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[vsnet-grb-info 41984] EP/WXT trigger 01709258681 (EP260227a?): LCO candidate afterglow
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43867 SUBJECT: EP/WXT trigger 01709258681 (EP260227a?): LCO candidate afterglow DATE: 26/02/27 23:33:12 GMT FROM: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani(a)nbi.ku.dk> R. A. J. Eyles-Ferris (Leicester), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), J. N. D. van Dalen (Radboud), J. Chacón (PUC), report on behalf of a larger collaboration: We observed the field of the EP/WXT trigger 01709258681 (likely EP260227a) using the Sinistro instrument on the 1-m telescope in Sutherland (South Africa), part of the LCO network. Six 300 s observations were secured in the SDSS r band, with mean time 2026 Feb 27.945 UT (2.47 hr after the WXT trigger). Comparison of our images with archival templates from the Legacy Survey reveals a new object at coordinates (J2000): RA = 14:52:19.94 Dec = -11:24:23.9 Photometry yields r = 20.18 +/- 0.04 (AB) calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction. We suggest this is the optical counterpart of the EP/WXT trigger 01709258681. View this GCN Circular online at https://gcn.nasa.gov/circulars/43867. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41983] IceCube cascade alert 142200_1517143 retraction
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43866 SUBJECT: IceCube cascade alert 142200_1517143 retraction DATE: 26/02/27 20:54:47 GMT FROM: Erik Blaufuss at University of Maryland, College Park <blaufuss(a)umd.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: On 27 Feb 26 01:26:48 UT IceCube issued a cascade alert (https://gcn.gsfc.nasa.gov/notices_amon_icecube_cascade/142200_1517143.amon) reporting the detection of a neutrino candidate event with a high probability of being of astrophysical origin. Upon further examination by the IceCube collaboration, the event was found to be likely due to unintended light emission from detector equipment. At the time of this alert, portions of the detector hardware were found to unintentionally producing light in the instrumented volume. Normally, these periods of operation should be excluded from generating alerts. We apologize for any confusion this error may have caused. The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc(a)icecube.wisc.edu View this GCN Circular online at https://gcn.nasa.gov/circulars/43866. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41982] GRB 260226A: Insight-HXMT/HE detection of the very bright burst
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43865 SUBJECT: GRB 260226A: Insight-HXMT/HE detection of the very bright burst DATE: 26/02/27 16:11:28 GMT FROM: yzh807926(a)163.com Zheng-Hang Yu, Chen-Wei Wang and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team: At 2026-02-26T10:38:11.100 (T0), Insight-HXMT/HE detected the long bright burst GRB 260226A, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #43840), Fermi/LAT(D. Depalo et al., GCN #43844) , AstroSat-CZTI (Harsha et al., GCN #43846), Glowbug (Woolf et al., GCN #43855), CALET-GBM (Kobayashi et al., GCN #43860), and Konus-Wind(D. Svinkin et al., GCN #43864) The Insight-HXMT/HE light curve mainly consists of two eposides with a T90 of about 76 s. The 1s peak rate, measured from T0+1.5 s, is 15858 cnts/sec. Insight-HXMT/HE detected a total of 264251 counts form this burst. However, the HE detector was suffered from saturation during the brightest peak due to the high-brightness. The HXMT/HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260226A.png All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 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/43865. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41981] Konus-Wind detection of GRB 260226A
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43864 SUBJECT: Konus-Wind detection of GRB 260226A DATE: 26/02/27 13:52:56 GMT FROM: Dmitry Svinkin at Ioffe Institute <svinkin(a)mail.ioffe.ru> D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, A. Tsvetkova, M. Ulanov, and T. Cline on behalf of the Konus-Wind team, report: The bright, long-duration GRB 260226A (Fermi-GBM detection: The Fermi GBM team, GCN 43840; Bissaldi eat al., GCN 43851; BALROG localization: Preis and Greiner, GCN 43843; Fermi-LAT detection: Depalo et al., GCN 43844, 43850; AstroSat-CZTI detection: Harsha et al., GCN 43846; NuSTAR-ACS detection: Waratkar et al., GCN 43854; Glowbug detection: Woolf et al., GCN 43855; CALET-GBM detection: Kobayashi et al., GCN 43860) triggered Konus-Wind at T0=38287.440 s UT (10:38:07.440). The burst light curve shows a very bright, multi-peaked emission pulse which starts at ~T0-9 s and has a duration of ~30 s. This pulse is followed by a weaker, smoothly decaying emission tail, visible up to the end of the KW triggered data record (~T0+250 s). The emission is seen up to ~20 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260226_T38287/ As observed by Konus-Wind, the burst had the total fluence of 7.27(-0.17,+0.17)x10^-4 erg/cm2, and the 64-ms peak flux, measured from T0+10.688 s, of 1.33(-0.06,+0.06)x10^-4 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+142.080 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 = -0.93(-0.02,+0.03), the high energy photon index beta = -2.33(-0.07,+0.06), the peak energy Ep = 613(-25,+26) keV (chi2 = 118/97 dof). The spectrum near the maximum count rate (measured from T0+10.240 to T0+10.752 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 = -0.64(-0.06,+0.07), the high energy photon index beta = -2.11(-0.08,+0.07), the peak energy Ep = 772(-77,+80) keV (chi2 = 48/57 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/43864. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41980] GRB 260223A: 7DT detection and Medium-band SED of Afterglow
