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[vsnet-grb-info 42219] Fermi GRB 260323A: Global MASTER-Net observations report
by GCN Circulars 24 Mar '26

24 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44098 SUBJECT: Fermi GRB 260323A: Global MASTER-Net observations report DATE: 26/03/24 00:15:25 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 260323A ( Fermi GBM team, GCN 44097) errorbox 143 sec after notice time and 173 sec after trigger time at 2026-03-23 23:54:21 UT, with upper limit up to 20.1 mag. Observations started at twilight. The observations began at zenith distance = 43 deg. The sun altitude is -16.3 deg. The galactic latitude b = -27 deg., longitude l = 289 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3206970 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 188 | 2026-03-23 23:54:21 | MASTER-OAFA | (06h 02m 40.77s , -72d 38m 38.2s) | C | 30 | 18.8 | 229 | 2026-03-23 23:54:57 | MASTER-OAFA | (06h 02m 40.97s , -72d 38m 38.5s) | C | 40 | 18.9 | 280 | 2026-03-23 23:55:43 | MASTER-OAFA | (06h 02m 41.22s , -72d 38m 38.9s) | C | 50 | 19.2 | 341 | 2026-03-23 23:56:39 | MASTER-OAFA | (06h 02m 41.55s , -72d 38m 39.0s) | C | 60 | 19.3 | 650 | 2026-03-24 00:01:17 | MASTER-OAFA | (06h 42m 20.00s , -76d 51m 39.3s) | C | 120 | 20.1 | 786 | 2026-03-24 00:03:23 | MASTER-OAFA | (06h 42m 20.80s , -76d 51m 40.5s) | C | 140 | 20.1 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/44098. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42218] GRB 260323A: Fermi GBM Final Real-time Localization
by GCN Circulars 24 Mar '26

24 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44097 SUBJECT: GRB 260323A: Fermi GBM Final Real-time Localization DATE: 26/03/24 00:02:09 GMT FROM: Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply(a)GIOC.nsstc.nasa.gov> The Fermi GBM team reports the detection of a likely LONG GRB At 23:51:27 UT on 23 Mar 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260323A (trigger 796002692.989066 / 260323994). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 100.1, Dec = -77.1 (J2000 degrees, equivalent to J2000 06h 40m, -77d 05'), with a statistical uncertainty of 5.5 degrees. The angle from the Fermi LAT boresight is 149.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260323994/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260323994/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260323994/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44097. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42217] GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44096 SUBJECT: GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days DATE: 26/03/23 22:07:59 GMT FROM: Daniel Perley at Liverpool JMU <d.a.perley(a)ljmu.ac.uk> GRB260310A: Palomar 60-inch photometry of an extremely bright late-time afterglow with a temporal break at 8 days D. A. Perley (LJMU), Y. Wagh (LJMU), R. Jayaraman (Cornell), J. Sollerman (Stockholm), A. Y. Q. Ho (Cornell), K.-R. Hinds (Caltech), and A. Bochenek (LJMU) report: We used the SED Machine Rainbow Camera optical imager mounted on the Palomar 60-inch telescope to obtain observations of AT2026fgk (Hinds et al., TNS AstroNote 2026-65), the optical counterpart (Konno et al., GCN 43974) of the Fermi/GBM gamma-ray burst GRB260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975). Observations began on the night of 2026-03-12 at 06:03 UT and have continued every night since in the SDSS g, r, and i filters; u-band imaging was also taken on certain nights. Photometry was performed following image subtraction of template images taken from Pan-STARRS survey imaging. For the first seven days after the GRB, the fading is generally consistent with a power-law evolution with a shallow temporal index of approximately -0.6. Beginning at around 7-8 days, the afterglow begins falling more sharply, with a decay index closer to -1.35. (A break in the X-ray light curve was seen at a similar time; Jayaraman et al., GCN 44095 ). Select photometry (AB, not corrected for extinction) is provided below: | UT date | dt (d) | filter | mag +/- unc | |---------------------|--------|--------|----------------| | 2026-03-12 06:03:41 | 2.047 | r | 18.14 +/- 0.04 | | 2026-03-13 06:33:38 | 3.067 | r | 18.19 +/- 0.03 | | 2026-03-15 06:08:19 | 5.050 | r | 18.59 +/- 0.03 | | 2026-03-18 06:01:43 | 8.045 | r | 18.83 +/- 0.03 | | 2026-03-23 05:20:09 | 13.016 | r | 19.58 +/- 0.04 | The behavior is consistent with other recent reports noting the continuation of the optical decay with no SN-like rebrightening yet (e.g., Volnova et al., GCN 44060; Bussman et al., GCN 44061). Accounting for Galactic extinction and converting to the equivalent host-frame bandpass, the absolute magnitude of the optical transient as of the most recent observation is M_V ~ -19.7. The fact that the light curve is still decaying steeply