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[vsnet-grb-info 42400] GRB 260310A / AT2026fgk: NuSTAR detection of the rebrightening
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44278 SUBJECT: GRB 260310A / AT2026fgk: NuSTAR detection of the rebrightening DATE: 26/04/12 20:47:14 GMT FROM: Gaurav Waratkar at Caltech <gauravwaratkar(a)iitb.ac.in> Gaurav Waratkar (Caltech), Elias Kammoun (Caltech), Rahul Jayaraman (Cornell), K-Ryan Hinds (Caltech), Anna Y. Q. Ho (Cornell) report: In response to our NuSTAR DDT request (PI Waratkar), NuSTAR observed GRB 260310A (Fermi GBM team, GCN 43951; Salunkhe et al., GCN 43958), associated with the afterglow AT2026fgk (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) beginning at 2026-04-11T13:45:07 UTC, about 32 days after the GRB. A total of 12.7 ks of observations were obtained from both the FPMA and FPMB modules. We clearly detect the X-ray afterglow at the location of AT2026fgk. We performed a preliminary spectral analysis in the 3–79 keV energy range, fitting an absorbed power law. We find a photon index of 1.64 ± 0.25 with an unabsorbed flux of (4.49 ± 0.67) x 10^-13 ergs/cm2/s (2-10 keV). All uncertainties are reported at the 90% CL. Our detection agrees with the deviation from the post-break afterglow decay reported in EP/FXT observations (Jayaraman et al., GCN 44234). We thank the entire NuSTAR Science & Mission operations teams for the rapid approval and execution of this DDT observation. NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington. View this GCN Circular online at https://gcn.nasa.gov/circulars/44278. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42399] GRB 260411B: Fermi-LAT detection
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44277 SUBJECT: GRB 260411B: Fermi-LAT detection DATE: 26/04/12 16:41:28 GMT FROM: Rahul Gupta at NASA GSFC <rahul.gupta(a)nasa.gov> F. Longo (Univ. and INFN Trieste), D. Depalo (Politecnico and INFN Bari), R. Gupta (NASA/GSFC), A. Holzmann Airasca (UniTrento and INFN Bari), N. Omodei (Stanford University), and N. Di Lalla (Stanford University) report on behalf of the Fermi-LAT team: At 08:04.35.31 UT on April 11th, 2026, Fermi-LAT detected high-energy emission from GRB 260411B, which was also detected by Fermi-GBM (trigger 797587480/260411337, GCN #44262 and #44266), and SVOM (Wang et al. 2026, GCN #44268). An optical counterpart candidate was detected by GOTO (O'Neill et al. 2026, GCN #44265), DOT (Gupta et al. 2026, GCN #44271), Ondrejov D50 (Jeiinek et al. 2026, GCN #44272), and COLIBRI' (Sánchez Álvarez et al. 2026, GCN #44274). The GBM location was initially inside the LAT field of view at an angle of 60 degrees to the LAT boresight, and remained in the LAT field of view until ~T0+500 s. No significant excess is seen using standard >100 MeV likelihood analysis procedures. Using the LAT Low Energy (LLE) data selection, over 190 counts above background were detected within a 20 s interval coinciding with the time of the GBM emission. This data selection has insufficient spatial resolution to provide a reliable LAT localization. Since an excess of events was not seen using the standard analysis selection, this detection is likely due to low-energy gamma-rays (below 100 MeV). The Fermi-LAT point of contact for this burst is Sara Cutini (sara.cutini(a)pg.infn.it) 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/44277. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42398] GRB 260412A: SVOM detection of a long burst
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44276 SUBJECT: GRB 260412A: SVOM detection of a long burst DATE: 26/04/12 06:57:17 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Ziqi Wang (GXU), Zhiyong Liu(GZNU), Hatsune GOTO(RIKEN), Wenjin XIE(NAOC), Ulysse Jacob (LUPM) report on behalf of the SVOM mission team: At 2026-04-12T06:13:46 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260412A (SVOM burst-id sb26041206), which is also detected by Fermi/GBM (Fermi GBM Team, GCN 44275). The following trigger information was received on the ground with low latency by the SVOM VHF Alert Network. The burst was detected both by the Count-Rate Trigger (CRT) and the Image Trigger (IMT), which produced a sequence of 67 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 42.64 in the [8-120] keV energy band over a time window of 81.92 seconds starting at 2026-04-12T06:13:35. The localization of the best alert is R.A., Dec. 79.9074, -44.5851 degrees: R.A. (J2000) = 5h19m37.78s Dec. (J2000) = -44d35m06.31s with a 90% confidence level (C.L.) radius of 2.69 arcmin (including systematic error of 2 arcmin added in quadrature). The SVOM/ECLAIRs preview VHF light curve showed multiple broad peak structures with a T90 duration of about 321.20 (-2.00 +1.89) s in the 5-120 keV energy band. Due to SVOM performing another ToO-EX during the trigger, no immediate slew was performed on this burst. No X-ray observation could be performed by SVOM/MXT for the time being.No optical observation could be performed by SVOM/VT for the time being. 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. SVOM/ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. SVOM/GRM was developed by the Institute of High Energy Physics (IHEP) of CAS. SVOM/MXT was developed jointly by CNES, CEA-IRFU, CNRS-IJCLab, University of Leicester, MPE. The Burst Advocate (BA) on shift for this alert is Ziqi Wang: ziqi.wang(a)st.gxu.edu.cn. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44276. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42397] GRB 260412A: Fermi GBM Final Real-time Localization
