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[vsnet-grb-info 42204] EP260321a: GOTO detection of blue variable source
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44083 SUBJECT: EP260321a: GOTO detection of blue variable source DATE: 26/03/22 21:30:52 GMT FROM: Shane Moran at University of Leicester <shane.moran(a)leicester.ac.uk> S. Moran, A. Kumar, G. Ramsay, D. O'Neill, K. Ulaczyk, S. Moran, B. P. Gompertz, R. Starling, M. Wortley, M. Kennedy, K. Ackley, M. Dyer, J. Lyman, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, M. Pursiainen, on behalf of GOTO collaboration: We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) that serendipitously covered the field of EP/WXT detected EP260321a (WXT01709259023; Huang et al., GCNs 44068, 44075). Observations covering the localisation area were taken at 2026-03-22 11:25:46 UT (+22.92h post trigger) utilising GOTO-South. Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm). We detect the previously reported variable source (Lee et al. GCN 44070, Tanvir et al. GCN 44082) with L = 19.51 ± 0.1 AB mag. GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC). View this GCN Circular online at https://gcn.nasa.gov/circulars/44083. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42203] EP260321a: VLT imaging and spectroscopy show the variable source to be at z = 0.0343
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44082 SUBJECT: EP260321a: VLT imaging and spectroscopy show the variable source to be at z = 0.0343 DATE: 26/03/22 19:50:53 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> N. R. Tanvir (Leicester), L. Izzo (INAF/OACN), A. J. Levan (Radboud and Warwick), D. Xu (NAOC), G. Corcoran (UCD), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), G. Pugliese (API), A. de Ugarte Postigo (LAM), K. Wiersema (Hertfordshire), report on behalf of the Stargate collaboration: We observed the field of EP260321a (Huang et al., GCNs 44068, 44075) with the ESO VLT UT1 (Antu) and UT4 (Yepun), obtaining R-band imaging with the FORS2 instrument (mid-time 00:44 UT on 2026-02-22; 12.23 hr after the EP trigger), and integral field spectroscopy with the MUSE instrument on UT4 (mid-time 05:29 UT on 2026-02-22; 16.98 hr after the EP trigger). Within the refined EP/FXT error circle (Huang et al., GCN 44075), the blue, historically variable source coincident with the outskirts of a galaxy at z = 0.0343, noted by Lee et al. (GCN 44070), is prominent. We measure a magnitude of R = 19.5, which is within the range of archival values, but is significantly brighter than reported by Lee et al. (GCN 44070). The source itself is consistent with being marginally resolved in 0.7" seeing imaging, though this may be due to contamination from the underlying galaxy. Our spectroscopy shows the variable source (covering the wavelength range 4750 - 9300 Å) to be characterised by a blue power-law continuum, with very strong narrow emission lines at z = 0.0343 (e.g., [O III] 5008 EW ~ 500 Å). No absorption features are detected at either z = 0 or z = 0.0343. While the emission lines are visible across the entire galaxy, they are strongly spatially concentrated at the location of the blue variable object, with fluxes approximately twenty times brighter than at the galaxy nucleus. This makes it unlikely for the source to be a random foreground superposition, suggesting that it lies itself within the galaxy at z = 0.0343. At this redshift, our measured flux corresponds to an absolute magnitude of approximately M_R = -16.8, and the past variability (Lee et al., GCN 44070) appears to be consistent with previous outbursts to a comparable flux level. It is possible that past variability was due to LBV-like outbursts from a star embedded within a dense, luminous cluster (as evidenced by the strong emission line component). The short-term variability following the outburst (Lee et al., GCN 44070; Aryan et al., GCN 44081), coupled with the location within the revised FXT error circle, suggest that this source is the optical counterpart of EP260321a. It is unclear if the current outburst is related to a further LBV flare or the terminal collapse of the star, as may be suggested by the possible shock breakout properties of the X-ray outburst (Huang et al., GCN 44075). Under this interpretation, this would make EP260321a the first example of an X-ray transient progenitor detected prior to explosion. We encourage further monitoring of this object at all wavelengths in order to clarify its enigmatic nature. We acknowledge excellent support of the ESO observing staff at Paranal, in particular Abel de Burgos Sierra, Alonso Luna Ruiz Fernandez, Elyar Sedaghati, Enrico Congiu, Israel Blanchard, and Rodrigo Palominos. We also thank the visitor observers on UT4, Silvia Piranomonte and Eleonora Parlanti, for generously allowing the execution of our observation. View this GCN Circular online at https://gcn.nasa.gov/circulars/44082. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42202] EP260321a: further Kinder optical observations
