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[vsnet-grb-info 42117] GRB 260310A/AT 2026fgk: WINTER NIR Detections
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43996 SUBJECT: GRB 260310A/AT 2026fgk: WINTER NIR Detections DATE: 26/03/13 21:05:01 GMT FROM: Robert David Stein at JSI <rdstein(a)umd.edu> Robert Stein (UMD), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Xander J. Hall (CMU), Tomas Ahumada (NOIRLab), Danielle Frostig (CfA), Mansi Kasliwal (Caltech), Nathan Lourie (MIT), Benjamin Schneider (LAM), Robert Simcoe (MIT), and report: We observed AT 2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132), identified as the likely counterpart to GRB 260310A (Fermi GBM Team, GCN 43951). We used the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations began at 2026-03-12T09:57:09.115 UTC (~53 hours after the GRB trigger), and continued over several expochs in both J-band and Hs-band. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) We clearly detect a source in all of our stacked images, with times given relative to burst time (2026-03-10T04:57:11): | t - t0 (hr) | filter | mag(AB) | magerr | limiting mag | |-------------|--------|---------|--------|--------------| | 53.00 | J | 17.16 | 0.09 | 19.28 | | 53.58 | J | 17.28 | 0.09 | 19.18 | | 54.13 | Hs | 17.10 | 0.21 | 17.91 | | 71.87 | J | 17.34 | 0.12 | 19.03 | | 72.17 | Hs | 16.97 | 0.20 | 17.77 | We do not perform image subtraction due to a lack of suitable reference images, so there might be some contamination from the host galaxy. However, given the large offset and relatively faint host, we do not expect this to substantially alter our measurements. We do not measure significant evolution over our ~20 hour baseline, after accounting for the uncertainty in each measurement. A slow fading would be consistent with these observations, and with expectations for a power-law decay of an afterglow at this relatively late (>2 days) phase. Our observations are consistent with values reported by Li et al (GCN 43993), who quote a value of J=16.7 +/- 0.1 mag in the Vega system (~17.6 mag AB) at a later phase of +81 hours. Further observations of this transient are planned. WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research. View this GCN Circular online at https://gcn.nasa.gov/circulars/43996. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42116] GRB 260313A: A Correction to the Fermi GBM Final Localization
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43995 SUBJECT: GRB 260313A: A Correction to the Fermi GBM Final Localization DATE: 26/03/13 18:31:37 GMT FROM: oliver.roberts(a)universityofgalway.ie O.J. Roberts (Uni. of Galway, Ireland) reports on behalf of the Fermi Gamma-ray Burst Monitor Team. "At 12:00:28.58 UT on 13 March 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260313A (trigger 795096033/260312500). An on-ground calculated location, using the Fermi GBM trigger data, was initially reported in GCN 43987 by the GBM Team. Further analysis found the location of the short GRB to be inaccurate. We report the updated/refined location to be RA = 241.16, Dec = 5.80 (J2000 degrees, equivalent to J2000 16h 05m, 05d 48'), with a statistical uncertainty of 5.8 degrees. The angle from the Fermi LAT boresight with this new position is 15 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260312500/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260312500/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260312500q…" View this GCN Circular online at https://gcn.nasa.gov/circulars/43995. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42115] GRB 260310A: Einstein Probe Follow-up X-ray Telescope (FXT) Detection
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43994 SUBJECT: GRB 260310A: Einstein Probe Follow-up X-ray Telescope (FXT) Detection DATE: 26/03/13 17:39:34 GMT FROM: Rahul Jayaraman at Cornell University <rj438(a)cornell.edu> Rahul Jayaraman (Cornell), Anna Y. Q. Ho (Cornell), K-Ryan Hinds (Caltech), 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) report: The Follow-up X-ray Telescope on board the Einstein Probe observed the position of AT2026fgk, the likely counterpart to GRB 260310A (Fermi GBM team, GCN 43951; Arya et al., GCN 43958; Konno et al., GCN 43974; Martin-Carrillo et al., GCN 43979; Méndez et al., GCN 43980; Becerra et al., GCN 43991), beginning at 2026-03-12T17:22:58 UTC. The exposure time of this observation was 1498 s. An uncatalogued X-ray source was detected at RA = 219.3158, DEC = 71.8417, with an uncertainty of 10 arcsec (radius, 90% CL statistical and systematic); this X-ray source is consistent with the optical position initially reported by Hinds et al., AstroNote 