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[vsnet-grb-info 42284] EP260329b: J-band upper limits with WINTER
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44163 SUBJECT: EP260329b: J-band upper limits with WINTER DATE: 26/03/31 00:28:16 GMT FROM: Geoffrey Mo at Caltech / Carnegie Observatories <gmo(a)mit.edu> Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), 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 EP260329b (Song et al., GCN 44142, GCN 44155) 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 2026-03-30T04:09:30 UTC in the J band (~17.6 hr after the GRB trigger), consisting of 15 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 (Li et al., GCN 44139; Eyles-Ferris et al., GCN 44144; Magnani et al., GCN 44146; Aryan et al., GCN 44147; Lipunov et al., GCN 44150). We obtain the following 5-sigma upper limit: J = 19.3 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/44163. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42283] EP260329a: J-band upper limit with WINTER
by GCN Circulars 31 Mar '26

31 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44162 SUBJECT: EP260329a: J-band upper limit with WINTER DATE: 26/03/31 00:27:44 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 EP260329a (Yang et al., GCN 44141) 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 2026-03-29T11:11:03 UTC in the J band (~7.7 hr after the GRB trigger), consisting of 15 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 (Sankar et al., GCN 44145; He et al., GCN 44152; Yang et al., GCN 44153; Angulo et al., GCN 44154). We obtain the following 5-sigma upper limit: J = 18.0 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/44162. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42282] GRB 260330A: Glowbug gamma-ray detection
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44161 SUBJECT: GRB 260330A: Glowbug gamma-ray detection DATE: 26/03/30 21:27:51 GMT FROM: C.C. Cheung at Naval Research Lab <Teddy.Cheung(a)nrl.navy.mil> C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report: The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, confirms the detection of GRB 260330A, which was detected by Fermi/GBM (GCN 44151). The burst onset is at 2026-03-30 03:40:18.4 with a burst duration of ~16s. The lightcurve comprises an initial faint peak at ~T0+0.2s, followed by bright multi-peaked structures from ~T0+3s to +6s and ~T0+9s to +15.5s. The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS. Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS and operated until 2024 April when it was put in safe storage on orbit. Glowbug was removed from storage and resumed operation on 2025 September 12. [1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959 [2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O [3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108 Distribution Statement A: Approved for public release. Distribution is unlimited. View this GCN Circular online at https://gcn.nasa.gov/circulars/44161. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42281] GRB 260310A: VLA Multi-frequency Radio Observations
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44160 SUBJECT: GRB 260310A: VLA Multi-frequency Radio Observations DATE: 26/03/30 20:08:21 GMT FROM: Daniel Perley at Liverpool JMU <d.a.perley(a)ljmu.ac.uk> D. A. Perley (LJMU), G. Schroeder (Cornell), and T. Laskar (Utah) report on behalf of a larger collaboration: The Karl G. Jansky Very Large Array observed the location of the afterglow (Hinds et al., TNS AstroNote 2026-65; Konno et al., GCN 43974) associated with GRB 260310A (Fermi GBM Team, GCN 43951; Hamburg & Meegan, GCN 43975) using all available receivers from S-band to Q-band, providing nearly continuous spectral coverage from 2 to 50 GHz. Observations were carried out on UT 2026-03-27 between 07:45 and 11:06 UT, approximately 17.2 days after the GRB. Consistent with previous reports (Rhodes et al., GCN 44005; Giarratana et al., GCN 44045; Ho et al., GCN 44057; Ho et al., GCN 44134) we detect very strong radio emission from the afterglow. Preliminary flux densities are: | freq(GHz) | flux(mJy) | |-----------|----------------| | 3 | 2.90 +/- 0.04 | | 6 | 7.52 +/- 0.05 | | 10 | 11.11 +/- 0.11 | | 15 | 11.73 +/- 0.16 | | 33 | 9.58 +/- 0.39 | | 45 | 7.98 +/- 0.27 | This indicates that the afterglow has brightened at low frequencies since the observations of Giarratana et al. and Rhodes et al. at ~4 days post-GRB, and that the peak of the f_nu spectral energy distribution during our observations was at approximately 15 GHz. We thank the NRAO for rapidly approving our DDT request for a public multifrequency campaign on this exceptional GRB, and Heidi Medlin for assistance with checking the SBs. Further observations are planned. View this GCN Circular online at https://gcn.nasa.gov/circulars/44160. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42280] Fermi GBM Sub-Threshold Detection of GRB 260330B