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44081 SUBJECT: EP260321a: further Kinder optical observations DATE: 26/03/22 15:49:58 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> A. Aryan, M.-H. Lee, T.-W. Chen, W.-J. Hou (all NCU), S. Yang (HNAS), A. K. H. Kong (NTHU), T. Moriya (NAOJ), S. J. Smartt, J. Gillanders (both Oxford), Y. J. Yang (NYUAD), A. Dutta, Y.-H. Lee, A. Sankar.K, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, C.-S. Lin H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), 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. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the fast X-ray transient EP260321a (Huang et al., GCN 44068) using the 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al., 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first LOT epoch of observations started at 11:34 UTC on the 22nd of March 2026 (MJD 61121.482), 23.07 hr after the EP-WXT trigger. We used Kinder pipeline (Yang et al. A&A 646, A22) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image. In the difference image, we still detected this stellar source SDSS J095942.88+002506.2 (Lee et al., GCN 44070), but no other new transient has emerged. Moreover, we used AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry for this variable star (without template subtraction). The details of the observations and the measured magnitude (in the AB system) are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | (med/avg.) Airmass LOT | r | 61121.482 | 23.07 | 300 * 6 | 19.18 +/- 0.04 | 0".96 | 1.27 LOT | g | 61121.520 | 23.99 | 300 * 2 | 18.77 +/- 0.04 | 1".11 | 1.17 Our 3-sigma upper limit for the observed field is r = 23.4 mag. EP260321a was identified as a possible supernova shock-breakout candidate in the EP refined analysis (Huang et al., GCN 44075). However, no optical counterpart has been reported to date (Lee et al., GCN 44070; Ma et al., GCN 44074; Aguilar-Ruiz et al., GCN 44076; Liang et al., GCN 44079). For comparison, in SN 2008D the optical counterpart was detected only ~1.4 hr after the initial X-ray detection (Soderberg et al. 2008). Given the similar X-ray duration to SN 2008D and the continued absence of any optical counterpart, we suspect that the EP260321a X-ray emission more likely originates from the variable star itself. If it is a Galactic X-ray binary, the spectrum suggests a supersoft (WD) accretor, but the X-ray flux is weak; a strong local stellar flare is also plausible. The presented magnitudes are calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105) and are not corrected for the expected Galactic foreground extinctions of A_r = 0.06 mag and A_g = 0.08 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission can be found in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/44081. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42201] GRB 260321A: SVOM/VT optical upper limits
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44080 SUBJECT: GRB 260321A: SVOM/VT optical upper limits DATE: 26/03/22 14:10:30 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, L. P. Xin, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), M. Brunet (IRAP), O. Godet (IRAP) report on behalf of the SVOM/VT team. SVOM/VT performed an observation with automatic slew to the field of GRB 260321A triggered by SVOM/ECLAIRs (sb26032123, Brunet et al., GCN 44071). The observation started at 2026-03-21T18:57:18 UTC, i.e., about 45 minutes post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. No credible sources were detected within the error box of ECLAIRs (Brunet et al., GCN 44071), compared to the Legacy survey catalog. The 5 sigma upper limits in AB magnitude are derived as follows: Mid time | Band | Exposure Time | 5 sigma upper limit 1.13 hour VT_B 53*50 sec > 22.7 mag 1.12 hour VT_R 42*50 sec > 22.5 mag More detailed analysis is still ongoing. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/44080. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42200] EP260321a: Las Cumbres upper limit