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44275 SUBJECT: GRB 260412A: Fermi GBM Final Real-time Localization DATE: 26/04/12 06:24:26 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 06:13:46 UT on 12 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260412A (trigger 797667231.169604 / 260412260). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 70.0, Dec = -47.3 (J2000 degrees, equivalent to J2000 04h 40m, -47d 17'), with a statistical uncertainty of 5.7 degrees. The angle from the Fermi LAT boresight is 31.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260412260/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260412260/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260412260/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44275. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42396] GRB 260411B: COLIBRÍ optical observations
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44274 SUBJECT: GRB 260411B: COLIBRÍ optical observations DATE: 26/04/12 05:49:16 GMT FROM: freddalvarez(a)astro.unam.mx Fredd Sánchez Álvarez (UNAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM), Rosa L. Becerra (UNAM), Noémie Globus (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM),Edilberto Aguilar-Ruiz (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), 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), Benjamin Schneider (LAM) report: We imaged the possible optical counterpart (O’Neill et al., GCN Circ. 44265) of the Fermi GRB 260411B (Fermi GBM Team, GCN Circ. 44262) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-12 02:56 to 2026-04-12 04:43 (from 18.86 to 20.68 hours after the trigger) and obtained 25/35/24/84 minutes of exposure in the g/r/i/z filters. The data were reduced, coadded, calibrated, and analyzed 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. In the stacked image, we detect the optical counterpart reported by O’Neill et al. (GCN Circ. 44265), Gupta et al. (GCN Circ. 44271), and Jelinek et al. (GCN Circ.44272) and obtain the following preliminary magnitudes: g = 21.44 +/- 0.04 r = 20.69 +/- 0.02 i = 20.35 +/- 0.03 z = 20.10 +/- 0.03 After correcting for Galactic extinction (A_g ≈ 0.92 and A_z ≈ 0.41), the g-z colour is 0.85 +/- 0.05. This and the other colours are consistent with an afterglow with moderate reddening from its host galaxy. Furthermore, the detection in g-band suggests that the redshift of the transient is likely z < 3.5, assuming typical IGM absorption. Comparing our r magnitude to the one reported by DOT (Gupta et al. GCN Circ. 44271), we estimate that the temporal decay index between 12.4 and 19.6 hours is α ≈ -2.3. The transient appears to show an initially rapid decay in the early GOTO observations (O’Neill et al. GCN Circ. 44265) at about 0.9 hours, followed by a shallower phase (α ≈ −0.8) between 0.9 and 12.4 hours, and a subsequent steepening at later times. In the Pan-STARRS DR1 images, there is an extended source at about 2” from the position of the transient with preliminary magnitudes of: g = 24.12 +/- 0.23 r = 24.06 +/- 0.31 i = 24.64 +/- 0.53 z = 23.44 +/- 0.38 After correcting for Galactic extinction, we estimate a probability of chance coincidence of about 2%, which suggests that this source is the host galaxy of GRB 260411B. We encourage spectroscopic observations of the source. Further observations are planned. 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/44274. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42395] GRB 260410B: Fermi GBM Observation
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44273 SUBJECT: GRB 260410B: Fermi GBM Observation DATE: 26/04/12 05:02:47 GMT FROM: rhamburg(a)usra.edu R. Hamburg (USRA) and U. Pathak (IIT Bombay) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 12:22:26.93 UT on 10 April 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260410B (trigger 797516551/260410516), which was also detected by SVOM/GRM (Wang et al. 2026, GCN 44257). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 4.22, Dec = 37.74 (J2000 degrees, equivalent to J2000 0h 16m, +37d 44'), with a statistical uncertainty of 5.61 degrees (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a mixture of two Gaussians, one with a radius of 1.8 degrees (52% contribution) and one with a radius of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]). The angle from the Fermi LAT boresight is 90 degrees. The GBM light curve single bright emission episode with a duration (T90) of about 0.19 s (50-300 keV). The time-averaged spectrum from T0-0.04 to T0+0.16 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.01 +/- 0.09 and the cutoff energy, parameterized as Epeak, is 1220 +/- 30 keV. The event fluence (10-1000 keV) in this time interval is (1.09 +/- 0.03)E-06 erg/cm^2. The 64-ms peak photon flux measured starting from T0+0.0 s in the 10-1000 keV band is 11.3 +/- 0.9 ph/s/cm^2. The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/" View this GCN Circular online at https://gcn.nasa.gov/circulars/44273. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42394] GRB 260411B: Ondrejov D50 optical afterglow detection
