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[vsnet-grb-info 42525] EP260321a: Upper limits from ATCA radio observations
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44403 SUBJECT: EP260321a: Upper limits from ATCA radio observations DATE: 26/04/23 17:14:05 GMT FROM: Nayana A J at University of California, Berkeley <nayan89deva(a)gmail.com> Nayana A.J., Eli Wiston, Raffaella Margutti, Itai Sfaradi, Ryan Chornock (UC Berkeley) report: We observed the field of EP260321a (Huang et al., GCN 44068) with the Australia Telescope Compact Array (ATCA) using the BIGCAT system. The observations were conducted on 2026 March 30.37 UT in the 15 mm band, utilizing a total instantaneous bandwidth of approximately 8 GHz, divided into four 2 GHz-wide sub-bands, each imaged independently. The resulting images have central observing frequencies of 17, 19, 21, and 23 GHz. In our preliminary analysis, we do not detect radio emission at the position of the optical counterpart (Lee et al., GCN 44070; Tanvir et al., GCN 44082). The corresponding 3-sigma flux density upper limits are 0.21, 0.15, 0.075, and 0.06 mJy at 17, 19, 21, and 23 GHz, respectively. We thank the ATCA operations staff for their prompt scheduling of these observations. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, 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/44403. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42524] GRB 260421B: Kilonova-Catcher optical afterglow detection
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44402 SUBJECT: GRB 260421B: Kilonova-Catcher optical afterglow detection DATE: 26/04/23 15:46:39 GMT FROM: Damien Turpin at CEA-Saclay <dturpin-astro(a)hotmail.com> M. Freeberg, R. Hellot, M. Serrau (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration: We observed the field of GRB 260421B detected by Fermi/GBM (Fermi GBM team, GCN 44360) and SVOM (Zhao et al., GCN 44362; Guo et al., GCN 44389) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with a CDK17 telescope located at AITP San Pedro Chile Observatory operated by R. Hellot and the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+1.9 hours and were taken with sdss-r filter. In our stacked frames, subtracted from the Legacy Survey DR10 template image, we marginally detect the optical counterpart (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; Mo et al., GCN 44384; and Shi et al., GCN 44388, Pawar et al., GCN 44396). We report our follow-up results in the table below: | Tmid-TGRB (hrs) | Exp (s) | Filter | Magnitude | Instrument | | ------------ | ------------ |------------ |------------ |------------ | | 2.48 | 20 x 180s | r (AB) | 21.12 +/- 0.30 | TEC160FL | | 3.87 | 9 x 300s | r (AB) | 20.94 (3 sig U.L.) | CDK17 | All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog. We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023). GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/) View this GCN Circular online at https://gcn.nasa.gov/circulars/44402. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42523] GRB 260421B: Insight-HXMT/HE detection
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44401 SUBJECT: GRB 260421B: Insight-HXMT/HE detection DATE: 26/04/23 14:06:52 GMT FROM: guohx(a)ihep.ac.cn Hao-Xuan Guo, Chen-Wei Wang, Cheng-Kui Li, Shao-Lin Xiong, and Chao Zheng (IHEP) report on behalf of the Insight-HXMT team: At 2026-04-21T04:13:23.400 (T0), Insight-HXMT/HE detected a long burst, GRB 260421B, which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44360), SVOM/ECLAIRs (D.Zhao et al., GCN #44362) and SVOM/GRM (Hao-Xuan Guo et al., GCN #44389). The Insight-HXMT/HE light curve mainly consists of multipe pulses with a total duration (T90) of 16.0 +3.0/-1.5 s. The 1s peak rate, measured from T0+12.75 s, is 3246 cnts/sec. Insight-HXMT/HE detected a total of 11625 counts from this burst. The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb260421B.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/44401. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42522] The EP-WXT trigger 01709258652: Mondy optical observations
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44400 SUBJECT: The EP-WXT trigger 01709258652: Mondy optical observations DATE: 26/04/23 13:31:19 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> M. Chutchikova (HSE), A. Volnova (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), N. Pankov (IKI) report on behalf of IKI-GRB-FuN: We observed the EP-WXT trigger 01709258652 (Yang et al., GCN 43849) with the AZT-33IK 1.5m telescope of the Mondy observatory starting on 2026-02-26 (UT) 21:07:17, i.e., ~8 hours after the trigger, and taking several 120-seconds frames in the R band. The photometry of suggested flare star LP 435-538 (High Proper Motion Star; Yang et al., GCN 43849; Wu et al., GCN 43857; Fang et al., GCN 43889) do no exhibit any significant variability within 1 hour of our observations. Preliminary photometry and observational details are the following: Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope (mid,days) (n*s) (3sigma) 2026-02-26 21:07:17 0.32452 30*120 R 14.10 0.01 23.2 Mondy/AZT-33IK The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction. Our measured R magnitude is consistent with the photometry of the USNO-B1.0 star 1099-00207147 (R2 = 14.16) corresponding by the coordinates to LP 435-538. View this GCN Circular online at https://gcn.nasa.gov/circulars/44400. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42521] GRB 250328A: VLT host galaxy discovery and redshift z = 1.250