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44079 SUBJECT: EP260321a: Las Cumbres upper limit DATE: 26/03/22 13:12:22 GMT FROM: aujust(a)mail.ustc.edu.cn Runduo Liang, Wenxiong Li (NAOC), Iair Arcavi (TAU), Ido Keinan (TAU), David Sand (U of Arizona) We observed the position of EP260321a (GCN 44068, GCN 44075) with a Las Cumbres 1m telescope at Cerro Tololo Inter-American Observatory, Chile, about 11.25 hours after the Einstein Probe WXT trigger. We took 2x300s exposures in the broad optical r-band. We didn’t detect any uncataloged source within the EP-FXT refined position down to a 3-sigma limiting r-band mag of ~20.8. The presented magnitudes are calibrated with Pan-STARRS sources in the field and not corrected for Galactic extinction. Additional follow-up is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/44079. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42199] GRB 260321A: SVOM/COLIBRÍ (FM-GFT) optical upper limit
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44078 SUBJECT: GRB 260321A: SVOM/COLIBRÍ (FM-GFT) optical upper limit DATE: 26/03/22 08:47:42 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo(a)gmail.com> Antonio de Ugarte Postigo (LAM), Edilberto Aguilar-Ruiz (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), 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), M. Brunet, O. Godet (IRAP) report: We imaged the field of the SVOM GRB 260321A (Brunet et al., GCN Circ. 44071) using the DDRAGO wide-field imager on the SVOM/COLIBRÍ (FM-GFT) telescope. We observed from 2026-03-22 06:35:41 to 07:59:54 UTC (from 12.39 to 13.80 hours after the trigger) and obtained 61 minutes of exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ pipeline and analyzed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked image, we do not detect any new source within the ECLAIRs source position (Brunet et al., GCN Circ. 44071), as compared to the Legacy Survey, down to the following 5-sigma limit: r > 23.7 mag z > 22.6 mag Further observations are ongoing. 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/44078. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42198] GRB 260320A: Insight-HXMT/HE detection
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44077 SUBJECT: GRB 260320A: Insight-HXMT/HE detection DATE: 26/03/22 08:44:50 GMT FROM: Xinghao Luo at IHEP <2952704891(a)qq.com> Xing-Hao Luo, Chen-Wei Wang, Chao Zheng, Cheng-Kui Li and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team: At 2026-03-20T19:40:04.700 (T0), Insight-HXMT/HE detected the burst GRB 260320A, which is also detected by NuSTAR (G. Waratkar et.al., GCN #44064). The Insight-HXMT/HE light curve mainly consists of a single pulse with a T90 of 4.4 +1.2/-0.8 s. The 1s peak rate, measured from T0-0.7 s, is 2511 cnts/sec. Insight-HXMT/HE detected a total of 5253 counts from this burst. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260320A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/ View this GCN Circular online at https://gcn.nasa.gov/circulars/44077. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42197] EP260321a: COLIBRÍ optical upper limit on any supernova shock breakout emission
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44076 SUBJECT: EP260321a: COLIBRÍ optical upper limit on any supernova shock breakout emission DATE: 26/03/22 07:01:20 GMT FROM: Rosa L. Becerra at Instituto de Astronomía, UNAM <rbecerra(a)astro.unam.mx> Edilberto Aguilar-Ruiz (UNAM), Antonio de Ugarte Postigo (LAM), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (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), Leonardo García García (UNAM), 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), and Benjamin Schneider (LAM) report: We imaged the field of the Einstein Probe X-ray transient EP260321a (Huang et al., GCN Circ. 44068) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-22 03:48 to 05:50 UTC (from 15.3 to 17.4 hours after the trigger) and obtained 24, 65, and 89 minutes of exposure, respectively, in the g, r, and z filters. The data were reduced and coadded with the COLIBRÍ ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction. In the stacked image, and after subtracting template images from the Legacy Survey DR10 (Dey et al. 2019) using STDWeb, we do not detect any new source at the FXT refined position (Huang et al., GCN Circ. 44075) down to the following 5-sigma limit: g > 23.8 r > 23.8 z > 22.8 Our upper limit in r is consistent with the ones reported by Lipunov et al. (GCN Circ. 44069) and Lee et al. (GCN Circ. 44070). We do see additional flux from the superposed star, also in agreement with Lee et al. In order to test the shock breakout scenario proposed by Huang et al. (GCN Circ. 44075), we compared our results against the light curve of the AT2016gkg (Bersten et al. 2018), shifted at the redshift of the galaxy 2dFGRS TGN352Z077 at z=0.034 (DESI Legacy Survey, Dey et al. 2019). At the time of our observations, we expect an AB magnitude about 19.2 at 570 nm, which lies between our g and r filters. Our upper limits are about 4.6 magnitudes fainter than this, and so we are unable to confirm the presence of a shock breakout. 