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[vsnet-grb-info 42796] GRB 260518A: AstroSat CZTI detection of a long burst
by GCN Circulars 19 May '26

19 May '26

TITLE: GCN CIRCULAR NUMBER: 44668 SUBJECT: GRB 260518A: AstroSat CZTI detection of a long burst DATE: 26/05/19 16:41:44 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> Harsha K. H. (IUCAA), S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), A. Goyal (IITB), A. Arya (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 long GRB 260518A. The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2026-05-18 07:44:29.50 UTC. The measured peak count rate associated with the burst is 380 (+84, -78) counts/s above the background in the combined data of all quadrants, with a total of 654 (+182, -192) counts. The local mean background count rate was 295 (+5, -6) counts/s. Using cumulative rates, we measure a T90 of 9 (+2, -2) s. The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-05-18 07:44:29.10 UTC. The measured peak count rate associated with the burst is 417 (+67, -72) counts/s above the background in the combined data of all quadrants, with a total of 1270 (+298, -329) counts. The local mean background count rate was 1301 (+5, -5) counts/s. We measure a T90 of 10 (+2, -3) s from the cumulative Veto light curve. 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 data products like interactive and downloadable light curves for this GRB can be found at: https://astrosat.iucaa.in/cift/cift_products/516786266.6/S516786266.6_detai… 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/44668. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42795] GRB 260516D: SVOM/ECLAIRs refined analysis
by GCN Circulars 19 May '26

19 May '26

TITLE: GCN CIRCULAR NUMBER: 44667 SUBJECT: GRB 260516D: SVOM/ECLAIRs refined analysis DATE: 26/05/19 15:29:15 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> M. Brunet, O. Godet, J.-L. Atteia (IRAP), A. Saccardi (CEA/ Irfu), report on behalf of the SVOM/ECLAIRs team Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260516D (SVOM burst-id sb260511616 – GCN 44649, trigger time T0 = 2026-05-16T20:39:50 UTC). The burst that triggered ECLAIRs shows a multiple peak light curve with most emission detected below 50 keV. The burst duration is estimated to be 152 s in the 4-20 keV energy band. The time-averaged spectrum from T0-78 s to T0+74 s in the energy range 5-50 keV is best fitted by a powerlaw model with alpha = -2.46 +0.32/-0.38. With this model, the 4-120 keV fluence is (5.3 +0.5/-2.4)e-7 erg/cm² and the 4-120 keV photon flux is 0.22 +0.1/-1.1 ph/cm²/s. We note that given the value of the photon index, the peak energy is likely to be below 5 keV, which classifies this GRB as an X-Ray Flash. By considering a Band model with alpha = -1, beta = -2.46 and a peak energy of 4 keV, a redshift of z=1.801 (GCN 44656) and a standard cosmology with H_0 = 67.3 km/s/Mpc and Omega_M = 0.315 (Planck Collaboration, 2014), we estimate the following rest-frame parameters: the isotropic energy E_iso is 8.9e51 erg, and the rest-frame peak energy of the time-averaged spectrum Ep,z is 11.2 keV. With these estimates, GRB 260516D lies below the track of Type-II GRBs on the Amati relation reported by Willingale et al. 2017. All the quoted errors are at the 68% confidence level. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44667. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42794] GRB 260515A: SVOM/ECLAIRs refined analysis
by GCN Circulars 19 May '26