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43863 SUBJECT: GRB 260223A: 7DT detection and Medium-band SED of Afterglow DATE: 26/02/27 12:05:22 GMT FROM: YoungPyo Hong at Seoul National University <evan77333(a)gmail.com> YoungPyo Hong (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Donggeun Tak (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Donghwan Hyun (SNU ARC/SNU), Gregory S.H. Paek (IfA, SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), Ji Hoon Kim (SNU ARC/SNU), and Won-Hyeong Lee (SNU ARC/SNU), report on behalf of the 7 Dimensional Telescope (7DT) team: We report optical follow-up observations of GRB26023A (GCN 43808; GCN 43809), conducted with the 7-Dimensional Telescope (7DT), located in Chile. The 7DT follow-up began at 04:44:14 UTC on 2026-02-23, corresponding to T0 + 33 minutes, pointing to a region covering the target localization provided by Fermi (GCN 43808) at RA, DEC = 110.5 deg, -29.4 deg with an uncertainty of 1.7 degree. Initial observations were made with 15, units in 19 medium-band filters as well as r-band filters (20 filters in total). The following table summarizes the observations and the derived 5-sigma upper limits: Filter Mag Mag_err Exposure Date Time ---------------------------------------------------------------------------------------------------- r 19.05 0.08 300.0 2026-02-23T04:49:44.000 m400 >19.02 N/A 300.0 2026-02-23T04:44:14.000 m425 19.28 0.16 300.0 2026-02-23T04:44:31.667 m450 19.48 0.24 300.0 2026-02-23T04:44:16.667 m475 18.97 0.12 300.0 2026-02-23T04:44:15.000 m500 19.62 0.27 300.0 2026-02-23T04:44:20.667 m525 19.48 0.24 300.0 2026-02-23T04:44:23.000 m550 >19.29 N/A 300.0 2026-02-23T04:49:38.000 m575 18.95 0.14 300.0 2026-02-23T04:51:58.000 m600 18.92 0.20 300.0 2026-02-23T04:49:39.333 m625 18.79 0.22 300.0 2026-02-23T04:49:37.000 m650 18.66 0.22 300.0 2026-02-23T04:44:38.000 m675 >18.95 N/A 300.0 2026-02-23T04:44:23.000 m700 18.13 0.18 300.0 2026-02-23T04:44:18.000 m725 >18.46 N/A 300.0 2026-02-23T04:44:23.000 m750 18.23 0.22 300.0 2026-02-23T04:44:20.667 m775 17.99 0.31 300.0 2026-02-23T04:44:43.333 m800 >17.52 N/A 300.0 2026-02-23T04:44:21.667 m825 >17.41 N/A 300.0 2026-02-23T04:44:21.333 m850 >17.05 N/A 300.0 2026-02-23T04:44:21.000 ---------------------------------------------------------------------------------------------------- * Upper limits (marked with '>') are 5-sigma limits The optical counterpart was detected in most filters. Photometric flux calibration was performed using synthetic photometry based on the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. Note that no extinction correction has been applied. The 7-Dimensional Telescope (7DT), located in Chile and comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters, aims to detect optical counterparts of GW sources and conduct the 7-Dimensional Sky Survey (7DS) of the Southern Hemisphere. Further information about the 7DT is available at https://7ds.snu.ac.kr/ and http://gwuniverse.snu.ac.kr/. View this GCN Circular online at https://gcn.nasa.gov/circulars/43863. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41979] EP260214a: VLT/FORS2 optical observations
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43862 SUBJECT: EP260214a: VLT/FORS2 optical observations DATE: 26/02/27 09:53:25 GMT FROM: N. Passaleva at Sapienza University of Rome <niccolo.passaleva(a)uniroma1.it> N. Passaleva, H. Waseem, Yu-Han Yang, and Eleonora Troja (U Rome) report on behalf of the ERC BHianca team: We observed the field of EP260214a (Li et al., GCN 43742) with the FORS2 imager on the ESO VLT UT1 (Antu). Observations began 11.3 days after the EP/WXT trigger and were carried out at an average airmass ~1.3 and with an average seeing ~0.45 arcsec in the R filter. At the position of the candidate radio counterpart reported by Ricci et al. (GCN 43845) we detect a faint source (r~26 AB mag). Assuming our observations are dominated by the host galaxy light, we compute a chance alignment of Pcc~1%. We thank the staff at the VLT for the rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/43862. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41978] GRB 260225A: WINTER J-band upper limit