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/44076. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42196] EP260321a: refined analysis of the EP-WXT and EP-FXT observations, implying a possible supernova shock breakout candidate
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44075 SUBJECT: EP260321a: refined analysis of the EP-WXT and EP-FXT observations, implying a possible supernova shock breakout candidate DATE: 26/03/22 04:32:32 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Q. J. Huang (PMO, CAS), Z.-C. Zou (NJU), D. Y. Li, X. Mao, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team: The fast X-ray transient EP260321a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 44068). The transient triggered WXT at T0=2026-03-21T12:23:07 (UTC). The WXT observation lasted for approximately 432 seconds, and was interrupted due to the autonomous follow-up observation. The WXT 0.5-4 keV spectrum was soft, and can be fitted with an absorbed black body with the absorption fixed at the Galactic value of 2.7 × 10^(20) cm^-2 and a temperature of 164 (-29/+40) eV. The derived average unabsorbed 0.5-4 keV flux is 8.0 (-1.9/+2.2) × 10^(-11) erg/s/cm^2. The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-03-21T12:35:02 (UTC, T0+12min). The exposure time of this observation is 4266s. On-ground analysis of the FXT data found an uncatalogued fading source within the WXT error circle at R.A., Dec. = 149.9287, 0.4177 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). A significant decay was detected in the FXT light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed black body with Galactic absorption and a temperature of 121 (-3/+3) eV. The derived average unabsorbed 0.5-10 keV flux is 9.8 (-0.3/+0.3) × 10^(-12) erg/s/cm^2. All quoted errors are at the 90% confidence level. We note that within the FXT error circle, there is a galaxy 2dFGRS TGN352Z077, at a redshift of 0.0345. If the transient is associated with this galaxy, the corresponding luminosity of the WXT detection would be approximately 2.2 × 10^(44) erg/s. Given its very soft spectrum, rapid decay and the corresponding luminosity at the assumed redshift, we tend to consider EP260321a as possibly a supernova shock breakout candidate. No optical counterpart of EP260321a has been detected so far (Lipunov et al., GCN 44069; Lee et al., GCN 44070; Ma et al., GCN 44074) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260321a. A follow-up observation with the EP-FXT was planned, and further information will be updated when the telemetry data are received. Launched on January 9, 2024, EP 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/44075. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42195] EP260321a: SVOM/VT optical upper limit
by GCN Circulars 22 Mar '26

22 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44074 SUBJECT: EP260321a: SVOM/VT optical upper limit DATE: 26/03/22 02:34:27 GMT FROM: Yinuo Ma <mayn(a)bao.ac.cn> Y. N. Ma, H. L. Li, Z. H. Yao, Y. L. Qiu, C. Wu, L. P. Xin, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team. SVOM/VT performed a Target of Opportunity observation of EP260321a detected by EP/WXT (Huang et al., GCN 44068). SVOM/VT began observing the field at 2026-03-21T13:10:39 UTC, 0.673 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously. With X-band data available, no uncataloged source is found within the error box of FXT (Huang et al., GCN 44068) compared with DESI Legacy Survey at the mid time of 1.92 hours after trigger, VT_R > 22.8 mag with exposure time of 53*50 sec, VT_B > 23.0 mag with exposure time of 54*50 sec. The stellar source and the galaxy reported by Kinder (Lee et al., GCN 44070) are detected in our observations, with no significant variability. Our photometry was not corrected for Galactic extinction. The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS. View this GCN Circular online at https://gcn.nasa.gov/circulars/44074. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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