19 May '26

TITLE: GCN CIRCULAR NUMBER: 44666 SUBJECT: GRB 260515A: SVOM/ECLAIRs refined analysis DATE: 26/05/19 14:14:04 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> M. Brunet, L. Bouchet (IRAP), A. Saccardi (CEA/ Irfu), report on behalf of the SVOM/ECLAIRs team Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260515A (SVOM burst-id sb26051504 – GCN 44622, trigger time T0 = 2026-05-15T19:08:19 UTC), which was also detected by Konus-Wind (GCN 44647). The burst that triggered ECLAIRs shows a FRED like shape light curve preceded by a faint hard precursor at T0-4.2s. The burst duration is T90 = 16.33 +0.54/-0.35 s in the 4-120 keV energy band. The time-averaged spectrum from T0-0.31 s to T0+16.0 s in the energy range 5-120 keV is best fitted by a powerlaw model with alpha = -1.69 +/-0.04. With this model, the 4-120 keV fluence is (1.09 -0.07/+0.04)e-6 erg/cm² and the 4-120 keV photon flux is 2.26 -0.06/+0.04 ph/cm²/s. All the quoted errors are at the 68% confidence level. The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44666. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42793] GRB 260421B: radio detection with the VLA
by GCN Circulars 19 May '26

19 May '26

TITLE: GCN CIRCULAR NUMBER: 44665 SUBJECT: GRB 260421B: radio detection with the VLA DATE: 26/05/19 13:17:22 GMT FROM: Stefano Giarratana at INAF-OAB <s.giarratana(a)ira.inaf.it> S. Giarratana (INAF-OAB), M. Giroletti (INAF-IRA), G. Ghirlanda (INAF-OAB), N. Di Lalla (Stanford Univ.), N. Omodei (Stanford Univ.), O. S. Salafia (INAF-OAB), L. Nava (INAF-OAB) At 01:30:46 UT on 2026 April 24 (T_mid = 2.92 days post-burst) and at 01:38:23 UT on 2026 April 30 (T_mid = 8.92 days post-burst) the Karl G. Jansky VLA observed the field of GRB 260421B (Fermi GBM team, GCN 44360, 44382; SVOM mission team, GCN 44362, GCN 44389, 44397; Insight-HXMT team, GCN 44401) at central frequencies of 6, 10 and 15 GHz. The standard 3C286 was used as bandpass and flux density calibrator, while J1239-1023 was used as phase calibrator. From a preliminary analysis, an unresolved radio source is detected at 8.92 days post-burst at a position (J2000): RA: 12:40:00.478 +- 0.002 Dec: -17:44:24.70 +- 0.03 consistent with the X-ray (SVOM mission team, GCN 44362; Einstein Probe team, GCN 44387) and optical (He et al., GCN 44363; Sánchez Álvarez et al., GCN 44364; SVOM mission team, GCN 44365, 44377; Wu et al., GCN 44366; Pérez-Fournon et al., GCN 44383; GRANDMA/Kilonova-Catcher collaboration, GCN 44402) position of the transient. The preliminary analysis yields the following results: ================================================================ T_mid Freq Peak r.m.s. Beam PA [days] [GHz] [uJy/b] [uJy/b] [arcsec^2] [deg] ================================================================ 2.92 6 - 8 0.72x0.28 -38 2.92 10 - 9 0.50x0.19 -43 2.92 15 - 12 0.41x0.11 -43 8.92 6 52 8 0.58x0.27 -33 8.92 10 81 9 0.40x0.21 -38 8.92 15 137 9 0.32x0.12 -38 ================================================================ We would like to thank the staff of the VLA for approving, executing, and processing the observations. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. These observations were carried out as part of project SF181027, approved in the framework of the Fermi - NRAO joint program agreement. View this GCN Circular online at https://gcn.nasa.gov/circulars/44665. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42792] GRID detection of GRB 260511A
by GCN Circulars 19 May '26

