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[vsnet-grb-info 42510] GRB260421B: J band upper limit by SYSU 80cm infrared telescope
by GCN Circulars 22 Apr '26

22 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44388 SUBJECT: GRB260421B: J band upper limit by SYSU 80cm infrared telescope DATE: 26/04/22 07:24:57 GMT FROM: shiy228(a)mail2.sysu.edu.cn Yun Shi, Chun Chen, Duo-Le Cao, Zhong-Nan Dong, Rui-Chen Gao, Wei-Sen Huang, Jia-Qi Lin, Pu Lin, Jin-Ji Li, Hao-Nan Yang, Yan Yu, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team: We observed the field of the long GRB 260421B, detected by Fermi-GBM (Fermi GBM team, GCN 44360) and SVOM/ECLAIRs (Zhao et al., GCN 44362), and followed up by Einstein Probe/FXT (Wang et al., GCN 44387), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Early near-infrared follow-up observations have also been reported (Mo et al., GCN 44384). Our observations were carried out from 2026 Apr 21 15:25 UT to Apr 21 17:06 UT, corresponding to 11.2–12.9 hours after the GBM trigger (2026-04-21 04:13:21 UT), with a total exposure time of 5200 s in the J band, obtained under poor conditions. We do not detect any near-infrared counterpart at the position of the reported afterglow (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383; Wang et al., GCN 44387). The 5-sigma limiting magnitude is approximately J ~ 16.9 Vega magnitudes. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University. View this GCN Circular online at https://gcn.nasa.gov/circulars/44388. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42509] GRB 260421B: EP-FXT follow-up observation
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44387 SUBJECT: GRB 260421B: EP-FXT follow-up observation DATE: 26/04/22 02:42:41 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> B. T. Wang (YNAO, CAS), J. P. Chen (SYSU), D. Y. Li, and W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team: EP-FXT performed a follow-up observation of the SVOM/ECLAIRs-detected burst GRB 260421B (SVOM/sb26042105; Zhao et al., GCN 44362), also triggered by Fermi-GBM (Fermi GBM team, GCN 44360; Preis & Greiner, GCN 44361; Mukherjee et al., GCN 44382), starting at 2026-04-21T06:08:31UTC, approximately 1.9 hours after the SVOM trigger, with a total exposure time of 5994 s. One uncataloged source is detected within the ECLAIRs error circle, which is spatially consistent with the optical counterpart (He et al., GCN 44363; Sanchez-Alvarez, GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366, Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383) at a redshift of z=2.115 (Schneider et al., GCN 44368). Preliminary analysis on this source is automatically conducted, and the details are listed as follows. EPF_J124000.4-174426 RA (J2000): 190.0018 Dec (J2000): -17.74055 Flux: 1.26e-11 erg/s/cm2 (observed, 0.5-10 keV) Flux_err: 6.27e-13 erg/s/cm2 (1 sigma) 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/44387. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42508] The EP-WXT trigger 01709260362 is likely a flaring star
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44386 SUBJECT: The EP-WXT trigger 01709260362 is likely a flaring star DATE: 26/04/22 01:30:57 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> J. Yang (ZZU), J. P. Chen (SYSU), B. T. Wang (YNAO, CAS), R. D. Liang, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team: The EP-WXT trigger 01709260362 at the time of 2026-04-21T20:52:02, is likely a stellar flare associated with G 151-9. The estimated flux of the flare is around 2.8e-10 erg/s/cm^2 in 0.5-4.0 keV, corresponding to an X-ray luminosity of around 2.7e31 erg/s. 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/44386. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42507] GOTO26djh/AT2026jpw: ATCA radio detection
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44385 SUBJECT: GOTO26djh/AT2026jpw: ATCA radio detection DATE: 26/04/22 00:39:54 GMT FROM: James Leung at U. Toronto / HUJI <jamesk.leung(a)utoronto.ca> J. K. Leung (U. Toronto/HUJI) We observed the field of the transient GOTO26djh/AT2026jpw (O'Neill et al., GCN 44301) with the Australia Telescope Compact Array (ATCA). We conducted the observation on 2026 April 19 at 15:30 - 16:30 UT. In our preliminary analysis, we detect an unresolved radio source coincident with the position of the transient (Rhodes et al., GCN 44379). The unresolved radio source had a flux density of ~0.2mJy at 10 GHz and a self-absorbed spectrum at and around this frequency. We thank the CSIRO Space and Astronomy staff for supporting these observations. We acknowledge the Gomeroi people as the traditional owners of the Observatory site. The Australia Telescope Compact Array is part of the Australia Telescope National Facility (https://ror.org/05qajvd42) which is funded by the Australian Government for operation as a National Facility managed by CSIRO. View this GCN Circular online at https://gcn.nasa.gov/circulars/44385. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42506] GRB 260421B: J-band upper limit with WINTER
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44384 SUBJECT: GRB 260421B: J-band upper limit with WINTER DATE: 26/04/21 23:32:26 GMT FROM: Geoffrey Mo at Caltech / Carnegie Observatories <gmo(a)mit.edu> Geoffrey Mo (Caltech/Carnegie), Benjamin Schneider (LAM), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Viraj Karambelkar (Columbia), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report: We observed the field of GRB 260421B (Fermi GBM Team, GCN 44360; Preis et al., GCN 44361; Zhao et al., GCN 44362; Mukherjee et al., GCN 44382) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations began at 2026-04-21T04:36:00 UTC in the J band (~23 min after the GRB trigger) under poor conditions, consisting of 10 x 120 s exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) We do not detect a source at the optical counterpart location (He et al., GCN 44363; Álvarez et al., GCN 44364; Turpin et al., GCN 44365; Wu et al., GCN 44366; Lipunov et al., GCN 44367; Schneider et al., GCN 44368; Li et al., GCN 44377; Pérez-Fournon et al., GCN 44383). We obtain the following 5-sigma upper limit: J = 16.4 mag (AB). WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research. View this GCN Circular online at https://gcn.nasa.gov/circulars/44384. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42505] GRB 260421B: LCO optical counterpart detection
