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[vsnet-grb-info 42556] EP260429a: LCO optical upper limits
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44433 SUBJECT: EP260429a: LCO optical upper limits DATE: 26/04/30 11:20:40 GMT FROM: Rob Eyles-Ferris at U of Leicester <raje1(a)leicester.ac.uk> R. A. J. Eyles-Ferris (Leicester), J. A. Chácon (PUC), F. E. Bauer (SSI and UTA), J. Quirola-Vasquez (Radboud), D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP26029a (Wang et al., GCN 44426) using LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) equipped with the SINISTRO instrument. A series of 6x300 s exposures were taken in each of the SDSS-r and SDSS-z filters, starting at 2026-04-30 08:23:54 UT (~18.0 hr after the trigger). No new source is detected within or at the border of the EP/WXT uncertainty region (Wang et al., GCN 44426) nor do we identify any significant residuals in subtractions using PanSTARRS DR1 templates. From the stacked images, we obtain the following 3-sigma upper limits:  r > 22.5 z > 20.9 These upper limits are in AB magnitudes, calibrated using nearby stars from the Pan-STARRS catalogue, and are not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44433. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42555] EP260430a: LCO optical upper limits
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44432 SUBJECT: EP260430a: LCO optical upper limits DATE: 26/04/30 10:50:21 GMT FROM: Jonathan Quirola at Radboud University <jaquirola1990(a)gmail.com> J. A. Chácon (PUC), F. E. Bauer (SSI and UTA), D. B. Malesani (DAWN/NBI and Radboud), J. Quirola-Vasquez (Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), R. A. J. Eyles-Ferris (Leicester), report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260430a (Wang et al., GCN 44427) using LCO 1m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) equipped with the SINISTRO instrument. A series of 6x300 s exposures was taken using the SDSS-r filter, starting at 2026-04-30 03:32:35 UT (~1 hr after the trigger). No new source is detected within or at the border of the EP/FXT uncertainty region (Wang et al., GCN 44427). From the stacked images, we obtain the following 3-sigma upper limit:  r > 22.1 This upper limit is in AB magnitudes, calibrated using nearby stars from the Pan-STARRS catalog, and is not corrected for Galactic extinction. View this GCN Circular online at https://gcn.nasa.gov/circulars/44432. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42554] GRB 260430A / EP260430a: COLIBRÍ optical upper limit
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44431 SUBJECT: GRB 260430A / EP260430a: COLIBRÍ optical upper limit DATE: 26/04/30 10:30:34 GMT FROM: F. Fortin at IRAP <ffortin.sci.edu(a)gmail.com> Francis Fortin (IRAP), Enrique Moreno Méndez (UNAM), 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), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), 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), 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 theEP GRB 260430A (Wang et al., GCN Circ. 44427) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-30 05:06:00 to 08:49:04 UTC (from 2.55 to 6.29 hours after the trigger) and obtained 43 minutes of simultaneous exposure in the r and z filters. The data were reduced and coadded with the COLIBRÍ 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 image, we do not detect any new source at the WXT source position (Wang et al., GCN Circ. 44427) down to the following 3-sigma limit: r > 22.38 z > 21.78 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/44431. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42553] GRB 260429A: Fermi GBM Detection
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44430 SUBJECT: GRB 260429A: Fermi GBM Detection DATE: 26/04/30 07:10:23 GMT FROM: Lorenzo Scotton at UAH <lscottongcn(a)outlook.com> L. Scotton (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 23:24:02.02 UT on 29 April 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260429A (trigger 799197847/260429975). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 283.92, Dec = 18.10 (J2000 degrees, equivalent to J2000 18h 55m, +18d 6'), with a statistical uncertainty of 2.61 degrees. (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a mixture of two Gaussians, one with a radius of 1.8 degrees (52% contribution) and one with a radius of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]). The angle from the Fermi LAT boresight is 76 degrees. The GBM light curve consists of a single emission episode with a duration (T90) of about 0.16 s (50-300 keV). The time-averaged spectrum from T0-0.1 to T0+0.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1 +/- 0.04 and the cutoff energy, parameterized as Epeak, is 900 +/- 20 keV. The event fluence (10-1000 keV) in this time interval is (2.41 +/- 0.07)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 69 +/- 2 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 600 +/- 20 keV, alpha = -0.92 +/- 0.03 and beta = -2 +/- 0.09. 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/44430. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42552] Fermi GRB 260429A: Global MASTER-Net observations report