by GCN Circulars 12 Apr '26

12 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44272 SUBJECT: GRB 260411B: Ondrejov D50 optical afterglow detection DATE: 26/04/12 01:09:43 GMT FROM: Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek(a)asu.cas.cz> M. Jelinek, J. Strobl, F. Novotny, A. Malenakova, R. Hudec (ASU CAS Ondrejov) report: We observed the position of the optical afterglow of GRB 260411B (Fermi GBM team, GCN 44262; O'Neill et al., GCN 44265) with the D50 robotic telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. We performed a series of 80x60s exposures in the SDSS r' filter. We faintly detect the optical afterglow (O'Neill et al., GCN 44265) in the 80x60s combined image with exposure mean time 11.45 h after the trigger. The AB magnitude of the object is: r' = 20.0 +/- 0.3 Photometric calibration was performed using the ATLAS REFCAT2 (Tonry et al.) catalog. The magnitude is not corrected for Galactic extinction in the direction of the burst. Comparing with the GOTO detection at t = 0.89 h (L = 17.55 +/- 0.02; O'Neill et al., GCN 44265), this would imply a temporal decay index of alpha ~ 0.9 (where F_nu ~ t^-alpha). View this GCN Circular online at https://gcn.nasa.gov/circulars/44272. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42393] GRB260411B: 3.6m DOT optical observation
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44271 SUBJECT: GRB260411B: 3.6m DOT optical observation DATE: 26/04/11 20:40:36 GMT FROM: ANSHIKA GUPTA at ARIES <anshika05180(a)gmail.com> Anshika Gupta, Aayushi Verma, Pankaj Pawar, Debalina Kar, Dhruv Jain, and Kuntal Misra (ARIES) report: We observed the field of GRB260411B detected by Fermi/GBM (Fermi GBM Team, GCN 44262) with the 3.6m Devasthal Optical Telescope (DOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations began on 2026-04-11 at 20:29:45.480 UT, i.e., ~12.41 hours after the Fermi/GBM trigger. We have two frames with an exposure time of 200s in the r filter. We stacked the images after the alignment. We detect an optical counterpart in our stacked image at the position given by O'Neill et al. (GCN 44265). We obtain the following preliminary magnitude in the stacked image: Date Start_UT T_start-T0 (hours) Filter Exp time (s) Magnitude ================================================================== 2026-04-11 20:29:45.480 ~12.41 r 200s*2 19.74 +/-0.01 Our detection is consistent with O'Neill et al. (GCN 44265). The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the Pan-STARRS catalog. View this GCN Circular online at https://gcn.nasa.gov/circulars/44271. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42392] GRB 260411A: SVOM/GRM observation
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44270 SUBJECT: GRB 260411A: SVOM/GRM observation DATE: 26/04/11 17:35:00 GMT FROM: xueyuan zao <zaoxueyuan(a)gmail.com> SVOM/GRM team: Xue-Yuan Zao, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Frédéric Daigne (IAP) Report on behalf of the SVOM team: SVOM/GRM detected the burst GRB 260411A at 2026-04-11T06:36:28.200 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44261). With the event-by-event data downloaded through the X-band ground station, we conducted the standard analysis pipeline of GRB 260411A. The GRM light curve shows that this burst consists of multiple pulses with a T90 of 2.0 +3.2/-1.6 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here： https://www.bursthub.cn//admin/static/svgrb260411A.png In addition, the position of this burst, as determined by Fermi/GBM (RA= 153.6, DEC=12.0, GCN#44261), is located at about 69.8 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. 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: Xue-Yuan Zao (IHEP) (zaoxy(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/44270. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42391] GRB 260407B: SVOM/GRM detection of a possible magnetar X-ray burst
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44269 SUBJECT: GRB 260407B: SVOM/GRM detection of a possible magnetar X-ray burst DATE: 26/04/11 16:25:49 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Tais Maiolino (LUPM) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a bright short burst GRB 260407B (SVOM trigger reference: sb26040702) at 2026-04-07T10:58:55.900 UTC (T0). ECLAIRs was not collecting data at the time of this burst. With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a narrow spike with a T90 of 0.11 +0.06/-0.03 s in the 15-5000 keV band. All the signal is detected lower than 100 keV. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260407B.png We note that both the soft spectrum and the short duration suggest a magnetar X-ray burst. 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 point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) View this GCN Circular online at https://gcn.nasa.gov/circulars/44269. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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