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44383 SUBJECT: GRB 260421B: LCO optical counterpart detection DATE: 26/04/21 23:18:19 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf(a)iac.es> I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), D. Cano-Morales, A.E. Hernández-Díaz, I. Correa-Plasencia, E. Lekaroz-Urriza, M. Quintana-Ansaldo (all ULL), A. López-Oramas (IAC and ULL), and D. Aguado (IAC and ULL) Following the detection of the long GRB 260421B by Fermi GBM (Fermi GBM team, GCN circ. 44360; Preis & Greiner, GCN circ. 44361; and Mukherjee, GCN circ. 44382) and by SVOM ECLAIRs, GBM, and MXT (Zhao et al., GCN circ. 44362), we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 200 sec in the SDSS r' filter, started on 2026-04-21 at 13:40:22 UT, about 9.45 hours after the Fermi trigger. The optical counterpart, first reported by He et al. (GCN circ. 44363), is detected in our image with an AB magnitude of 21.55 +/- 0.21, calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Other optical detections or upper limits have been reported by Sánchez Álvarez et al. (GCN circ. 44364), Turpin et al. (GCN circ. 44365), Wu et al. (GCN circ. 44366), Lipunov et al. (GCN circ. 44367), Schneider et al. (GCN circ. 44368, redshift of z = 2.115), and Li et al. (GCN circ. 44377). This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2026A-011, SGLF and Superluminous Supernovae surveys). This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5). View this GCN Circular online at https://gcn.nasa.gov/circulars/44383. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42504] GRB 260421B: Fermi GBM Observation
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44382 SUBJECT: GRB 260421B: Fermi GBM Observation DATE: 26/04/21 19:36:16 GMT FROM: oindabimukherjee(a)gmail.com O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 04:13:21.17 UT on 21 April 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421B (trigger 798437606/260421176), which was also detected by SVOM/ECLAIRs (Zhao et al. 2026, GCN 44362), and VLT/X-shooter with spectroscopic redshift z = 2.115 (Schneider et al. 2026, GCN 44368). The Fermi GBM on-ground location (GCN 44360) is consistent with the SVOM/ECLAIRs position. The angle from the Fermi LAT boresight is 58 degrees. The GBM light curve consists of a single emission episode with a duration (T90) of about 18 s (50-300 keV). The time-averaged spectrum from T0-1.6 to T0+24.9 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.11 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 159 +/- 1 keV. The event fluence (10-1000 keV) in this time interval is (1.06 +/- 0.01)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+15 s in the 10-1000 keV band is 15.4 +/- 0.3 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 130 +/- 1 keV, alpha = -1 +/- 0.005 and beta = -2.31 +/- 0.02. 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/44382. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42503] Fermi Gamma-ray Burst Monitor trigger 798482368/260421694 is not a GRB
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44381 SUBJECT: Fermi Gamma-ray Burst Monitor trigger 798482368/260421694 is not a GRB DATE: 26/04/21 19:24:18 GMT FROM: atrigg2(a)lsu.edu A. C. Trigg (NPP ORAU, NASA MSFC) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "The Fermi Gamma-ray Burst Monitor (GBM) trigger 798482368/260421694 at 16:39:23.09 UT on 21 April 2026, tentatively classified as a GRB, is in fact not due to a GRB. This trigger is likely due to local particles (SAA Exit)." View this GCN Circular online at https://gcn.nasa.gov/circulars/44381. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42502] GRB 260421A: AstroSat CZTI detection
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44380 SUBJECT: GRB 260421A: AstroSat CZTI detection DATE: 26/04/21 15:59:15 GMT FROM: Anuraag Arya at IIT Bombay <aryaanuraag910(a)gmail.com> M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (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 short GRB 260421A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 44358). The source was detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2026-04-21 02:12:43.91 UTC. The measured peak count rate associated with the burst is 251 (+66, -70) counts/s above the background in the combined data of all quadrants, with a total of 267 (+133, -138) counts. The local mean background count rate was 1340 (+8, -10) counts/s. Due to the intrinsic 1s binning of Veto data, we cannot reliably estimate T90 for this burst. 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 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/44380. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42501] GOTO26djh/AT2026jpw: radio detection at 15GHz
by GCN Circulars 21 Apr '26

21 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44379 SUBJECT: GOTO26djh/AT2026jpw: radio detection at 15GHz DATE: 26/04/21 15:44:07 GMT FROM: Lauren Rhodes at McGill <lauren.rhodes(a)mcgill.ca> Lauren Rhodes (McGill), Andrew Hughes, Rob Fender (Oxford), Dave Green, Dave Titterington (Cambridge) report: We observed the position of GOTO26djh/AT2026jpw (GCN 44301) with the Arcminute Microkelvin Imager Large-Array (AMI-LA) at 15.5 GHz beginning for three epochs starting 2026-04-16 22:18UTC; 2026-04-17 21:14UTC and 2026-04-18 21:38UTC, each epoch lasting 5, 6.5, and 7 hours, respectively. We detect an unresolved source, in each epoch with coordinates consistent with those of GOTO26djh/AT2026jpw. We measure a peak flux density of 0.28±0.07, 0.44±0.08, and 0.36±0.09mJy//beam in epochs 1, 2 and 3, respectively. We thank the staff at the Mullard Radio Astronomy Observatory for carrying out these observations and operating the AMI-LA. (edited) View this GCN Circular online at https://gcn.nasa.gov/circulars/44379. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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