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[vsnet-grb-info 42387] GRB 260411B: GOTO optical counterpart candidate
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44265 SUBJECT: GRB 260411B: GOTO optical counterpart candidate DATE: 26/04/11 10:56:59 GMT FROM: d.s.oneill(a)bham.ac.uk D. O'Neill, R. Starling, G. Ramsay, 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. Gompertz, 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 Fermi/GBM alert GRB 260411B (Fermi GBM team, GCN #44262). Observations covering the localisation area began at 2026-04-11 08:57:53 UT (t0 +0.89h). 8 images consisting of 4 x 90s exposures were taken in the L band (400-700 nm), covering 38.5 sq deg within the 90% localisation contour. ~32.3% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.4 mag. Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. A new optical source GOTO26dam is identified within the GBM 90% localisation region (22% contour) with co-ordinates: RA,DEC (J2000) = 85.567893, 1.984983, 05:42:16.29, +01:59:05.94 The source was detected with L = 17.55 ± 0.02 AB mag (+0.89h) and is rapidly fading with a decay index alpha ~ -2.1±0.4. We find no evidence of the source prior to the GRB trigger time in previous ATLAS observations at 2026-04-07 17:36:17.597 (t0 -86.47h) down to c > 19.37 mag using the forced photometry server (Shingles et al. 2021). 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/44265. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42386] EP260409a: Gemini/FLAMINGOS2 J-band counterpart
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44264 SUBJECT: EP260409a: Gemini/FLAMINGOS2 J-band counterpart DATE: 26/04/11 09:24:03 GMT FROM: P.G. Jonker at Radboud University <p.jonker(a)astro.ru.nl> J. Quirola-Vasquez (Radboud), J. Sanchez-Sierras (Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), F. E. Bauer (SSI and UTA), J. Chácon (PUC), B. Lemaux (Gemini) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260409a (Jiang et al. 2026, GCN 44242) using the FLAMINGOS2 instrument mounted on the Gemini-South telescope. Two J-band observations were obtained, one on 2026-04-09 06:52:40 (~0.27 d after the EP trigger, T0, 8x40 s exposures) and one on 2026-04-11 02:13:36 UT (i.e., ~2.08 d after T0, 16x40 s exposures). In our first epoch stacked images, we detect a candidate counterpart at coordinates (J2000): RA: 11:31:37.985 DEC: -9:20:43.00 Photometry yields J = 21.06 +/- 0.10 (Vega) calibrated against nearby stars from the 2MASS catalog (not corrected for Galactic extinction). During the second epoch, the target is no longer detected, with a 3sigma limit of J>22.65 (Vega), implying it faded significantly. No source is detected at this location in the Legacy Survey in any band, nor in archival J-, H-, and K-band images from the VISTA Hemisphere Survey. Given the lack of optical detection (Globus et al., GCN 44243; Li et al., GCN 44244) and from our Las Cumbres observations we report a 3 sigma limit of r>23.5 and z>21.5 AB mag (obtained at ~1.37 and 3.26 hours, respectively, after T0 with an exposure time of 6x300 s), this candidate counterpart is significantly red, implying EP260409a could be a high-redshift or high extinction Fast X-ray Transient. We thank the Gemini-South staff, for their rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44264. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42385] GRB 260411B: BALROG localization (Fermi Trigger 797587480 / GRB 260411337)
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44263 SUBJECT: GRB 260411B: BALROG localization (Fermi Trigger 797587480 / GRB 260411337) DATE: 26/04/11 08:50:14 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 797587480 at 08:04:35 on 11 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) = 85.7 deg Decl.(2000.0) = 3.1 deg The 1 sigma statistical error radius is 2.5 deg. We estimate an additional systematic error of 2 deg. Further details are available at: https://grb.mpe.mpg.de/grb/GRB260411337/ The Healpix map can be downloaded from: https://grb.mpe.mpg.de/grb/GRB260411337/healpix The location parameters are available as JSON at: https://grb.mpe.mpg.de/grb/GRB260411337/json View this GCN Circular online at https://gcn.nasa.gov/circulars/44263. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42384] GRB 260411B: Fermi GBM Final Real-time Localization
