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[vsnet-grb-info 42296] GRB 260401A: BALROG localization (Fermi Trigger 796733611 / GRB 260401454)
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44175 SUBJECT: GRB 260401A: BALROG localization (Fermi Trigger 796733611 / GRB 260401454) DATE: 26/04/01 12:11:04 GMT FROM: Jochen Greiner at MPE <jcgrog(a)mpe.mpg.de> T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report: The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger 796733611 at 10:53:26 on 01 April 2026 were automatically fitted for spectrum and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427; Berlato et al. 2019, ApJ 873, 60). The best-fit position is: RA(2000.0) = 220.7 deg Decl.(2000.0) = -16.9 deg The 1 sigma statistical error radius is 1.3 deg. We estimate an additional systematic error of 1 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB260401454/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB260401454/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB260401454/json View this GCN Circular online at https://gcn.nasa.gov/circulars/44175. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42295] GRB 260401A: Fermi GBM Final Real-time Localization
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44174 SUBJECT: GRB 260401A: Fermi GBM Final Real-time Localization DATE: 26/04/01 11:02:02 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 10:53:26 UT on 1 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260401A (trigger 796733611.002678 / 260401454). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 219.5, Dec = -17.4 (J2000 degrees, equivalent to J2000 14h 37m, -17d 23'), with a statistical uncertainty of 1.6 degrees. The angle from the Fermi LAT boresight is 60.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260401454/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260401454/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260401454/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44174. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42294] EP260329b: Mondy and Assy-Turgen optical upper limits
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44173 SUBJECT: EP260329b: Mondy and Assy-Turgen optical upper limits DATE: 26/04/01 09:25:28 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> A. Volnova (IKI), E. Klunko (ISTP), D. Anarbek (FAI), M. Krugov (FAI), A. Pozanenko (IKI), N. Pankov (IKI) report on behalf of IKI-GRB-FuN: We observed the field of the fast X-ray transient EP260329b (Song et al., GCN 44142; EP Team, GCN 44155) with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) taking several 40-second expositions in R-band, and with the AZT-20 telescope of Assy-Turgen observatory taking several 30-second exposures in r' filter on Mar. 29. In the stacked frames we do not detect the optical counterpart reported previously (Li et al., GCN 44139; Zhu et al., GCN 44140; Eyles-Ferris et al., GCN 44144; Aryan et al., GCN 44147; Lipunov et al., GCN 44150; Mo et al., GCN 44163; O'Neill et al., GCN 44169). Preliminary photometry and observational details are the following: Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope (mid,days) (n*s) (3sigma) 2026-03-29 15:24:19 0.22110 90*40 R n/d n/d 21.1 Mondy/AZT-33IK 2026-03-29 19:57:04 0.40444 80*30 r n/d n/d 21.6 Assy-Turgen/AZT-20 The photometry is based on several nearby stars from the SDSS-DR16 catalogue (Lupton transformations were used for the R-band values) and is not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44173. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42293] GRB 260310A / AT2026fgk: Luminance-band optical follow-up with a 28cm telescope in Daocheng
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44172 SUBJECT: GRB 260310A / AT2026fgk: Luminance-band optical follow-up with a 28cm telescope in Daocheng DATE: 26/04/01 07:07:05 GMT FROM: Y.C. Kang at Peking University <yckang(a)stu.pku.edu.cn> Zhuokai Liu, Zexuan Wu, Chenxi Bao, Yacheng Kang (PKU) report on behalf of the Interestar Collaboration and the PKU HiTF (High-energy Transients Follow-up) group: We observed the field of GRB 260310A, detected by Fermi/GBM (GCN 43951; T0 = 2026-03-10T04:57:10.81) and AstroSat/CZTI (Salunke et al., GCN 43958), using a 28cm optical telescope located in Daocheng, Sichuan, China. Observations began on 2026 Mar 17, approximately ~ 7.4 days after the GRB trigger, and spanned six epochs in the Luminance filter. In the stacked Luminance-band images, we detect the optical counterpart at a position consistent with AT2026fgk, the reported GRB afterglow candidate at z = 0.153 (O'Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65; Konno et al., GCN 43974; Hinds et al., GCN 43977), later found to show Type Ic-BL SN features (de Ugarte Postigo et al., GCN 44124; Guelfand et al., GCN 44125; O'Connor et al., GCN 44137). Preliminary photometry was calibrated using nearby catalogued field stars and is reported as