 -2. With the SN Ia spectral templates from Hsiao et al. (2007) and software SNCosmo (Barbary et al. 2024), we find that the SED of event α can be explained by SN Ia at z ∼ 1 . 32 with χ 2 ∼ 8, for which the age is t ∼ +129days post-peak (+300 days in the observer frame; Fig 4, bottom-left panel). At this redshift, the multiple images are expected as nondetections in obtained JWST data, either because of large difference of arrival time or proximity to bright cluster member galaxies. Therefore, we do not rule out the possibility of event α being a hostless SN at z ∼ 1 . 3.  \nAs a final remark, naturally there are other transient sources (e.g., tidal disruption events) that could be considered to interpret event α . However, we believe that the above discussions cover most of the known possibilities, and we conclude that event α could be also inter-  \neted as a nebular-phase SN Ia at the Abell 370 redshift or a 'hostless' SN at z ∼ 1 . 3.", "4.3.2. Event β and β '": "The event β and event β ' are found in the 2022 and 2023 observations, respectively, at the most south-east position of the Dragon arc (Figure 2). Both event β and event β ' are some of the brightest events among the current microlensing events found in the 2022 and 2023 images. As the locations of these events are in the outer disk region of the Dragon arc, we expect low dust attenuation for these two sources. Thus, we assume no dust attenuation for sources observed in both event β and event β ' .  \nRight panel of Figure 4 shows the observed fluxes of event β and event β ' . Using the spectra of stellar atmosphere models of Lejeune et al. (1997), we performed χ 2 minimization. We find that low temperature stars with surface temperature of T = 3000K to T = 3300K fit well and a T = 3200K star gives the minimum χ 2 for the SED of event β . Similarly, the observed SED of event β ' is well represented with a star with surface temperature of T = 3500 -3700 K while a T = 3500K star gives the minimum χ 2 . With these fits, we find the apparent luminosity of the event β and event β ' to be µL bol ∼ 1 . 1 × 10 8 L ⊙ and µL bol ∼ 7 . 8 × 10 7 L ⊙ , respectively.  \nDue to their low temperatures represented by their red colors, these events would be microlensed red giant/supergiant stars that are strongly magnified with µ ≳ 1000. The different spectral shapes of event β and event β ' suggest that they would be different stars, rather than multiple images of a single star. The existence of such massive stars suggests that there could be a star-forming region under the critical curve of this position where we can expect more microlensing events in future observations. At the same time, as low temperature stars at high-redshift are only bright in λ obs ≳ 2 µ m, these sources demonstrate the importance of such nearIR observations to build complete stellar samples at high-redshift.  \nAn alternative interpretation would be that these two stars are indeed the same star multiply imaged as a result of astrometric perturbation induced by dark matter subhalos. Such events are predicted by Dai et al. (2018) and will occur specifically around the critical crossing region such as the location near event β and event β ' (see also Kelly et al. 2018). In this case, the difference of the observed SEDs are interpreted by intrinsic variability of the magnified star. In fact, such variability of stars accompanying changes of surface temperature and SEDs are observed in nearby stars such as Cepheid. Although  \na definitive conclusion is difficult to obtain with existing photometry using single epochs, confirmations of such magnified close-paired star in large numbers allow to investigate the subhalo structure of dark matter as well as to constrain dark matter physics. Future JWST's time-domain observations using matched filters allow us to obtain accurate multi-wavelength photometry of individually magnified stars and allow us to conduct such studies.", '5. CONCLUSION': "In this study, we report the discovery of multiple microlensing events in a strongly lensed z = 0 . 725 galaxy Abell 370 'Dragon' arc. Using JWST observations taken in 2022 and 2023, the microlensing events are found as transients occurring in each epoch. In total, 46 microlensing events are detected with > 5 σ across the Dragon arc, showing the high rate of microlensing events that can be captured by JWST observations.  \nAlthough the locations of the majority of microlensing events are consistent with the macrolensing critical curves of existing mass models, a few of them are significantly away from the expected critical curves. Events farther away from the critical curve can be obtained in two ways; i) adding substructure on small scales that can boost the magnification (and hence the probability of microlensing) around them, such as millilenses, or ii) a shallow luminosity function for the lensed stars increases the number of lensed stars farther away from the critical curve (see Diego et al. 2024, for a discussion of these two competing effects). A detailed study of these two scenarios based on the events presented in this work is left for a future work.  \nUsing multi-wavelength photometry, we compare restframe colors of the microlensing events. Many of the detected sources have relatively red colors due to dust attenuation from host galaxy's interstellar medium as well as the low atmospheric temperatures of the detected microlensed stars.  \nWe then study multi-wavelength SEDs of a few transient events that are relatively free from the background emission of the Dragon arc. In particular, we study the SED of the event α that has the largest angular offset from the Dragon arc (1 . 8 '' ), and conclude that it is unlikely a microlensed star because of the low surface density of massive stars in the halo of Dragon arc. Instead, it may be a nebular-phase SN Ia at the redshift of Abell 370 cluster ( z = 0 . 375), or a lensed SN Ia at z ∼ 1 . 32, for which the host galaxy is undetected. We also investigate SEDs of two events (event β/β ' ). We find their SEDs are consistent with stars with atmospheric temperatures of T = 3200 -3500 K. Their proximity to the critical  \ncurve and the similarity of their SEDs suggest that they could be a multiply-imaged single variable star, while it is also possible that they are two different stars individually microlensed.  \nOur findings demonstrate the unique power of time-domain observations in near-IR wavelength using JWST . Further JWST multiple-epoch observations of the Dragon arc, including the F182M and F210M imaging observations of GO-2883 (PI: Sun), as well as other strongly lensed galaxies at high redshift will provide us with essential information about stellar populations in high-redshift galaxies. Further detailed analysis of such microlensing events will also provide essential information about the lensing properties of dark matter sub-halo distribution in galaxy clusters.", 'ACKNOWLEDGMENTS': "We thank Lindsey Kwok for kindly sharing the JWST spectrum of SN 2021aefx, and Jordi Miralda-Escude for insightful comments. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program #1208, 3538. The authors acknowledge the JWST GO-3538 team led by PI E. Iani for developing their observing program with a zero-exclusive-access period. All the BUFFALO HST data used in this paper can be found in MAST: 10.17909/t9-w6tj-wp63. This work was supported by JSPS KAKENHI Grant Number JP22K21349, JP23K13149, JP20H05856, JP22H01260, JP22J21. F.S. and E.E. acknowledge JWST/NIRCam contract to the University of Arizona NAS5-02015. F.S. acknowledges support for program #2883 provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. J.M.D. acknowledges the support of project PID2022-138896NBC51 (MCIU/AEI/MINECO/FEDER, UE) Ministerio de Ciencia, Investigaci'on y Universidades. L.D. acknowledges research grant support from the Alfred P. Sloan Foundation (Award Number FG-2021-16495), and support of Frank and Karen Dabby STEM Fund in the Society of Hellman Fellows. A.Z. acknowledges support by Grant No. 2020750 from the United StatesIsrael Binational Science Foundation (BSF) and Grant No. 2109066 from the United States National Science Foundation (NSF); by the Ministry of Science &  \nTechnology, Israel; and by the Israel Science Foundation Grant No. 864/23. E.Z. acknowledges project grant 2022-03804 from the Swedish Research Council (Vetenskapsr˚adet) and has also benefited from a sabbatical at the Swedish Collegium for Advanced Study. M.J. is supported by the United Kingdom Research and Innovation (UKRI) Future Leaders Fellowship 'Using Cosmic Beasts to uncover the Nature of Dark Matter' (grants number MR/S017216/1 & MR/X006069/1). D.J.L. acknowledges support from the UKRI FLF grants number MR/S017216/1 & MR/X006069/1. D.J.L. is also supported by Science and Technology Facilities Council (STFC) grants ST/T000244/1 and ST/W002612/1. E.I. acknowledges funding from the Netherlands Research School for Astronomy (NOVA). R.A.W. and S.H.C. acknowledge support from NASA JWST Interdisciplinary Scientist grants NAG5-12460, NNX14AN10G and 80NSSC18K0200 from GSFC. C.C.C. acknowledges support from the National Science and Technology Council of Taiwan (111-2112M001-045-MY3), as well as Academia Sinica through the Career Development Award (AS-CDA-112-M02).  \nD.E. acknowledges support from a Beatriz Galindo senior fellowship (BG20/00224) from the Spanish Ministry of Science and Innovation, projects PID2020114414GB-100 and PID2020-113689GB-I00 financed by MCIN/AEI/10.13039/501100011033, project P20-00334 financed by the Junta de Andaluc'ıa, and project AFQM-510-UGR20 of the FEDER/Junta de Andaluc'ıaConsejer'ıa de Transformaci'on Econ'omica, Industria, Consejer'a, Universidades. K.K. acknowledges the support by JSPS KAKENHI Grant Number JP17H06130, JP22H04939, and JP23K20035. G.E.M. acknowledges financial support from the Villum Young Investigator grants 37440 and 13160 and the Cosmic Dawn Center (DAWN), funded by the Danish National Research Foundation (DNRF) under grant No. 140. A.N. acknowledges funding from Olle Engkvists Stiftelse.  \nFacilities: JWST(NIRCam)  \nSoftware: astropy (Astropy Collaboration et al. 2013, 2018), DAOFIND (Stetson 1987), Photutils (Bradley et al. 2023), Pypher (Boucaud et al. 2016), SciPy (Virtanen et al. 2020), SNCosmo (Barbary et al. 2024), Source Extractor (Bertin & Arnouts 1996)", 'REFERENCES': 'Abbott, B. P., Abbott, R., Abbott, T. D., et al. 2017, ApJL, 848, L12, doi: 10.3847/2041-8213/aa91c9 Adamo, A., Bradley, L. D., Vanzella, E., et al. 2024, arXiv e-prints, arXiv:2401.03224, doi: 10.48550/arXiv.2401.03224 Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A, 558, A33, doi: 10.1051/0004-6361/201322068 Astropy Collaboration, Price-Whelan, A. M., Sip"ocz, B. 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Diego, 3 Liang Dai, 4 Masamune Oguri, 1, 5 Adi Zitrin, 6 Erik Zackrisson, 7, 8 Mathilde Jauzac, 9, 10, 11, 12 David J. Lagattuta, 9, 10 Eiichi Egami, 2 Edoardo Iani, 13 Rogier A. Windhorst, 14 Katsuya T. Abe, 1 Franz Erik Bauer, 15, 16, 17 Fuyan Bian, 18 Rachana Bhatawdekar, 19 Thomas J. Broadhurst, 20, 21, 22 Zheng Cai, 23 Chian-Chou Chen, 24 Wenlei Chen, 25 Seth H. Cohen, 26 Christopher J. Conselice, 27, 28 Daniel Espada, 27, 28 Nicholas Foo, 26 Brenda L. Frye, 29 Seiji Fujimoto, 30, ∗ Lukas J. Furtak, 6 Miriam Golubchik, 6 Tiger Yu-Yang Hsiao, 31 Jean-Baptiste Jolly, 32 Hiroki Kawai, 33, 1 Patrick L. Kelly, 34 Anton M. Koekemoer, 35 Kotaro Kohno, 36, 37 Vasily Kokorev, 38 Mingyu Li, 23 Zihao Li, 23 Xiaojing Lin, 23, 2 Georgios E. Magdis, 39, 40, 41 Ashish K. Meena, 6 Armin Nabizadeh, 7 Johan Richard, 42 Charles L. Steinhardt, 43, 44 Yunjing Wu, 23 Yongda Zhu, 2 and Siwei Zou 23  \n1  \nCenter for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan  \n2  \nSteward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA  \n- 3 Instituto de F'ısica de Cantabria (CSIC-UC). Avda. Los Castros 39005 Santander, Spain\n- 4 Department of Physics, University of California, 366 Physics North MC 7300, Berkeley, CA. 94720, USA\n- 5 Department of Physics, Graduate School of Science, Chiba University, Chiba 263-8522, Japan\n- 6 Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Be'er-Sheva 84105, Israel  \nObservational Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden  \n8  \nSwedish Collegium for Advanced Study, Linneanum, Thunbergsvagen 2, SE-752 38 Uppsala, Sweden  \n9  \nCentre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK  \n- 10 Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK  \nAstrophysics Research Centre, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa  \n- 12 School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa 13 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands 14 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA\n- 15 Instituto de Astrof'ısica and Centro de Astroingenier'ıa, Facultad de F'ısica, Pontificia Universidad Cat'olica de Chile, Campus San Joaqu'ın, Av. Vicu˜na Mackenna 4860, Macul Santiago, Chile, 7820436\n- 16 Millennium Institute of Astrophysics, Nuncio Monse˜nor S'otero Sanz 100, Of 104, Providencia, Santiago, Chile 17 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301\n- 18 European Southern Observatory, Alonso de C'ordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile\n- 19 European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Ca˜nada, Madrid, Spain\n- 20 Dept. of Physics, University of the Basque Country, Bilbao, Spain\n- 21 DIPC, Donostia International Physics Centre, San Sebastian, Spain\n- 22 Ikerbasque, Basque Foundation for Science, Bilbao, E-48011, Spain\n- 23 Department of Astronomy, Tsinghua University, Beijing 100084, China\n- 24 Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan 25 Department of Physics, Oklahoma State University, 145 Physical Sciences Bldg, Stillwater, OK 74078, USA 26 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA\n- 27 Departamento de F'ısica Te'orica y del Cosmos, Campus de Fuentenueva, Edificio Mecenas, Universidad de Granada, E-18071, Granada, Spain  \n28 Instituto Carlos I de F'ısica Te'orica y Computacional, Facultad de Ciencias, E-18071, Granada, Spain  \n- 29 Department of Astronomy/Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA 30 Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA\n- 31 Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University, 3400 N Charles St. Baltimore, MD 21218, USA\n- 32 Max-Planck-Institut fur Extraterrestrische Physik (MPE), Giessenbachstraße 1, D-85748 Garching, Germany  \n33 Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan  \n- 34 Minnesota Institute for Astrophysics, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA 35 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA\n- 36 Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan\n- 37 Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan\n- 38 Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands  \n39 Cosmic Dawn Center (DAWN), Denmark  \n- 40 DTU-Space, Technical University of Denmark, Elektrovej 327, 2800, Kgs. Lyngby, Denmark\n- 41 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200, Copenhagen N, Denmark  \nUniv Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval,  \n42 France  \n43 Dark Cosmology Centre  \n44 Niels Bohr Institute, University of Copenhagen, Jagtvej 120, DK-2100 Copenhagen Ø"}