suggests that any associated supernova is likely to be significantly dimmer than the benchmark SN1998bw, which peaked at a similar magnitude and phase (e.g., Galama et al. 1998, Nature, 395, 670). The color of the afterglow indicates only modest dust extinction in the host galaxy, so the subluminous nature of the SN is likely intrinsic. A fainter SN comparable to SN2006aj is not yet ruled out, but will be tightly constrained by observations during the next 1-2 weeks. Additionally, we note that the optical afterglow is both extremely bright and very luminous at late times. To our knowledge only one[1] previous GRB (GRB030329 at z=0.168) has remained brighter than r=20 mag more than 10 observer-frame days after the event, excluding events dominated by the rebrightening of the SN on this timescale (see e.g., Kann et al. 2023, ApJL, 948, 12). The luminosity is also notable: despite its relatively weak prompt emission (E_iso ~ 10^51 erg: Minaev et al., GCN 44053), the afterglow luminosity of GRB260310A at the current time is at the upper end of the afterglow luminosity distribution at this time post-burst, comparable to many high-luminosity (E_iso ~ 10^54 erg) GRBs such as 080319B, 130427A, and 221009A. We encourage continued monitoring of this exceptional GRB afterglow (and any potential associated SN), which is likely to remain accessible to small- to medium-size optical facilities for many more weeks and possibly months. [1] GRB221009A would have been observed to have a similar apparent magnitude at this time but was heavily obscured by Galactic dust. Based on observations obtained with the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and the German Center for Astrophysics (DZA), Germany. Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA. DAP acknowledges the work, legacy, and friendship of D. Alexander Kann, who doubtless would have been tremendously excited by this GRB afterglow. View this GCN Circular online at https://gcn.nasa.gov/circulars/44096. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42216] GRB 260310A/AT2026fgk: Detection of temporal steepening in Einstein Probe X-ray data
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44095 SUBJECT: GRB 260310A/AT2026fgk: Detection of temporal steepening in Einstein Probe X-ray data DATE: 26/03/23 21:59:34 GMT FROM: Rahul Jayaraman at Cornell University <rj438(a)cornell.edu> Rahul Jayaraman (Cornell), Gaurav Waratkar (Caltech), K-Ryan Hinds (Caltech), Yogesh Wagh (LJMU), Daniel Perley (LJMU), Anna Y. Q. Ho (Cornell), S. Y. Fu (HUST), R. D. Liang, M. J. Liu, Z. X. Ling, Hui Sun, Weimin Yuan (NAO, CAS), Genevieve Schroeder (Cornell), Xander J. Hall (CMU), Michael Coughlin (UMN), Aleksandra Bochenek (LJMU), and Jesper Sollerman (Stockholm) report: The Follow-up X-ray Telescope on board the Einstein Probe has continued to monitor the X-ray afterglow of GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Jayaraman et al., GCN 43994), which is associated with the optical transient AT2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65). Observations with duration ~3 ks were taken at intervals of roughly every 2 days after the initial detection of this source in X-rays (Jayaraman et al., GCN 43994). With four further epochs of data since the initial detection, we find evidence for a steepening in the X-ray light curve at around 8 ± 2 days after the burst, with the decay slope steepening from roughly –0.3 to –1.6. The photon index measured by EP remains at roughly 1.5 throughout these observations. This steepening appears contemporaneous with an observed steepening at a similar epoch post-burst in r-band photometric data taken by the SED Machine on the Palomar P60 telescope, which will be reported in a forthcoming Circular. Launched on January 9, 2024, the Einstein Probe is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is supported by the Strategic Priority Program on Space Sciénce of the Chinese Academy of Sciences, in collaboration with the European Space Agency, the Max-Planck-Institute for Extraterrestrial Physics (Germany), and the Centre National d'études Spatiales (France). View this GCN Circular online at https://gcn.nasa.gov/circulars/44095. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42215] GRB 260322A: Fermi GBM Final Localization
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44094 SUBJECT: GRB 260322A: Fermi GBM Final Localization DATE: 26/03/23 17:56:30 GMT FROM: Utkarsh Pathak at IIT Bombay <utkarshpathak.07(a)gmail.com> The Fermi GBM team reports the detection of a likely SHORT GRB "At 10:45:40 UT on 22 March 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260322A (trigger 795869144/260322448). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 301.0, Dec = -61.8 (J2000 degrees, equivalent to J2000 20h 3m, -61d 46'), with a statistical uncertainty of 7.9 degrees. The angle from the Fermi LAT boresight is 59.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260322448/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260322448/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260322448/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/44094. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42214] GRB 260320A: Glowbug gamma-ray detection