19 May '26

TITLE: GCN CIRCULAR NUMBER: 44664 SUBJECT: GRID detection of GRB 260511A DATE: 26/05/19 08:18:09 GMT FROM: GRID Student Team at Tsinghua University <grid(a)tsinghua.edu.cn> Longhao Li, Siqi Wu, Chenyu Wang, report on behalf of the GRID Collaboration: GRID-10B and GRID-11B reports the detection of the long-duration GRB 260511A, which was also detected by Fermi GBM (GCN Circular 44529), SVOM (GCN Circular 44532), NuSTAR(GCN Circular 44555), MASS-Cube(GCN Circular 44584), Konus-Wind (GCN Circular 44585), Glowbug(GCN Circular 44586). The event was triggered with GRID on 2026-05-11 at 06:13:30 UTC. The measured burst duration (T90) in the 30-2000 keV range is approximately T90 = 26.0 +/- 1.3 seconds. The GRID light curve of this event can be found at https://mirrors.tuna.tsinghua.edu.cn/GRID/data/GRID-GCN/GRB260511A/GRID_260…. GRID is a student-led project to monitor the transient gamma-ray sky with multiple detectors onboard different nanosatellites in the era of multi-messenger astronomy. For more information about GRID, please refer to the following references: https://doi.org/10.1007/s10686-019-09636-w and https://doi.org/10.1007/s10686-021-09819-4. View this GCN Circular online at https://gcn.nasa.gov/circulars/44664. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42791] GRB 260509A: CrAO ZTSh optical upper limit
by GCN Circulars 18 May '26

18 May '26

TITLE: GCN CIRCULAR NUMBER: 44663 SUBJECT: GRB 260509A: CrAO ZTSh optical upper limit DATE: 26/05/18 13:50:25 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin (SAO RAS), A. Pozanenko (IKI), V. Rumyantsev (CrAO), N. Pankov (IKI), A. Volnova (IKI) report on behalf of IKI GRB-FuN. We observed the field of the GRB 260509A (Götz et al., GCN 44504) with the Shajn 2.6-meter telescope (ZTSh) of the Crimean Astrophysical Observatory (CrAO). We obtained 40*120 s exposures R filter on 2026-05-15 19:45:59--21:09:48 UT (5.94218 days after trigger). We did not detect the OT (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506; Wu et al., GCN 44507; Sánchez Álvarez et al., GCN 44509; Zheng et al., GCN 44510; Kechin et al., GCN 44512; Belkin, GCN 44513; Corcoran et al., GCN 44514; Bochenek and Perley, GCN 44516; Moskvitin and Spiridonova, GCN 44563; Mo et al., GCN 44590; Li et al., GCN 44598) down to the 3-sigma limiting magnitude of R_lim = 24.2. This preliminary photometry is based on nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44663. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42790] GRB 260509A: CrAO ZTSh optical upper limit
by GCN Circulars 18 May '26

18 May '26

TITLE: GCN CIRCULAR NUMBER: 44662 SUBJECT: GRB 260509A: CrAO ZTSh optical upper limit DATE: 26/05/18 13:49:55 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin (SAO RAS), A. Pozanenko (IKI), V. Rumyantsev (CrAO), N. Pankov (IKI), A. Volnova (IKI) report on behalf of IKI GRB-FuN. We observed the field of the GRB 260509A (Götz et al., GCN 44504) with the Shajn 2.6-meter telescope (ZTSh) of the Crimean Astrophysical Observatory (CrAO). We obtained 40*120 s exposures R filter on 2026-05-15 19:45:59--21:09:48 UT (5.94218 days after trigger). We did not detect the OT (O'Neill et al., GCN 44505; Saccardi et al., GCN 44506; Wu et al., GCN 44507; Sánchez Álvarez et al., GCN 44509; Zheng et al., GCN 44510; Kechin et al., GCN 44512; Belkin, GCN 44513; Corcoran et al., GCN 44514; Bochenek and Perley, GCN 44516; Moskvitin and Spiridonova, GCN 44563; Mo et al., GCN 44590; Li et al., GCN 44598) down to the 3-sigma limiting magnitude of R_lim = 24.2. This preliminary photometry is based on nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44662. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42789] GRB 260516D: EP-FXT follow-up observation
by GCN Circulars 17 May '26