approximate AB-equivalent magnitudes, without correction for Galactic extinction. Our measurements show that the optical counterpart continues to fade, from ~ 19.0 mag to ~ 19.8 mag between 7.4 and 14.3 days after the GRB trigger. The observation log is summarized below: | Date | Start_UT | T_mid - T0 (days) | Filter | Exposure (s) | Magnitude (AB) | | :--------: | :------: | :---------------: | :-------: | :----------: | :------------: | | 2026-03-17 | 13:28:40 | 7.40 | Luminance | 7400 | 19.02 +/- 0.04 | | 2026-03-18 | 13:52:51 | 8.42 | Luminance | 7600 | 19.10 +/- 0.04 | | 2026-03-19 | 14:14:50 | 9.42 | Luminance | 6000 | 19.18 +/- 0.04 | | 2026-03-20 | 12:37:02 | 10.36 | Luminance | 7400 | 19.39 +/- 0.04 | | 2026-03-21 | 12:39:41 | 11.35 | Luminance | 5000 | 19.65 +/- 0.05 | | 2026-03-24 | 12:33:31 | 14.33 | Luminance | 2200 | 19.84 +/- 0.08 | Further analysis and follow-up observations are ongoing. The 28cm optical telescope is located in Daocheng, Sichuan, China, and is operated by the Interestar Collaboration. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/44172. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42292] EP260321a :Xinglong optical observations
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44171 SUBJECT: EP260321a :Xinglong optical observations DATE: 26/04/01 05:05:57 GMT FROM: Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong(a)nao.cas.cn> Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Yuguang-Sun(NAOC), Pengliang-Du (NAOC), Jie-Zheng(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration: We performed optical observations of the field of a fast X-Ray transient EP260321a(Q.J. Huang et al., GCN 44068) in the g, r and i filter using Tsinghua-NAOC 0.8-m telescope (TNT) located at Xinglong, Hebei, China. A series of 300-s exposures were obtained on three epochs (March 25, 26, and 27, 2026), with the first observation at 2026-03-25T14:37:57, approximately 4 days after the EP FXT trigger (2026-03-21T12:30:18). We measure a preliminary magnitude calibrated against nearby stars from the Pan-STARRS catalog. We summarize our observation results as follows: Obs. No. | Time (UTC) | Exposure Time (s) | Filter | Apparent mag (AB) | Telescope Name 1 | 2026-03-25T14:37:57 | 300 s | g |17.70 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT) 2 | 2026-03-25T14:53:18 | 300 s | r |17.69 +/- 0.20| Tsinghua-NAOC 0.8-m telescope (TNT) 3 | 2026-03-25T15:08:39 | 300 s | i |17.42 +/- 0.25| Tsinghua-NAOC 0.8-m telescope (TNT) 4 | 2026-03-26T16:00:16 | 300 s | g |17.17 +/- 0.24| Tsinghua-NAOC 0.8-m telescope (TNT) 5 | 2026-03-26T16:15:37 | 300 s | r |17.30 +/- 0.23| Tsinghua-NAOC 0.8-m telescope (TNT) 6 | 2026-03-26T16:40:01 | 300 s | i |17.38 +/- 0.30| Tsinghua-NAOC 0.8-m telescope (TNT) 7 | 2026-03-27T13:25:21 | 300 s | g |16.61 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT) 8 | 2026-03-27T13:40:42 | 300 s | r |16.53 +/- 0.01| Tsinghua-NAOC 0.8-m telescope (TNT) 9 | 2026-03-27T13:56:03 | 300 s | i |16.43 +/- 0.02| Tsinghua-NAOC 0.8-m telescope (TNT) View this GCN Circular online at https://gcn.nasa.gov/circulars/44171. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42291] EP260327a: Xinglong optical upper limit
by GCN Circulars 01 Apr '26

01 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44170 SUBJECT: EP260327a: Xinglong optical upper limit DATE: 26/04/01 04:32:23 GMT FROM: Xinglong Observatory at National Astronomical Observatories (NAOC), Chinese Academy of Sciences (CAS) <xinglong(a)nao.cas.cn> Yu-Zhang(NAOC), Junjie-Jin(NAOC), Haiyang-Mu(NAOC), Junjun-Jia(NAOC), Jie-Zheng(NAOC), Zhou-Fan(NAOC), Hong-Wu(NAOC) report on behalf of a large collaboration: Following the detection of EP260327a by EP-WXT (C. Y. Dai, GCN 44126), we observed the field of EP260327a using the 2.16-m telescope at Xinglong Observatory, NAOC. We obtained 6x300s clear-band frames with a median time of 2026-03-27T14:06:25 i.e., 8.2 hr after the EP trigger. No uncatalogued optical transient is detected in the stacked images within the 20 arcsec EP/FXT error circle (C. Y. Dai, GCN 44126), down to 3-sigma limiting magnitudes of clear ~ 20.87 mag, calibrated with Pan-STARRS sources in the field. Also there is no apparent brightening for the catalogued sources within the error circle. View this GCN Circular online at https://gcn.nasa.gov/circulars/44170. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42290] EP260329b: GOTO detection of optical counterpart
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44169 SUBJECT: EP260329b: GOTO detection of optical counterpart DATE: 26/03/31 17:39:44 GMT FROM: d.s.oneill(a)bham.ac.uk D. O'Neill, R. Starling, A. Kumar, K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Gompertz, 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) in response to the EP/WXT alert EP260329b (Song et al; GCN 44142). We detect the counterpart candidate previously reported (Li et al., GCN 44139; Eyles-Ferris et al., GCN 44144; Magnani et al., GCN 44146; Aryan et al., GCN 44147; Lipunov et al., GCN 44150) with an L-band magnitude of 18.73 ± 0.09 AB mag (+0.74h) at 2026-03-29 11:20:38 UT. We find no evidence of the source prior to the GRB trigger time in previous GOTO observations taken at 2026-03-28 11:41:26 (-22.91h) down to a 3-sigma depth of L>20.10 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/44169. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42289] GRB 260330B: SVOM/VT optical upper limits