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44093 SUBJECT: GRB 260320A: Glowbug gamma-ray detection DATE: 26/03/23 17:34:44 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung(a)nrl.navy.mil> C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, confirms the detection of GRB 260320A, which was detected by NuSTAR (GCN 44064), SVOM/GRM (sb26032003), Insight-HXMT/HE (GCN 44077), and Konus-Wind (Trig_Time 19:40:01.257). The burst onset is at 2026-03-20 19:40:02, with a peak at ~T0+1.5s, and a burst duration of ~7s. Note that data throughout the detection interval suffered from deadtime in various detectors. The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O [3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/44093. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42213] EP260321a / AT2026gzf: VLT/X-shooter detection of supernova-like spectral features at z = 0.0344
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44092 SUBJECT: EP260321a / AT2026gzf: VLT/X-shooter detection of supernova-like spectral features at z = 0.0344 DATE: 26/03/23 17:12:25 GMT FROM: nrt3(a)le.ac.uk D. Xu (NAOC), G. Corcoran (UCD), J. An (NAOC), L. Izzo (INAF/OACN), N. Habeeb (Leicester), N. R. Tanvir (Leicester), A. J. Levan (Radboud and Warwick), C. C. Thoene (AbAO), S. Schulze (Weizmann), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI), A. Saccardi (CEA/Irfu), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), A. L. Thakur (INAF-IAPS) report on behalf of the Stargate collaboration: We observed the variable point source, AT2026gzf, (Tanvir et al., GCN 44082; see also Lee et al., GCN 44070; Aryan et al., GCN 44081; Ahumada et al., GCN 44084; Liu et al., GCN 44087) associated with X-ray transient EP260321a (Huang et al., GCN 44068) with the X-shooter spectrograph on the ESO VLT UT3 (Melipal) beginning on 2026-02-23 at 00:38 UT (36.1 hr post-trigger). Our observations consisted of 4x600 s exposures, and span the wavelength range 3000-20600 AA. Bright continuum flux is visible in all three arms of the spectrum. In addition to the blue continuum and narrow emission lines reported for the previous MUSE observation (Tanvir et al., GCN 44082), our X-shooter spectrum shows broad features below 5000 AA, similar to those seen in the early spectra of core-collapse supernovae, and inconsistent with those seen in typical young stellar populations. However, we caution that at this early phase there are limited templates for comparison, combined with some contribution from any underlying cluster light, and a search for matches within GELATO and SNID does not, at this time, yield a firm classification. Furthermore, the detection of narrow absorption features due to Ca II at z = 0.0344 conclusively excludes a Milky Way origin for this transient. We acknowledge excellent support of the ESO observing staff at Paranal, in particular Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Lorena Faundez and Marcela Espinoza. View this GCN Circular online at https://gcn.nasa.gov/circulars/44092. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42212] EP260321a: COLIBRÍ further optical observations and brightening
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44091 SUBJECT: EP260321a: COLIBRÍ further optical observations and brightening DATE: 26/03/23 16:06:10 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> Antonio de Ugarte Postigo (LAM), Leonardo García García (UNAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of EP260321a (Huang et al., GCN Circ. 44068, 44075) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope, following our previous observation (Aguilar-Ruiz et al. GCN Circ. 44076). We observed in two epochs between 2026-03-23 04:40 to 10:04 UTC (between 40.17 and 45.57 hours after the trigger) and obtained imaging in the g, r, i, z and y bands. The data were reduced and coadded with the ASU COLIBRÍ pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. From our first observation, at a mid epoch T+16.33 h after the EP detection, to the second epoch of last night, at a mid epoch of T+43.61 h, the transient reported by Lee et al. (GCN Circ. 44070), has increased in brightness in r-band by 1.3+/-0.1 mag consistently with the reported evolution of the source (Lee et al. GCN Circ. 44070, Aryan et al. GCN Circ. 44081, Tanvir et al. GCN Circ. 44082, Moran et al. GCN Circ. 44083, Ahumada et al. GCN Circ. 44084, Liu et al. GCN Circ. 44087 and Sankar et al. GCN Circ. 44089). Our latest photometric measurement is r = 18.67 +/- 0.02 mag but we caution that the value is likely affected by the contribution of the underlying galaxy. Further observations are planned. We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM. COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico. View this GCN Circular online at https://gcn.nasa.gov/circulars/44091. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42211] Fermi Gamma-ray Burst Monitor trigger 795918973/260323025 is not a GRB