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43861 SUBJECT: GRB 260225A: WINTER J-band upper limit DATE: 26/02/27 09:35:03 GMT FROM: Benjamin Schneider at MIT <bschn(a)mit.edu> Benjamin Schneider (LAM), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report: We observed the field of the SVOM GRB 260225A (Ma et al., GCN Circ. 43829) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2026-02-25 at 09:09:10 UT (17.92 min after the trigger) and consisted of 15 exposures of 120 s in the J-band. 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) and analysed with STDWeb/STDPipe (Karpov 2025). The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitude is not corrected for Galactic extinction. In the stacked image, the optical counterpart reported by Ducoin et al., GCN Circ. 43839 and Li et al., GCN Circ. 43838, consistent with the uncatalogued X-ray source detected by EP/FXT position (Yang et al., GCN 43839), is not detected down to the following 3-sigma AB magnitude: J > 19.5 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/43861. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41977] GRB 260225A: WINTER J-band upper limit
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43861 SUBJECT: GRB 260225A: WINTER J-band upper limit DATE: 26/02/27 09:35:03 GMT FROM: Benjamin Schneider at MIT <bschn(a)mit.edu> Benjamin Schneider (LAM), Geoffrey Mo (Caltech/Carnegie), Viraj Karambelkar (Columbia), Tomas Ahumada (NOIRLab), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report: We observed the field of the SVOM GRB 260225A (Ma et al., GCN Circ. 43829) in the near-infrared with the Palomar 1-m telescope, equipped with the 1-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on 2026-02-25 at 09:09:10 UT (17.92 min after the trigger) and consisted of 15 exposures of 120 s in the J-band. 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) and analysed with STDWeb/STDPipe (Karpov 2025). The photometric calibration was performed using nearby stars from the 2MASS catalog and the magnitude is not corrected for Galactic extinction. In the stacked image, the optical counterpart reported by Ducoin et al., GCN Circ. 43839 and Li et al., GCN Circ. 43838, consistent with the uncatalogued X-ray source detected by EP/FXT position (Yang et al., GCN 43839), is not detected down to the following 3-sigma AB magnitude: J > 19.5 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/43861. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41976] GRB 260226A: CALET Gamma-Ray Burst Monitor detection
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43860 SUBJECT: GRB 260226A: CALET Gamma-Ray Burst Monitor detection DATE: 26/02/27 08:55:16 GMT FROM: Yuta Kawakubo at Aoyama Gakuin University <kawakubo(a)phys.aoyama.ac.jp> K. Kobayashi (Waseda U), A. Yoshida, T. Sakamoto, S. Sugita, Y. Kawakubo (AGU), K. Yamaoka (Nagoya U), S. Nakahira (JAXA), Y. Asaoka (ICRR), S. Torii, Y. Akaike, Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC), M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena), and the CALET collaboration: The long GRB 260226A (Fermi GBM Final Real-time Localization: Fermi GBM team, GCN Circ. 43840; BALROG localization: Preis et al., GCN Circ 43843; Fermi-LAT detection: Depalo et al., GCN Circ 43844; AstroSat CZTI detection: Harsha et al., GCN Circ 43846; Fermi-LAT refined analysis: Depalo et al., GCN Circ 43850; Fermi GBM observation: Bissaldi et al., GCN Circ 43851; NuSTAR detection: Waratkar et al., GCN Circ 43854; Glowbug gamma-ray detection: Woolf et al., GCN Circ 43855) triggered the CALET Gamma-ray Burst Monitor (CGBM) at 10:38:04.46 UTC on 26 February 2026 (https://cgbm.calet.jp/cgbm_trigger/flight/1456137494/index.html) The burst signal was seen by all CGBM detectors. The burst light curve shows a multi-peaked structure that starts at T-3.9 sec, peaks at T+11.1 sec, and ends at T+37.0 sec. The T90 and T50 durations measured by the SGM data are 16.0 +/- 0.3 sec and 8.2 +/- 0.1 sec (40-1000 keV), respectively. The ground-processed light curve is available at https://cgbm.calet.jp/cgbm_trigger/ground/1456137494/ The CALET data used in this analysis are provided by the Waseda CALET Operation Center located at Waseda University. View this GCN Circular online at https://gcn.nasa.gov/circulars/43860. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 41975] GRB 260226A: COLIBRÍ optical upper limits
by GCN Circulars 27 Feb '26

27 Feb '26

TITLE: GCN CIRCULAR NUMBER: 43859 SUBJECT: GRB 260226A: COLIBRÍ optical upper limits DATE: 26/02/27 06:16:42 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo(a)astro.unam.mx> Camila Angulo (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Asuka Kuwata (UNAM), 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 260226A (Fermi GBM Team, GCN Circ. 43840, Fermi LAT, GCN Circ. 43844) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-02-27 02:54:20 to 03:52:25 UTC (from 16.27 to 17.25 hours after the trigger) and obtained 45 minutes of simultaneous exposures in the r and z filters. The data were reduced and coadded with the ASU pipeline and analysed with STDWeb/STDPipe (Karpov 2025). 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 images, comparing to LS (Dey et al. 2019), we do not detect any new source at the LAT source position (Depalo et al., GCN Circ. 43850) down to the following 5-sigma limits: r > 22.86 z > 21.99 These upper limits are consistent with the ones reported by Lipunov et al. (GCN Circ. 43847, 43848), Konno et al. (GCN Circ. 43852), Saikia et al. (GCN Circ. 43853), and Strausbaugh et al. (GCN Circ. 43858). We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams. 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/43859. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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