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44159 SUBJECT: Fermi GBM Sub-Threshold Detection of GRB 260330B DATE: 26/03/30 19:48:13 GMT FROM: atrigg2(a)lsu.edu A.C. Trigg (NPP ORAU, NASA MSFC) reports on behalf of the Fermi-GBM Team: SVOM/ECLAIRs detected GRB 260330B on 2026-03-30 at 14:19:57 UTC (GCN 44157). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified no candidates. The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient approximately 16 s after the ECLAIRs best image SNR time of 2026-03-30T14:19:31. The Targeted Search candidate was found most significantly on the 32 s timescale using the "soft" spectral template (i.e., Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7) and has a false alarm rate of 1.0-04 Hz. The Fermi-MET of this transient is 796573176.806 s. The Targeted Search localization is spatially consistent with the ECLAIRs location. [1] Goldstein et al. 2019 arXiv:1903.12597 View this GCN Circular online at https://gcn.nasa.gov/circulars/44159. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42279] GRB 260330B: LCO optical upper limits
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44158 SUBJECT: GRB 260330B: LCO optical upper limits DATE: 26/03/30 18:58:32 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> D. Turpin (CEA/Irfu), A. Saccardi (CEA/Irfu), G. X. Lan (IRAP), S. Guillot (IRAP), report on behalf of the SVOM mission team: We observed the field of GRB 260330B detected by SVOM (Lan et al., GCN 44157) with the LCO 1m telescope at Siding Spring Observatory equipped with the Sinistro instrument. Our observation started on 2026-03-30 at 17:05:41 UT (about 2.76 hr after the trigger) and we obtained 3x200 s exposures in the SDSS r and 3x200 s exposures in the Pan-STARRS z filters. In the stacked image, we do not detect any new source within the SVOM/ECLAIRs error box. We measure the following upper limit calibrated against the Pan-STARRS DR1 catalog, and not corrected for Galactic extinction: r > 20.4 AB (5-sigma, mid-time 2.85 hr after the trigger); z > 19.2 AB (5-sigma, mid-time 2.86 hr after the trigger). This project is funded by the SVOM collaboration. View this GCN Circular online at https://gcn.nasa.gov/circulars/44158. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42278] GRB 260330B: SVOM detection of a burst
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44157 SUBJECT: GRB 260330B: SVOM detection of a burst DATE: 26/03/30 14:50:52 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> G. X. Lan, S. Guillot, H. Yang, O. Godet (IRAP), report on behalf of the SVOM mission team: At 2026-03-30T14:19:57 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260330B (SVOM burst-id sb26033003). 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 9 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 12.13 in the [5-20] keV energy band over a time window of 40.96 seconds starting at 2026-03-30T14:19:31. The localization of the best alert is R.A., Dec. 252.555, 36.862 degrees: R.A. (J2000) = 16h50m13s Dec. (J2000) = 36d51m44s with a 90% confidence level (C.L.) radius of 6.66 arcmin (including systematic error of 2 arcmin added in quadrature). The SVOM/ECLAIRs light curve showed multiple broad peak structures with a T90 duration of about 20.844 (-3.025 / +7.55) s. This burst was also detected by SVOM/GRM with a significance of 9.10. SVOM slewed to the 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 Guangxuan Lan: glan(a)irap.omp.eu. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44157. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42277] EP260324a / AT2026hir: g- and r-band optical follow-up with the PKU 60cm Telescope
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44156 SUBJECT: EP260324a / AT2026hir: g- and r-band optical follow-up with the PKU 60cm Telescope DATE: 26/03/30 13:44:34 GMT FROM: Y.C. Kang at Peking University <yckang(a)stu.pku.edu.cn> Yacheng Kang, Zexuan Wu, Qiang Wang, Ruize Shi, Xinmiao Zhao, Zhuokai Liu, Chenxi Bao, Yiming Dong, Ziming Wang, Fangzhou Ren, Lijing Shao, Zhuo Li, Subo Dong, Xian Chen (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group: We observed the field of EP260324a (Wu et al., ATel 17728; T0 = 2026-03-24T07:04:20) using the PKU 60cm telescope at Xinglong Observatory, NAOC. Observations began on 2026 Mar 27, corresponding to ~ 3.4 days after the EP-WXT detection, and