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[vsnet-grb-info 42165] GRB 260310A / AT2026fgk: SVOM/VT optical observations
by GCN Circulars 18 Mar '26

18 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44044 SUBJECT: GRB 260310A / AT2026fgk: SVOM/VT optical observations DATE: 26/03/18 07:47:29 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, L. P. Xin, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, 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. T. Palmerio (CEA/Irfu)，J. X. Cao (GXU) report on behalf of the SVOM/VT team. SVOM/VT performed several ToO observations of the field of GRB 260310A detected by Fermi (Fermi GBM team, GCN 43951) and AstroSat(Salunke et al., GCN 43958) from 2026-03-14 to 2026-03-17 in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. The reported optical counterpart AT2026fgk discovered by GOTO(O'Neill et al. 2026, TNS Discovery Report 294132) and observed by other observatories (Lipunov et al., GCN 43954; Konno et al., GCN 43974, Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979, Moren Méndez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984, Pursiainen et al., Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; L. Izzo, GCN 44003; Brivio et al., GCN 44004; Rhodes et al., GCN 44005; Moskvitin et al., GCN 44006; Brosio et al., GCN 44021; Pawar et al., GCN 44022; Belkin and Shrestha, GCN 44043) is clearly detected in single frames in both channels. The preliminary measurements in AB magnitudes are given below: Mid_time Band Exposure Time Magnitude (AB) 5.26 day VT_R 70 sec 18.44 +/- 0.05 mag 5.26 day VT_B 70 sec 18.94 +/- 0.05 mag 7.35 day VT_R 70 sec 18.65 +/- 0.05 mag 7.35 day VT_B 70 sec 19.15 +/- 0.05 mag 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/44044. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42164] GRB 260310A / AT2026fgk: LCO optical observations
by GCN Circulars 18 Mar '26

18 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44043 SUBJECT: GRB 260310A / AT2026fgk: LCO optical observations DATE: 26/03/18 06:59:09 GMT FROM: Sergey Belkin at Monash University <sergey.belkin(a)monash.edu> S. Belkin and M. Shrestha (Monash U) report: We observed the optical counterpart AT2026fgk of GRB 260310A, detected by Fermi/GBM (Fermi GBM Team, GCN 43951) and AstroSat/CZTI (Salunke et al., GCN 43958), with the Las Cumbres Observatory 1m telescope at McDonald Observatory equipped with the Sinistro instrument. Observations began on 2026-03-14 at 06:12:09 UT, approximately 4.05 days after the trigger. We obtained 4 x 60 s exposures in the r band and 6 x 60 s exposures in the u band. In the stacked images we clearly detect the reported optical counterpart AT2026fgk (e.g. Lipunov et al., GCN 43954; Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Mendez et al., GCN 43980; de Ugarte Postigo et al., GCN 43984; Pursiainen et al., GCN 43990; Becerra et al., GCN 43991; Martin-Carrillo et al., GCN 44000; Watson et al., GCN 44001; Lai et al., GCN 44002; Izzo, GCN 44003; Moskvitin et al., GCN 44006; Romanov, GCN 44020; Brosio et al., GCN 44021; Pawar et al., GCN 44022), discovered by GOTO (O’Neill et al., TNS Discovery Report 294132). The preliminary photometry from the stacked images is: Date (UT) T-T0 (days) Filter Exp. time (s) Magnitude 3-sigma UL ======================================================================================== 2026-03-14 06:14:13 4.0535 r 60 x 4 18.58 +/- 0.02 21.79 2026-03-14 06:21:49 4.0588 u 60 x 6 19.47 +/- 0.04 21.12 The r-band photometry is calibrated against Pan-STARRS field stars. The u-band photometry is calibrated against Gaia DR3 synthetic SDSS u-band stars. The quoted values are not corrected for Galactic extinction. Further follow-up is encouraged. View this GCN Circular online at https://gcn.nasa.gov/circulars/44043. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42163] IceCube-260315A/AT2026fpm: EP-FXT follow-up observation
by GCN Circulars 18 Mar '26