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44399 SUBJECT: GRB 250328A: VLT host galaxy discovery and redshift z = 1.250 DATE: 26/04/23 12:50:54 GMT FROM: Nusrin Habeeb at University of Leicester <nh312(a)leicester.ac.uk> A. Saccardi (CEA/Irfu), B. Schneider (LAM), S. D. Vergani (LUX-Paris Obs.), D. B. Malesani (DAWN/NBI and Radboud), N. Habeeb (U. Leicester), R. Brivio (INAF/OAB), G. Corcoran (UCD), J.-L. Atteia (IRAP), report on behalf of the Stargate collaboration: We observed the location of the optical/NIR counterpart (Schneider et al., GCN 39919, 39920; Li et. al, GCN 39923; Li et. al, GCN 39936; Kumar et. al, GCN 39946; Klingler et. al, GCN 39968; Pankov et al., GCN 40001) of the SVOM GRB 250328A (Brunet et al., GCN 39910), using the ESO VLT UT1 (Antu) equipped with the FORS2 imager. A 20 min exposure R-band observation was carried out at 04:05:01 UT on 2026 April 19 (386.46 days after the SVOM trigger). Consistent with the afterglow position, an object is clearly identified with a magnitude r = 24.51 +/- 0.09, calibrated against the Pan-STARRS catalog and not corrected for Galactic extinction. Its coordinates (J2000) are: RA = 14:23:50.45 Dec = +25:55:55.09 Given the spatial coincidence with the GRB afterglow (<0.5"), this is the likely host galaxy of GRB 250328A. Spectroscopic follow-up observations of this object were secured using the ESO VLT UT3 (Melipal) equipped with the X-shooter spectrograph, covering the wavelength range 3000-21000 AA. We obtained four spectra of 1200 s each starting at 03:28:04 on 2026 April 23, i.e. about 390.44 days after the SVOM trigger. Faint emission lines are detected in the visible and near-infrared arms, which we identify as the [O III] 4959,5007 doublet, H-beta, and the [O II] 3726,3729 doublet, all at a common redshift of z = 1.250. This is thus the redshift of the likely host galaxy of GRB 250328A. We acknowledge excellent support from the ESO staff in Paranal, in particular Boris Haeussler, Diego Parraguez, Felipe Gaete, Hannah Osborne, Luca Sbordone, Matias Jones, and Thomas Rivinius. View this GCN Circular online at https://gcn.nasa.gov/circulars/44399. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42520] EP260416a: Mondy optical observations
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44398 SUBJECT: EP260416a: Mondy optical observations DATE: 26/04/23 12:39:12 GMT FROM: Alina Volnova at IKI RAS <alinusss(a)gmail.com> A. Volnova (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), N. Pankov (IKI) report on behalf of IKI-GRB-FuN: We observed the field of the fast X-ray transient EP260416a (Hu et al., GCN 44307, 44324) at z = 1.909 (He et al., GCN 44336; An et al., GCN 44339) with the AZT-33IK 1.5m telescope of the Mondy observatory starting on 2026-04-21 (UT) 15:28:28 and taking several 120-seconds frames in the R band. The optical counterpart (Ducoin et al., GCN 44310; Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Zheng et al., GCN 44321; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334; He et al., GCN 44336; An et al., GCN 44339; Jelinek et al., GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354; Pawar et al., GCN 44373; Li et al., GCN 44376) is clearly detected in the stacked frame. Preliminary photometry and observational details are the following: Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope (mid,days) (n*s) (3sigma) 2026-04-21 15:28:28 5.43677 27*120 R 22.72 0.09 23.3 Mondy/AZT-33IK The photometry is based on nearby USNO-B1.0 stars (R2-magnitudes) and is not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44398. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42519] GRB 260421A: SVOM/ECLAIRs refined analysis
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44397 SUBJECT: GRB 260421A: SVOM/ECLAIRs refined analysis DATE: 26/04/23 10:06:59 GMT FROM: SVOM_group <svomgroup(a)bao.ac.cn> M. Brunet (IRAP), L. Delfosse (CEA), O. Godet (IRAP), S. Schanne (CEA), W. L. Zhang (PMO), Z. Y. Liu (GZNU) report on behalf of the SVOM/ECLAIRs team Using the event-by-event data downloaded through the X-band ground station, we report further analysis of SVOM/ECLAIRs observations of GRB 260421A (SVOM burst-id sb26042105 – GCN 44362, trigger time T0 = 2026-04-21T04:13:25 UTC), which was also detected by Fermi (GCN 44360, 44361, 44382) and SVOM/GRM (GCN 44389). The burst that triggered ECLAIRs onboard shows a multiple peak lightcurve. The burst duration is T90 = 26.3 +4.4/-6.0 s in the 4-120 keV energy band. The time-averaged spectrum from T0-12 s to T0+20 s in the energy range 5-120 keV is best fitted (chi²/dof = 20.1/16) by a broken powerlaw model with best-fit parameters: alpha = -1.00 +0.35/-0.11, beta = -1.42 +0.07/-0.06 and a break energy of 13.7 +2.7/-4.4 keV. With this model, the 4-120 keV fluence is (8.6 +0.1/-1.1)e-6 erg/cm^2 and the 4-120 keV photon flux is 6.4 +0.1/-0.7 ph/cm²/s. Using a simple powerlaw provides a bad fit (chi²/dof = 31.3/18). 