17 May '26

TITLE: GCN CIRCULAR NUMBER: 44661 SUBJECT: GRB 260516D: EP-FXT follow-up observation DATE: 26/05/17 10:45:05 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y.-H. I. Yin (HKU), Z.M. Wang (BNU), W.F. Wen (SZTU), Z.-X. Ling (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed an automatic follow-up observation of GRB 260516D detected by SVOM/ECLAIRs (SVOM/sb26051616, Brunet et al., GCN 44649). The follow-up obervation started at 2026-05-16 22:46:58 UTC, approximately 2.1 hours after the SVOM trigger, with a total exposure time of 6080 s. On-ground analysis of the FXT data found an uncatalogued fading source within the ECLAIRs error circle at R.A., Dec. = 233.8275, 6.5288 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). This position is consistent with the optical counterpart (Li et al., GCN 44654, Mohan et al., GCN 44655, Izzo et al., GCN 44656, García et al., GCN 44657, Wu et al., GCN 44659, Belkin et al., GCN 44660). The average FXT 0.3-10 keV spectrum can be fitted with an absorbed power-law model, with the Galactic hydrogen column density fixed at 4.71 x 10^20 cm^-2, an intrinsic hydrogen column density of 6.29 (-1.71/+1.71) x 10^21 cm^-2, and a photon index of 2.43 (-0.08/+0.08). The derived unabsorbed average flux in the 0.3-10 keV band is 3.99 (-0.14/+0.15) x 10^(-12) erg/s/cm^2. 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/44661. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42788] GRB 260516D: GOTO optical counterpart detection
by GCN Circulars 17 May '26

17 May '26

TITLE: GCN CIRCULAR NUMBER: 44660 SUBJECT: GRB 260516D: GOTO optical counterpart detection DATE: 26/05/17 08:55:51 GMT FROM: Sergey Belkin at Monash University <sergey.belkin(a)monash.edu> S. Belkin, B. P. Gompertz, M. Wortley, G. Ramsay, D. O'Neill, R. Starling, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, 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) in response to the SVOM/ECLAIRs detection of the possible short faint burst GRB 260516D (Brunet et al., GCN 44649). Observations of the field were obtained with GOTO-N on 2026-05-16 and 2026-05-17 UT in the GOTO L-band (400-700 nm). Images were processed immediately after acquisition using the GOTO pipeline (Lyman et al. 2026). Difference imaging was performed using deeper template observations of the same pointings. We detect the optical afterglow candidate reported by SVOM/VT (Li et al., GCN 44654), the GROWTH-India Telescope (Mohan et al., GCN 44655), GTC/OSIRIS+ (Izzo et al., GCN 44656), SVOM/COLIBRÍ (García García et al., GCN 44657), and LCO (Wu et al., GCN 44659). The source was initially detected with L = 19.29 +/- 0.05 AB mag at 2026-05-16 22:03:56 UT (T0 +1.40 hr), before fading to L = 21.01 +/- 0.34 AB mag at 2026-05-17 01:26:52 UT (T0 +4.78 hr). These two detections imply a preliminary temporal decay index of alpha_L = 1.30 +/- 0.26, consistent with the optical decay reported by García García et al. (GCN 44657). 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/44660. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42787] GRB 260516D: LCO optical observations
by GCN Circulars 17 May '26

17 May '26

TITLE: GCN CIRCULAR NUMBER: 44659 SUBJECT: GRB 260516D: LCO optical observations DATE: 26/05/17 08:22:30 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> C. Wu (NAOC), A. Saccardi (CEA/Irfu), L.P. Xin (NAOC) report on behalf of the SVOM mission team: We observed the field of the SVOM/ECLAIRs GRB 260516D (Brunet et al., GCN 44649) with the LCO 1m telescope at McDonald Observatory and Cerro Tololo equipped with the Sinistro instrument. Our observation started on 2026-05-17 at 06:26:56 UT (about 9.79 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters. The optical counterpart (Li et al., GCN 44654; Mohan et al., GCN 44655; García et al., GCN 44657) with redshift of 1.801 (Izzo et al., GCN 44656) is faintly detected in our images. We measure the following magnitudes calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction: r = 21.75 +/- 0.10 AB (mid-time 9.88 hr after the trigger); z > 21.2 AB (3-sigma, mid-time 10.00 hr after the trigger). This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/44659. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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