2026-65. We performed a preliminary analysis of the FXT spectrum in the 0.5-10 keV energy range from both the A and B modules. We fit an absorbed power-law with a line-of-sight hydrogen column density of 2.65 x 10^20 cm^-2. We find a photon index of 1.55 ± 0.09, with an unabsorbed flux of (5.51 ± 0.22) x 10^-12 erg/s/cm^2. At the redshift of z = 0.153 from Ugarte Postigo et al. (GCN 43984), this would correspond to an X-ray luminosity of (3.74 ± 0.15) x 10^44 erg/s. Parameter uncertainties are reported at the 90% C.L. We consider this source the likely X-ray afterglow to GRB 260310A, and encourage further follow-up. The Einstein Probe is a space mission 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/43994. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42114] GRB 260310A/AT 2026fgk: J-band observations by the SYSU 80 cm infrared telescope
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43993 SUBJECT: GRB 260310A/AT 2026fgk: J-band observations by the SYSU 80 cm infrared telescope DATE: 26/03/13 17:35:21 GMT FROM: lijj328(a)mail2.sysu.edu.cn Jin-Ji Li, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Yan Yu, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team: We observed the field of GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations were carried out on 2026 Mar 13, 13:58 UT, 81.0 hours after the GRB trigger, using stacked images in the J band. We clearly detect an infrared counterpart at the position of the optical afterglow candidate AT 2026fgk (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43980; Atteia et al., GCN 43981; de Ugarte Postigo et al., GCN 43984; Hsu et al., GCN 43986; Malesani et al., GCN 43990; Becerra et al., GCN 43991; discovered by GOTO; O'Neill et al. 2026, TNS Discovery Report 294132), and we measure J = 16.7±0.1 Vega mag. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University. View this GCN Circular online at https://gcn.nasa.gov/circulars/43993. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42113] GRB 260310B: Insight-HXMT/HE detection
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43992 SUBJECT: GRB 260310B: Insight-HXMT/HE detection DATE: 26/03/13 15:05:40 GMT FROM: renyz16607(a)163.com Yang-Zhao Ren, Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng (IHEP) report on behalf of the Insight-HXMT team: At 2026-03-10T21:41:23.850 (T0), Insight-HXMT/HE detected a long burst, GRB 260310B, which is also detected by Fermi/GBM (Fermi GBM team, GCN #43955), and GECAM-B (Ren et al., GCN #43967). The Insight-HXMT/HE light curve mainly consists of multi-pulses with T90 of 43.5 +5.5/-7.5 s. The 1s peak rate, measured from T0+0.25 s, is 1147 cnts/sec. Insight-HXMT/HE detected a total of 11204 counts from this burst. The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260310B.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/43992. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42112] GRB 260310A/AT 2026fgk: DDOTI Optical Detection
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43991 SUBJECT: GRB 260310A/AT 2026fgk: DDOTI Optical Detection DATE: 26/03/13 14:38:23 GMT FROM: Rosa Leticia Becerra Godínez at Instituto de Astronomía, UNAM <rbecerra(a)astro.unam.mx> Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report: We observe the field of the Fermi/GBM GRB 260310A (Fermi GBM Team, GCN 43951) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the nights of 2026-03-10 and 2026-03-11 UTC. DDOTI observed from 05:11 UTC to 9:34 on 2026-03-10 (from T+0.2 hours to T+4.6 hours after the trigger) with a total exposure time of 2.1 hours and from 03:32 UTC to 12:20 on 2026-03-11 (from T+22.6 hours to T+31.4 hours after the trigger), with a total exposure time of 3.1 hours. On both nights we detect the source AT2026fgk (discovered by GOTO; O’Neill et al. 2026, TNS Discovery Report 294132), proposed as the optical counterpart (Konno et al.; GCN Circ. 43974), with preliminary AB magnitudes of: w ≈ 16.6 (in our first epoch); w = 17.52 +/- 0.01 (in our second epoch). The photometry of the second night is consistent with a temporal decay of alpha≈-0.6 between the magnitudes reported by the LAST collaboration (Konno et al.; GCN Circ. 43974) and COLIBRÍ (Moreno Méndez et al.; GCN Circ. 43980). Due to technical problems during the first night, the photometry from that epoch should be considered tentative. These values are not corrected for the Galactic extinction. We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir. View this GCN Circular online at https://gcn.nasa.gov/circulars/43991. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42111] GRB 260310A: NOT optical polarimetry