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. ECLAIRs was developed jointly by CNES, CEA-IRFU, CNRS-IRAP, CNRS-APC. The SVOM/ECLAIRs point of contact for this burst is: Marius Brunet (IRAP) (marius.brunet at utoulouse.fr) View this GCN Circular online at https://gcn.nasa.gov/circulars/44397. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42518] GRB 260421B: 3.6m DOT optical observation
by GCN Circulars 23 Apr '26

23 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44396 SUBJECT: GRB 260421B: 3.6m DOT optical observation DATE: 26/04/23 10:00:19 GMT FROM: pawarpankaj1492(a)gmail.com Pankaj Pawar, Anshika Gupta, Debalina Kar, Dhruv Jain and Kuntal Misra (ARIES) report: We observed the field of GRB 260421B (Fermi GBM team, GCN 44360; Zhao et al., GCN 44362) using the 3.6m Devasthal Optical Telescope (DOT) located at Devasthal of the Aryabhatta Research Institute of observational sciencES (ARIES), India. The observations began on 2026-04-21 at 21:24:53 UT, approximately 17 hours after the trigger time. We obtained 2*300s frames in the SDSS r filter. We do not detect the optical counterpart (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; Mo et al., GCN 44384; and Shi et al., GCN 44388) within the error circle, nor do we see a source at the reported location of the transient. The limiting magnitude in the stacked image is: r > 22.4 mag (AB) The magnitude is calibrated against the Pan-STARRS PS1 catalog. We thank the staff of Devasthal for their assistance during these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44396. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42517] EP260416a: ZTSh R band observations
by GCN Circulars 22 Apr '26

22 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44395 SUBJECT: EP260416a: ZTSh R band observations DATE: 26/04/22 23:58:50 GMT FROM: Alexander Moskvitin at SAO RAS <mosk(a)sao.ru> A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO), A. Volnova, N. Pankov, A. Pozanenko (IKI), Tao An, Yuanqi Liu (SHAO), Ankur Ghosh, Soebur Razzaque (U. of Johannesburg, RSA) report on behalf of GRB follow-up collaboration. We observed the field of EP260416a (Hu et al., GCN 44307; GCN 44324) with 2.6-m ZTSh of CrAO equipped with BVRI photometer and FLI PL4240 camera. We obtained a series of images in R band on 2026-04-22, 21:11:50--22:14:42 UT (t_mid - T0 = 6.6762 days). The optical transient discovered by COLIBRÍ (Ducoin et al., GCN 44310; and also observed by Li et al., GCN 44315; Hua et al., GCN 44318; Lee et al., GCN 44319; Martin-Carrillo et al., GCN 44328; Li et al., GCN 44334; He et al., GCN 44336; An et al., GCN 44339; Jelinek et al., GCN 44341; Guziy et al., GCN 44351; Ho et al., GCN 44354) is clearly detected in the stacked image with the following brightness. Date, UT_start, t_mid - T0, filter, exp, mag +/- err UL days s 2026-04-22T21:11:50 6.67620 R 30*120 23.12 +/- 0.17 24.1 This preliminary photometry is based on the nearby USNO-B1 stars and not corrected for the Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44395. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42516] EP-WXT trigger 01709260362: FRAM-ORM optical observations confirm flaring star G 151-9
by GCN Circulars 22 Apr '26

22 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44394 SUBJECT: EP-WXT trigger 01709260362: FRAM-ORM optical observations confirm flaring star G 151-9 DATE: 26/04/22 18:53:47 GMT FROM: Martin Jelinek at Astro.Inst-AVCR,Ondrejov <martin.jelinek(a)asu.cas.cz> Martin Jelinek, Jan Strobl and Filip Novotný (ASU CAS Ondrejov, CZ), Sergey Karpov, Martin Masek, Petr Janecek, Jakub Jurysek, Jan Ebr, Ronan Cunniffe, Petr Travnicek and Michael Prouza (Institute of Physics, Prague, CZ) report: The 25cm robotic telescope FRAM-ORM at La Palma (Spain) reacted robotically to the EP-WXT alert of trigger 01709260362 (Yang et al., GCN 44386), obtaining a series of 60 s unfiltered images starting at 21:24:16.6 UT on 2026-04-21, i.e. ~32.2 min post trigger (T0 = 2026-04-21T20:52:02.749 UTC, JD 2461152.36948). We clearly detect the flaring star G 151-9 identified by the EP team (Yang et al., GCN 44386). The source fades by approximately 0.25 mag over the course of our observations, which span from T0+32.2 min to T0+2h 13.6 min. The overall decay is consistent with a power-law of index alpha = 0.67 +/- 0.16, superimposed on the quiescent brightness of the star at an unfiltered magnitude of 12.12 +/- 0.07. This is consistent with the stellar flare interpretation. View this GCN Circular online at https://gcn.nasa.gov/circulars/44394. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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