18 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44042 SUBJECT: IceCube-260315A/AT2026fpm: EP-FXT follow-up observation DATE: 26/03/18 04:57:04 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. P. Feng, B. Zhang (USTC), H. N. Yang, Y. Liu (NAO, CAS) The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) performed a follow-up observation of AT2026fpm (Segura et al., GCN 44023) at UTC 2026-03-16T23:31:53 (T0+45.21h) (The IceCube Collaboration, GCN 44017). An X-ray source was detected at R.A. = 176.1586 deg, DEC = -2.6035 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic) and the flux of 1.1×10^(-13) erg/s/cm^2 in 0.5-10 keV band. The X-ray position is consistent with the optical transient AT2026fpm, which is spatially consistent with a QSO at z=1.389. 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/44042. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42162] GRB 260317A: Fermi GBM Final Localization
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44041 SUBJECT: GRB 260317A: Fermi GBM Final Localization DATE: 26/03/17 21:55:00 GMT FROM: rhamburg(a)usra.edu The Fermi GBM team reports the detection of a likely SHORT GRB. "At 20:01:51.98 UT on 17 March 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260317A (trigger 795470516/260317835). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 255.62, Dec = -18.51 (J2000 degrees, equivalent to J2000 17h 2m, -18d 30'), with a statistical uncertainty of 13.33 degrees. The angle from the Fermi LAT boresight is 93 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260317835/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260317835/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260317835/…" View this GCN Circular online at https://gcn.nasa.gov/circulars/44041. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42161] GRB 260316A: GECAM-B observation
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44040 SUBJECT: GRB 260316A: GECAM-B observation DATE: 26/03/17 15:46:38 GMT FROM: Yue Wang <m18509381757(a)163.com> Yue Wang, Chao Zheng, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team: GECAM-B was triggered in-flight by GRB 260316A at 2026-03-16T17:12:53.100 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44031) and SVOM/GRM (Yue Wang et. al., GCN #44039). According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a single pulse with a duration (T90) of 2.4 +0.4/-0.4 s. The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecambgrb260316A.png Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS). View this GCN Circular online at https://gcn.nasa.gov/circulars/44040. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42160] GRB 260316A: SVOM/GRM observation
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44039 SUBJECT: GRB 260316A: SVOM/GRM observation DATE: 26/03/17 15:43:47 GMT FROM: Yue Wang <m18509381757(a)163.com> SVOM/GRM team: Yue Wang, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Sebastien Guillot, Hui Yang (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by GRB 260316A at 2026-03-16T17:12:54.000 (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #44031). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 1.6 +1.6/-0.8 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260316A.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 305.5, Dec = -79.3, Error = 3.2, GCN #44031), is located at about 113 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-1.2 to T0+0.6 s is best fitted by a power law function. The power law index is -2.23 +0.17/-0.16. The event fluence (10-1000 keV) in this time interval is (3.21 +0.50/-0.51)E-06 erg/cm^2. 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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM/GRM point of contact for this burst is: Yue Wang (IHEP) (yuewang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/44039. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42159] EP260316a / GRB 260316A: SVOM/VT optical upper limits
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44038 SUBJECT: EP260316a / GRB 260316A: SVOM/VT optical upper limits DATE: 26/03/17 15:21:28 GMT FROM: Huali Li at at NAOC, SVOM <lhl(a)nao.cas.cn> H. L. Li, L. P. Xin, Y. N. Ma, C. Wu, Z. H. Yao, Y. L. Qiu, 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. T. Palmerio (CEA/Irfu) report on behalf of the SVOM/VT team. SVOM/VT performed ToO observation of the field of EP260316a detected by Einstein Probe (Jiang et al., GCN 44027, GCN 44034), and GRB 260316A with the sub-threshold detection by Fermi (Ravasio et al., GCN 44029). The observation started at 2026-03-16T22:31:53 UTC, i.e., about 9.98 hours post trigger in the VT_B (400-650 nm) and VT_R (650-1000 nm) channels simultaneously. No uncatalogued optical source was detected in our stacked images. The 3-sigma upper limits are: Mid_time Band Exposure Time Magnitude (AB) 10.35 hour VT_R 37*70 sec > 23.2 mag 10.35 hour VT_B 39*70 sec > 23.3 mag The faint optical candidate reported by COLIBRÍ (Fortin et al., GCN 44037) was not detected in our stacked images. 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/44038. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42158] EP260316a: COLIBRÍ faint optical counterpart candidate
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44037 SUBJECT: EP260316a: COLIBRÍ faint optical counterpart candidate DATE: 26/03/17 13:06:02 GMT FROM: F. Fortin at IRAP <ffortin.sci.edu(a)gmail.com> Francis Fortin (IRAP), Fredd Sánchez Álvarez (UNAM), Jean-Luc Atteia (IRAP), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Missimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the EP260316a (Jiang et al., GCN Circ. 44027, GCN Circ. 44034), also detected by Fermi (Ravasio et al., GCN Circ. 44029) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-17 07:56:51 to 10:00:54 UTC (from 19.4 to 21.5 hours after the trigger) and obtained 94 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ ASU 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. We detect a faint (SNR = 5.7) uncatalogued source located at the edge of the EP/FXT 10 arcsec error circle (Jiang et al., GCN Circ. 44034) at: RA(J2000) = 15:07:55.62 = 226.98176 degrees Dec(J2000) = +27:26:53.9 = +27.44830 degrees with an uncertainty of 0.5 arcsec. The preliminary magnitude derived for that source is: r = 24.40 +/- 0.19 z > 23.38 (3-sigma) We note that our images are deeper than the PS1 and LS catalogues, hence we cannot fully conclude whether or not this source is a transient. Further observations are planned. Followup of this source with other facilities is also encouraged. 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/44037. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42157] Fermi-LAT Gamma-ray Observations of IceCube-260315A
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44036 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-260315A DATE: 26/03/17 12:07:57 GMT FROM: Leo Pfeiffer at University of Würzburg <pfeiffer.leo(a)gmail.com> L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg) and S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC260315A neutrino event (GCN 44017) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 26-03-15 at 02:19:06.60 UTC (T0) with J2000 position RA = 175.69 (+0.49, -0.46) deg, Decl. = -1.90 (+0.53, -0.51) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC260315A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546). We searched for the existence of intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (>5sigma) new excess emission (> 100 MeV) within the IC260315A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC260315A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 2.7e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <1.6e-08 (<5.0e-08) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this analysis, the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer at uni-wuerzburg.de). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. View this GCN Circular online at https://gcn.nasa.gov/circulars/44036. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42156] GRB 260316A: AstroSat CZTI detection
by GCN Circulars 17 Mar '26

17 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44035 SUBJECT: GRB 260316A: AstroSat CZTI detection DATE: 26/03/17 09:51:41 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), A. Arya (IITB), A. Goyal (IITB), 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 GRB 260316A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 44031), Calet (Trigger Num. 1457716347), and SVOM (Trigger Num. sb26031602). The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-03-16 17:12:53.75 UTC. The measured peak count rate associated with the burst is 524 (+158, -53) counts/s above the background in the combined data of all quadrants, with a total of 818 (+139, -151) counts. The local mean background count rate was 306 (+5, -8) counts/s. Using cumulative rates, we measure a T90 of 4.91 (+1.7, -2.5) 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/44035. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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