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44262 SUBJECT: GRB 260411B: Fermi GBM Final Real-time Localization DATE: 26/04/11 08:15:05 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 08:04:35 UT on 11 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260411B (trigger 797587480.306254 / 260411337). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 83.2, Dec = 2.8 (J2000 degrees, equivalent to J2000 05h 32m, 2d 47'), with a statistical uncertainty of 1.0 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/bn260411337/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260411337/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260411337/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44262. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42383] GRB 260411A: Fermi GBM Final Real-time Localization
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44261 SUBJECT: GRB 260411A: Fermi GBM Final Real-time Localization DATE: 26/04/11 06:46:58 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 SHORT GRB At 06:36:28 UT on 11 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260411A (trigger 797582193.127706 / 260411275). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 153.6, Dec = 12.0 (J2000 degrees, equivalent to J2000 10h 14m, 12d 00'), with a statistical uncertainty of 9.6 degrees. The angle from the Fermi LAT boresight is 32.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260411275/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260411275/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260411275/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44261. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42382] GRB 260410A: COLIBRÍ optical upper limit
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44260 SUBJECT: GRB 260410A: COLIBRÍ optical upper limit DATE: 26/04/11 06:40:10 GMT FROM: a.kuwata(a)irya.unam.mx Asuka Kuwata (UNAM), Damien Dornic (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the Fermi GRB 260410A (A. Holzmann Airasca et al., GCN Circ. 44253) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-11 03:50:46 to 04:47:10 UTC (from 20.7805 to 21.7371 hours after the trigger) and obtained 2460 seconds of simultaneous exposure in the r/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 do not detect any new source at the Fermi-LAT source position (A. Holzmann Airasca et al., GCN Circ. 44253) down to the following 5-sigma limit: r > 23.26 z > 21.83 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/44260. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42381] EP-WXT01709259790: Optical Upper limits with kinder observations
by GCN Circulars 11 Apr '26

11 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44259 SUBJECT: EP-WXT01709259790: Optical Upper limits with kinder observations DATE: 26/04/11 02:08:30 GMT FROM: Janet Chen at National Central University <janetstars(a)gmail.com> A. Aryan, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. Yang(HNAS), S. J. Smartt, J. Gillanders (both Oxford), A. Sankar.K, M.-H. Lee, A. Dutta, Y.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, W.-J. Hou, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, K. W. Smith, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report: We observed the field of the WXT trigger 01709259790 using the 40cm SLT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 12:10 UTC on 10th of April 2026 (MJD 61140.5068), 42 min after the EP-WXT detection. The EP-WXT transient was later identified as the star Gaia DR3 782359908221356544 during its flaring stage (Huang et al. GCN 44256). The star has an archival SDSS magnitude of r = 16.20. In our observations, we captured the stellar flare and the star varies from r = 15.561 +/- 0.042 in our first frame to r = 15.838 +/- 0.028 in the final (12th) image. The detailed photometric measurements using AutoPhOT are as follows: Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude| SLT | r | 61140.5068 | 0.70 | 300 * 1 | 15.561 +/- 0.042 SLT | r | 61140.5104 | 0.78 | 300 * 1 | 15.582 +/- 0.048 SLT | r | 61140.5141 | 0.87 | 300 * 1 | 15.614 +/- 0.049 SLT | r | 61140.5177 | 0.96 | 300 * 1 | 15.718 +/- 0.043 SLT | r | 61140.5215 | 1.05 | 300 * 1 | 15.740 +/- 0.045 SLT | r | 61140.5251 | 1.14 | 300 * 1 | 15.750 +/- 0.038 SLT | r | 61140.5289 | 1.23 | 300 * 1 | 15.762 +/- 0.038 SLT | r | 61140.5326 | 1.31 | 300 * 1 | 15.786 +/- 0.041 SLT | r | 61140.5363 | 1.40 | 300 * 1 | 15.793 +/- 0.043 SLT | r | 61140.5400 | 1.49 | 300 * 1 | 15.805 +/- 0.035 SLT | r | 61140.5437 | 1.58 | 300 * 1 | 15.817 +/- 0.033 SLT | r | 61140.5473 | 1.67 | 300 * 1 | 15.838 +/- 0.028 We also utilized our Kinder Pipeline for the combined 300 sec*12 frames, we obtained an r-band limit of >20.6 mag at a median airmass of 1.13 and an average seeing of 1".5. Our limit is consistent with He et al. (GCN 44254). The presented magnitudes are calibrated using the field stars from the PanSTARRS catalog. The reported upper limit is not corrected for an expected galactic extinction of A_r = 0.02 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69. View this GCN Circular online at https://gcn.nasa.gov/circulars/44259. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42380] GRB 260410B: SVOM/GRM detection of a short faint burst
by GCN Circulars 10 Apr '26

10 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44257 SUBJECT: GRB 260410B: SVOM/GRM detection of a short faint burst DATE: 26/04/10 15:59:54 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Chen-Wei Wang (IHEP), F. Piron (LUPM) and C. Lachaud (APC) report on behalf of the SVOM mission team: At 2026-04-10T12:22:27 UTC (T0), SVOM/GRM triggered on the gamma-ray burst GRB 260410B (SVOM burst-id sb26041001) on a timescale of 1 seconds and with a signal-to-noise ratio (SNR) of 5.90. This transient was also detected by Fermi/GBM (GCN Circulars #44255). At the time of this burst, SVOM/ECLAIRs was not taking data. With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that the short burst GRB 260410B consists a single pulse with T90 of 0.20 +0.12/-0.06 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260410B.png 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 Chenwei Wang: cwwang AT ihep.ac.cn. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44257. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42379] GRB 260410A: Fermi GBM Observation
by GCN Circulars 10 Apr '26

10 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44258 SUBJECT: GRB 260410A: Fermi GBM Observation DATE: 26/04/10 17:00:13 GMT FROM: rhamburg(a)usra.edu R. Hamburg (USRA), U. Pathak (IIT Bombay) and R. Sonawane (IISER TVM) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 07:03:57.00 UT on 10 April 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260410A (trigger 797497441/260410294), which was also detected by Fermi-LAT (Holzmann Airasca et al. 2026, GCN 44253) and SVOM/GRM (Wang et al. 2026, GCN 44257). The Fermi GBM real-time localization (GCN 44252) is consistent with the Fermi-LAT position. The angle from the Fermi LAT boresight is 49 degrees. The GBM light curve consists of a single bright emission episode with a duration (T90) of about 1 s (50-300 keV). The time-averaged spectrum from T0-0.07 to T0+0.30 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.17 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 960 +/- 30 keV. The event fluence (10-1000 keV) in this time interval is (1.33 +/- 0.01)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 12.0 +/- 1.0 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/44258. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42378] GRB 260410A: SVOM/GRM detection of a short faint burst
by GCN Circulars 10 Apr '26

10 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44257 SUBJECT: GRB 260410A: SVOM/GRM detection of a short faint burst DATE: 26/04/10 15:59:54 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> Chen-Wei Wang (IHEP), F. Piron (LUPM) and C. Lachaud (APC) report on behalf of the SVOM mission team: At 2026-04-10T12:22:27 UTC (T0), SVOM/GRM triggered on the gamma-ray burst GRB 260410A (SVOM burst-id sb26041001) on a timescale of 1 seconds and with a signal-to-noise ratio (SNR) of 5.90. This transient was also detected by Fermi/GBM and Fermi/LAT (GCN Circulars #44252 and #44253). At the time of this burst, SVOM/ECLAIRs was not taking data. The SVOM/GRM light curve shows a single narrow peak structure with a duration of about 0.5 s. SVOM did not slew to the burst. An exceptional ToO observation has been scheduled which started about 3 hours after the trigger time. 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 Chenwei Wang: cwwang AT ihep.ac.cn. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44257. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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