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43990 SUBJECT: GRB 260310A: NOT optical polarimetry DATE: 26/03/13 14:33:29 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> M. Pursiainen (Warwick), D. B. Malesani (DAWN/NBI and Radboud), G. Leloudas (DTU Space), Dimple (Birmingham), A. Martin-Carrillo (UCD), B. P. Gompertz (Birmingham), D. A. Perley (LJMU), N. Pyykkinen (NOT and UTU), report on behalf of a larger collaboration: We observed the likely optical counterpart AT 2026fgk (Konno et al., GCN 43974; Hinds et al., AstroNote 2026-65; O’Neill et al. 2026, TNS Discovery Report 294132) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958), using the Nordic Optical Telescope (NOT) equipped with ALFOSC, carrying out imaging polarimetry. In an image taken on 2026 Mar 13.09 UT (2.89 days after trigger), the transient had an AB magnitude of r = 18.12 +- 0.02 (calibrated against Pan-STARRS sources, not corrected for Galactic extinction). Unfortunately the seeing conditions were poor (~2.8" FWHM). A sequence of 4x900 s exposures was secured in linear imaging polarimetry (mean epoch Mar 13.09 UT), employing four different retarder plate angles and an SDSS r filter. The data were reduced with a custom-built reduction pipeline (Pursiainen et al. 2025, doi:10.1093/mnras/staf232) based on the photutils python library (Bradley et al. 2024, doi:10.5281/zenodo.13989456), with reduction steps detailed in Pursiainen et al. (2023, doi:10.1051/0004-6361/202345945). No significant polarization is found for the transient, down to a 3-sigma upper limit of P < 1.5%. No correction for Galactic interstellar polarization has been applied, though we note that the line of sight to GRB 260310A only suffers from very modest extinction (A_V = 0.05 mag). View this GCN Circular online at https://gcn.nasa.gov/circulars/43990. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42110] GRB 260312B: AstroSat CZTI detection
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43989 SUBJECT: GRB 260312B: AstroSat CZTI detection DATE: 26/03/13 13:07:30 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> A. Goyal (IITB), A. Arya (IITB), M. Tembhurnikar (IUCAA), Harsha K. H. (IUCAA), S. Salunke (IUCAA), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration: Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long GRB 260312B which was also detected by Fermi GBM (Fermi GBM Team , GCN Circ. 43969), and Glowbug (Cheung et. al., GCN Circ. 43976). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-03-12 09:46:53.50 UTC. The measured peak count rate associated with the burst is 131 (+29, -30) counts/s above the background in the combined data of three quadrants (out of four), with a total of 605 (+164, -182) counts. The local mean background count rate was 186 (+3, -4) counts/s. Using cumulative rates, we measure a T90 of 14 (+1, -6) s. The source was also faintly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project. CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb View this GCN Circular online at https://gcn.nasa.gov/circulars/43989. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42109] Swift BAT trigger 1500295: A solar flare
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43988 SUBJECT: Swift BAT trigger 1500295: A solar flare DATE: 26/03/13 12:49:55 GMT FROM: Amy <yarleen(a)gmail.com> C. Gronwall (PSU), R. Gupta (NASA GSFC) and A. Y. Lien (U Tampa) report on behalf of the Neil Gehrels Swift Observatory Team: BAT trigger 1500295 is not a GRB, but was caused by a solar flare. Swift's automatic slews are currently disabled, so there will be no XRT or UVOT data for this trigger. View this GCN Circular online at https://gcn.nasa.gov/circulars/43988. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42108] GRB 260313A: Fermi GBM Final Real-time Localization
by GCN Circulars 13 Mar '26

13 Mar '26

TITLE: GCN CIRCULAR NUMBER: 43987 SUBJECT: GRB 260313A: Fermi GBM Final Real-time Localization DATE: 26/03/13 12:11:03 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 12:00:28 UT on 13 Mar 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260313A (trigger 795096033.584074 / 260313500). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 53.4, Dec = 33.3 (J2000 degrees, equivalent to J2000 03h 33m, 33d 17'), with a statistical uncertainty of 14.1 degrees. The angle from the Fermi LAT boresight is 152.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260313500/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260313500/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260313500/… View this GCN Circular online at https://gcn.nasa.gov/circulars/43987. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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