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44429 SUBJECT: Fermi GRB 260429A: Global MASTER-Net observations report DATE: 26/04/30 06:31:44 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) started inspect of the Fermi GRB 260429A ( Fermi GBM team, GCN 44425) errorbox 24026 sec after notice time and 24067 sec after trigger time at 2026-04-30 06:05:09 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 65 deg. The sun altitude is -63.1 deg. The galactic latitude b = 7 deg., longitude l = 50 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3281490 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 24098 | 2026-04-30 06:05:09 | MASTER-OAFA | (18h 56m 08.86s , +18d 11m 55.3s) | C | 60 | 17.6 | 24177 | 2026-04-30 06:06:28 | MASTER-OAFA | (18h 56m 09.20s , +18d 12m 58.1s) | C | 60 | 17.6 | 24255 | 2026-04-30 06:07:47 | MASTER-OAFA | (18h 56m 14.55s , +18d 12m 00.2s) | C | 60 | 17.5 | 24363 | 2026-04-30 06:09:04 | MASTER-OAFA | (18h 56m 09.34s , +18d 11m 03.2s) | C | 120 | 17.7 | 24498 | 2026-04-30 06:11:20 | MASTER-OAFA | (18h 56m 15.66s , +18d 11m 31.9s) | C | 120 | 17.8 | 24636 | 2026-04-30 06:13:37 | MASTER-OAFA | (18h 56m 11.82s , +18d 12m 47.8s) | C | 120 | 17.9 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/44429. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42550] EP260430a: Global MASTER-Net observations report
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44428 SUBJECT: EP260430a: Global MASTER-Net observations report DATE: 26/04/30 04:09:48 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov(a)xray.sai.msu.ru> V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) MASTER-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the EP260430a ( EP Team et al., GCN 44427) errorbox 320 sec after trigger time at 2026-04-30 02:38:07 UT, with upper limit up to 19.0 mag. The observations began at zenith distance = 23 deg. The sun altitude is -59.3 deg. The galactic latitude b = 50 deg., longitude l = 281 deg. Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3281929 We obtain a following upper limits. Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment _________|_____________________|_____________________|____________________________________|_____|_______|_______|________ 350 | 2026-04-30 02:38:07 | MASTER-OAFA | (11h 53m 04.26s , -09d 58m 33.8s) | C | 60 | 18.5 | 420 | 2026-04-30 02:38:07 | MASTER-OAFA | (11h 53m 04.26s , -09d 58m 33.8s) | C | 200 | 18.8 | Coadd 426 | 2026-04-30 02:39:13 | MASTER-OAFA | (11h 53m 04.09s , -09d 58m 36.3s) | C | 80 | 15.7 | 1038 | 2026-04-30 02:49:35 | MASTER-OAFA | (11h 53m 02.59s , -09d 59m 04.2s) | C | 60 | 18.5 | 1104 | 2026-04-30 02:50:40 | MASTER-OAFA | (11h 53m 02.42s , -09d 59m 07.3s) | C | 60 | 18.5 | 1164 | 2026-04-30 02:50:40 | MASTER-OAFA | (11h 53m 02.42s , -09d 59m 07.3s) | C | 180 | 18.9 | Coadd 1170 | 2026-04-30 02:51:46 | MASTER-OAFA | (11h 53m 02.26s , -09d 59m 10.5s) | C | 60 | 18.5 | 1236 | 2026-04-30 02:52:52 | MASTER-OAFA | (11h 53m 02.09s , -09d 59m 13.9s) | C | 60 | 18.4 | 1301 | 2026-04-30 02:53:57 | MASTER-OAFA | (11h 53m 01.92s , -09d 59m 17.1s) | C | 60 | 18.5 | 1361 | 2026-04-30 02:53:57 | MASTER-OAFA | (11h 53m 01.92s , -09d 59m 17.1s) | C | 180 | 19.0 | Coadd 1366 | 2026-04-30 02:55:03 | MASTER-OAFA | (11h 53m 01.76s , -09d 59m 20.5s) | C | 60 | 18.5 | 1432 | 2026-04-30 02:56:09 | MASTER-OAFA | (11h 53m 01.60s , -09d 59m 24.0s) | C | 60 | 18.5 | 1498 | 2026-04-30 02:57:14 | MASTER-OAFA | (11h 53m 01.42s , -09d 59m 27.3s) | C | 60 | 18.4 | 1558 | 2026-04-30 02:57:14 | MASTER-OAFA | (11h 53m 01.42s , -09d 59m 27.4s) | C | 180 | 19.0 | Coadd 1564 | 2026-04-30 02:58:20 | MASTER-OAFA | (11h 53m 01.25s , -09d 59m 30.6s) | C | 60 | 18.4 | 1630 | 2026-04-30 02:59:27 | MASTER-OAFA | (11h 53m 01.08s , -09d 59m 34.3s) | C | 60 | 18.5 | 1696 | 2026-04-30 03:00:32 | MASTER-OAFA | (11h 53m 00.91s , -09d 59m 37.8s) | C | 60 | 18.5 | 1761 | 2026-04-30 03:01:38 | MASTER-OAFA | (11h 53m 00.74s , -09d 59m 41.3s) | C | 60 | 18.5 | Filter C is a clear (unfiltred) band. The observation and reduction will continue. The message may be cited. [1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html View this GCN Circular online at https://gcn.nasa.gov/circulars/44428. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42549] EP260430a: Einstein Probe detection of an X-ray transient
