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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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[vsnet-grb-info 42390] GRB 260411B: SVOM/GRM observation of a long burst
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44268 SUBJECT: GRB 260411B: SVOM/GRM observation of a long burst DATE: 26/04/11 16:24:34 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Chao Zheng, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Frédéric Daigne (IAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight GRB 260411B (SVOM trigger reference: sb26041103) at 2026-04-11T08:04:37.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN#44262). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of two emission episodes with a T90 of 16.2 +2.1/-1.4 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260411B.png In addition, the position of this burst, as determined by GOTO (RA= 85.567893 deg, DEC= 1.984983 deg, GCN#44265), is located at about 81 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-2.5 to T0+16.4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.81 +0.09/-0.09 and the cutoff energy, parameterized as Epeak, is 385 +44/-37 keV. The event fluence (10-1000 keV) in this time interval is (2.37 +0.09/-0.09)E-05 erg/cm^2. The 1s peak spectrum, measured from T0+6.7 to T0+7.7 s, is also best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.41 +0.09/-0.09 and the cutoff energy, parameterized as Epeak, is 331 +20/-17 keV. The flux (10-1000 keV) in this time interval is (6.34 +0.20/-0.20)E-06 erg/cm^2/s. The localization of GRB 260411B in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb260411B_amati.png And the localization in the 'Yonetoku' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb260411B_yonetoku.png With the brightness of the prompt emission, the predicted soft X-ray afterglow [1] is shown at: https://www.bursthub.cn//admin/static/svgrb260411B_xag_pre.png 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) [1] Chen-Wei Wang et al. ApJ 997 353 https://doi.org/10.3847/1538-4357/ae2eae View this GCN Circular online at https://gcn.nasa.gov/circulars/44268. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42389] GRB 260409A: Insight-HXMT/HE detection
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44267 SUBJECT: GRB 260409A: Insight-HXMT/HE detection DATE: 26/04/11 15:14:51 GMT FROM: zhangjinpeng(a)ihep.ac.cn Jin-Peng Zhang, Chen-Wei Wang, Chao Zheng, and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team: At 2026-04-09T17:32:46.000 (T0), Insight-HXMT/HE detected GRB 260409A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44249). The Insight-HXMT/HE light curve mainly consists of several pulses with a T90 of 43 +6/-3 s. The 1 s peak rate, measured from T0+8.5 s, is 919 cnts/sec. Insight-HXMT/HE detected a total of 11547 counts from this burst. The Insight-HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260409A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the low gain mode with the energy range of about 300-3000 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/44267. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42388] GRB 260411B: Fermi GBM Detection
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44266 SUBJECT: GRB 260411B: Fermi GBM Detection DATE: 26/04/11 13:46:30 GMT FROM: Utkarsh Pathak at IIT Bombay <utkarshpathak.07(a)gmail.com> U. Pathak (IITB) and R. Sonawane (IISER TVM) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 08:04:35.31 UT on 11 April 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260411B (trigger 797587480/260411337). The Fermi GBM final real-time Localization is reported in GCN 44262. An optical counterpart is also detected by GOTO (O'Neill et al. 2026 GCN 44265). The angle from the Fermi LAT boresight is 60 degrees. The GBM light curve consists of two spikes in a single emission episode, with a duration (T90) of about 33 s (50-300 keV). The time-averaged spectrum from T0-2.9 to T0+29.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.62 +/- 0.02 and the cutoff energy, parameterized as Epeak, is 348 +/- 4 keV. The event fluence (10-1000 keV) in this time interval is (2.31 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+8.3 s in the 10-1000 keV band is 21.8 +/- 0.4 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 293 +/- 3 keV, alpha = -0.54 +/- 0.05 and beta = -2.3 +/- 0.1. 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/44266. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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