spanned two epochs. In the stacked g- and r-band images, we detect the optical counterpart in the difference image at a position consistent with that of ZTF26aapviim / AT2026hir (Anumarlapudi et al., GCN 44120; Mohan et al., GCN 44128). Preliminary photometry is calibrated against nearby Pan-STARRS field stars and reported in the AB magnitude system, without correction for Galactic extinction. The observation log is summarized below: | Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (AB) | | :--------: | :---------: | :---------------: | :----: | :----------: | :------------: | | 2026-03-27 | 16:33:34.32 | 3.41 | g | 4 * 600 | 17.88 +/- 0.02 | | 2026-03-27 | 17:16:14.28 | 3.44 | r | 4 * 600 | 18.22 +/- 0.03 | Further analysis and follow-up observations are ongoing. The PKU 60cm Telescope is operated by the Department of Astronomy, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients. View this GCN Circular online at https://gcn.nasa.gov/circulars/44156. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42276] EP260329b: refined analysis of the EP-WXT and EP-FXT observations
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44155 SUBJECT: EP260329b: refined analysis of the EP-WXT and EP-FXT observations DATE: 26/03/30 13:25:23 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> The fast X-ray transient EP260329b triggered the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Song et al., GCN 44142) , and followed by several optical telescopes (Li et al., GCN 44139, Zhu et al., GCN 44140, Eyles-Ferris et al., GCN 44144, Magnani et al., GCN 44146, Aryan et al., GCN 44147, Lipunov et al., GCN 44150). The refined analysis of the WXT data shows that the event started at T0=2026-03-29T10:35:25 (UTC) and lasted for 35 s with a single pulse, before the observation was interrupted by the autonomous follow-up observation. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.42 +/- 0.65. The derived average unabsorbed 0.5-4 keV flux is 1.07 (-0.37/+0.56) x 10^(-9) erg/s/cm^2. The autonomous observation by the Follow-up X-ray Telescope (FXT) was performed at 2026-03-29T10:38:50.16 (UTC), about 3 minutes after T0. The exposure time of this observation is 4388 s. The on-ground analysis shows that an uncatalogued source was detected at R.A., Dec. = 181.3131, 4.3505 deg (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). The FXT detected a sharp pulse lasting about 200 s. About 400 s after the end of the pulse, the light curve started to rise again. The rising phase was slower than that of the initial pulse and lasted for roughly 800 s, until the observation was interrupted due to Earth occultation. The average FXT 0.5-10 keV spectrum can be fitted with an absorbed power law with a fixed Galactic equivalent hydrogen column density of 1.51 x 10^20 cm^-2 and a photon index of 1.91 +/-0.04. The derived average unabsorbed 0.5-10 keV flux is 1.54 +/-0.05 x 10^(-11) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 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/44155. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42275] EP260329a: COLIBRÍ optical upper limit
by GCN Circulars 30 Mar '26

30 Mar '26

TITLE: GCN CIRCULAR NUMBER: 44154 SUBJECT: EP260329a: COLIBRÍ optical upper limit DATE: 26/03/30 07:26:18 GMT FROM: Camila Angulo Valdez at UNAM <camiangulo(a)astro.unam.mx> Camila Angulo (UNAM), Stéphane Basa (UAR Pytheas), Rosa L. Becerra (UNAM), William H. Lee (UNAM), Alan M. Watson (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), 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), Noémie Globus (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), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report: We imaged the field of the EP260329a (Yang et al., GCN Circ. 44141) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-03-29 11:41:47 to 12:23:53 UTC (from 8.19 to 8.89 hours after the trigger, starting 1.3 hours after the notice) and obtained 29 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the 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 position of the two EP/FXT sources (assuming a 10″ error) reported by Yang et al. (GCN Circ. 44153), as well as at the position of the optical counterpart reported by NOT (He et al., GCN Circ. 44152), down to the following 3-sigma limit: r > 22.45 z > 21.06 Our non-detection, together with the FXT observation, is consistent with the candidate reported by NOT (He et al., GCN Circ. 44152) being a plausible optical counterpart of EP260329A. 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/44154. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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