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44168 SUBJECT: GRB 260330B: SVOM/VT optical upper limits DATE: 26/03/31 15:25:54 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, J. R. Xu, 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. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA/Irfu), G. X. Lan, S. Guillot (IRAP) report on behalf of the SVOM/VT team. SVOM/VT performed observations with automatic slew to the field of GRB 260330B detected by SVOM/ECLAIRs and SVOM/GRM (sb26033003, Lan et al., GCN 44157). It was also a sub-threshold detection by Fermi/GBM (Trigg et al., GCN 44159). The observation started at 2026-03-30T15:06:18 UTC, i.e., about 46 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 (Lan et al., GCN 44157), compared to the Legacy Survey. The 5 sigma upper limits in AB magnitude are derived as follows: Mid time | Band | Exposure Time | 5 sigma upper limit 1.03 hour VT_B 38*50 sec > 22.7 mag 1.00 hour VT_R 34*50 sec > 22.5 mag Our photometry was in AB magnitude and was not corrected for Galactic extinction. This result is consistent with the nondetection report by LCO (Turpin et al., GCN 44158). More detailed analysis is 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/44168. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42288] Konus-Wind detection of GRB 260320A
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44167 SUBJECT: Konus-Wind detection of GRB 260320A DATE: 26/03/31 13:34:05 GMT FROM: Valentina Panteleeva at Ioffe Institute <panteleevav228(a)gmail.com> V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long-duration GRB 260320A (NuSTAR detection: Waratkar et al., GCN 44064, Insight-HXMT/HE detection: Luo et al., GCN 44077, Glowbug gamma-ray detection: Cheung et al., GCN 44093, GRID detection: Li et al., GCN 44109, IPN triangulation: Kozyrev et al., GCN 44121) triggered Konus-Wind at T0=70801.257 s UT (19:40:01.257). The burst light curve shows a single pulse which starts at ~T0-0.4 s and has a total duration of ~13.1 s. The emission is seen up to ~1 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260320_T70801/ As observed by Konus-Wind, the burst had the total fluence of 1.88(-0.20,+0.19)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+0.896 s, of 6.92(-0.78,+0.80)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+16.640 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.18(-0.10,+0.10), the high energy photon index beta = -2.44(-0.43,+0.18), the peak energy Ep = 267(-32,+45) keV (chi2 = 86/91 dof). The spectrum near the maximum count rate (measured from T0+0.256 to T0+8.448 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -1.33(-0.06,+0.06) and Ep = 359(-36,+44) keV (chi2 = 82/77 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.29 (chi2 = 82/76 dof). All the quoted errors are at the 68% confidence level. All the quoted values are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/44167. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42287] Konus-Wind detection of GRB 260316B
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44166 SUBJECT: Konus-Wind detection of GRB 260316B DATE: 26/03/31 13:32:31 GMT FROM: Valentina Panteleeva at Ioffe Institute <panteleevav228(a)gmail.com> V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long-duration GRB 260316B (Fermi-GBM detection: The Fermi GBM team, GCN 44031; AstroSat CZTI detection: Harsha et al., GCN 44035, SVOM/GRM detection: Wang et al., GCN 44039, GECAM-B detection: Wang et al., GCN 44040, CALET-GBM detection: Tamura et al., GCN 44100, IPN triangulation: Kozyrev et al., GCN 44111) triggered Konus-Wind at T0=61972.457 s UT (17:12:52.457). The burst light curve shows a single pulse which starts at ~T0-0.9 s and has a total duration of ~2.5 s. The emission is seen up to ~5 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260316_T61972/ As observed by Konus-Wind, the burst had the total fluence of 2.66(-0.45,+0.56)x10^-6 erg/cm2, and the 64-ms peak flux, measured from T0+0.230 s, of 2.87(-0.74,+0.85)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.24(-0.65,+0.76), the high energy photon index beta = -2.64(-7.36,+0.34), the peak energy Ep = 123(-22,+58) keV (chi2 = 64/77 dof). All the quoted errors are at the 68% confidence level. All the quoted values are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/44166. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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