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44090 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 795918973/260323025 is not a GRB DATE: 26/03/23 15:55:17 GMT FROM: Suman Bala at USRA <sumanbala2210(a)gmail.com> S. Bala (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 795918973/260323025 at 00:36:08.73 UT on 23 March 2026, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to particle event." View this GCN Circular online at https://gcn.nasa.gov/circulars/44090. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42210] EP260321a: Continued brightening from Kinder follow-up of the emerging supernova candidate
by GCN Circulars 23 Mar '26

23 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44089 SUBJECT: EP260321a: Continued brightening from Kinder follow-up of the emerging supernova candidate DATE: 26/03/23 15:36:09 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> A. Sankar.K, A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), M. Nicholl (QUB), T. Moriya (NAOJ), M. Saillenfest (LTE, Paris Observatory), Y.-N. Lee (NTNU), M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Y. J. Yang (NYUAD), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz M. Huber, K. Chambers (all IfA, Hawaii) report: We obtained further optical observations of the field of the fast X-ray transient EP260321a (Huang et al., GCN 44068, 44075) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86; Aryan et al. 2025, ApJS, 281, 20). The new LOT observations started at UTC 12:42 on March 23rd 2026 (MJD 61122.5295), corresponding to 48.20 hr after the EP-WXT trigger. At the refined EP/FXT position, coincident with the blue variable source first noted by Lee et al. (GCN 44070) and further discussed by Tanvir et al. (GCN 44082), Ahumada et al. (GCN 44084), and Liu et al. (GCN 44087), we detect the source in the g band. We used the AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry for this object: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | Seeing | Airmass LOT | g | 61122.5295 | 48.20 | 300 * 1 | 18.30 +/- 0.03 | 1".02 | 1.14 The presented magnitude is calibrated using field stars from the ATLAS-RefCat2 catalog from MAST (Tonry et al. 2018, ApJ, 867, 105), is reported in the AB system, and is not corrected for the expected Galactic foreground extinction of A_g = 0.08 mag in the direction of the transient (Schlafly & Finkbeiner 2011). Compared with our previous LOT measurement of g = 18.77 +/- 0.04 mag at 23.99 hr post-trigger (Aryan et al., GCN 44081), the source has brightened by approximately 0.47 mag. This is also consistent with, and extends, the re-brightening trend reported by Liu et al. (GCN 44087), indicating that the source is still rising. Our time resolved monitoring of this source during the six hours after the FXT detection with the LOT (Lee et al., GCN 44070, Aryan et al., GCN 44081) indicates a sharp decline of 0.3 mag within the first 2 hours followed by a continued, monotonic rise, now at g = 18.3 at t-t0 = 48.2 hr. With the spectroscopic observation from Tanvir et al. (GCN 44082) indicating that the source may be associated the z = 0.0343, we propose the LOT optical lightcurve could be explained by the shock cooling and a rise from a supernovae associated with the blue point like source visible in the archive images, also proposed by Liu et al. (GCN 44087). At M_g ~ -17.7, this is now at supernova-like luminosities, which would be consistent with Huang et al. (GCN 44075) interpretaiton of the EP260321a as shock breakout signature. The luminosity of the blue progenitor source would be very large for a single star, lying between M_r = -15 to -16. Elevated, high luminosity has been seen in LBV type precursors (e.g. SN1961V, Kochanek et al. 2011, ApJ 737, 76; and other LBV precursors, Smith et al. 2010, AJ 139, 1451), but this case would be extreme. We note that long term Pan-STARRS monitoring shows this source to be variable over 7 years, with flux variations in the w-band difference imaging around 21 < w < 22. Further spectroscopic observations are required to determine if the transient shows high expansion velocities and lines that definitively place it in the z=0.0343 host. View this GCN Circular online at https://gcn.nasa.gov/circulars/44089. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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