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44427 SUBJECT: EP260430a: Einstein Probe detection of an X-ray transient DATE: 26/04/30 03:45:21 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. L. Wang (NAO, CAS; ICE, CSIC), Y. Wang (PMO), G. J. Yang, C.C Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260430a. The transient triggered EP-WXT (ID: 01709261081) at 2026-04-30T02:32:47 (UTC). A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP was performed automatically following the WXT trigger. Within the WXT error circle, an uncatalogued X-ray source was detected at R.A. = 178.3341 deg, DEC = -10.2136 deg (J2000) with an uncertainty of 20 arcsec in radius (90% C.L. statistical and systematic). Further information will be updated when the telemetry data is received. 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/44427. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42551] EP260429a: EP-WXT detection of a fast X-ray transient
by GCN Circulars 30 Apr '26

30 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44426 SUBJECT: EP260429a: EP-WXT detection of a fast X-ray transient DATE: 26/04/30 03:05:55 GMT FROM: EP Team at NAOC/CAS <ep_ta(a)bao.ac.cn> Y. Wang (PMO), Y. L. Wang (NAO, CAS; ICE, CSIC), C.C Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team: We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260429a. The WXT light curve shows a single pulse with a duration of about 250 s. The transient started at 2026-04-29 14:41:11 UTC. The WXT position of the source is R.A. = 240.629 deg, DEC = -9.588 deg (J2000) with an uncertainty of 1.9 arcmin in radius (90% C.L. statistical and systematic). The average WXT 0.5 - 4 keV spectrum can be fitted with an absorbed power law with a Galactic hydrogen column density of 4.31 (-0.08/+0.09) x 10^21 cm^-2 and a photon index of 1.44 (-0.04/+0.04). The derived average unabsorbed 0.5-4 keV flux is 3.96 (-0.32/+0.37) x 10^-9 erg/s/cm^2. Follow-up observation with the Follow-up X-ray Telescope on board EP has been scheduled. Further information will be updated when the telemetry data is received. 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/44426. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42548] GRB 260429A: Fermi GBM Final Real-time Localization
by GCN Circulars 29 Apr '26

29 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44425 SUBJECT: GRB 260429A: Fermi GBM Final Real-time Localization DATE: 26/04/29 23:34:38 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 23:24:02 UT on 29 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260429A (trigger 799197847.01658 / 260429975). The on-ground calculated location, using the Fermi GBM trigger data, is RA = 283.9, Dec = 18.1 (J2000 degrees, equivalent to J2000 18h 55m, 18d 06'), with a statistical uncertainty of 2.6 degrees. The angle from the Fermi LAT boresight is 76.0 degrees. The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260429975/… The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260429975/… The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260429975/… View this GCN Circular online at https://gcn.nasa.gov/circulars/44425. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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[vsnet-grb-info 42545] GRB 260428A: SVOM/GRM observation of a possible short burst with extended emission
by GCN Circulars 29 Apr '26

29 Apr '26

TITLE: GCN CIRCULAR NUMBER: 44424 SUBJECT: GRB 260428A: SVOM/GRM observation of a possible short burst with extended emission DATE: 26/04/29 15:26:34 GMT FROM: Chenwei Wang at IHEP <cwwang(a)ihep.ac.cn> SVOM/GRM team: Chen-Wei Wang, Chao Zheng, Zheng-Hang Yu, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP) SVOM/ECLAIRs team: Olivier GODET (IRAP) Report on behalf of the SVOM team: SVOM/GRM was triggered in-flight by a burst GRB 260428A (SVOM trigger reference: sb26042803) at 2026-04-28T14:20:37.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN#44415), AstroSat (A. Goyal et al., GCN#44421) and GECAM-B (Wang et al., GCN#44423). With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of mutli-pulses hard spikes followed by a soft weak extended emission with a T90 of 4.1 +3.1/-2.1 s in the 15-5000 keV band. The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb260428A.png In addition, the position of this burst, as determined by Fermi/GBM (RA = 277.0 deg, Dec = 1.9 deg, Err= 2.1 deg, GCN#44415), is located at about 100 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. With this localization, the time-averaged spectrum from T0-1.4 to T0+8.4 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.11 +0.33/-0.25 and the cutoff energy, parameterized as Epeak, is 1380 +159/-66 keV. The event fluence (10-1000 keV) in this time interval is (4.82 +0.83/-0.79)E-06 erg/cm^2. GRB 260428A falls into the Type I region in the 'Amati' relation diagram, as shown at: https://www.bursthub.cn//admin/static/svgrb260428A_amati.png With the brightness of the prompt emission, the predicted soft X-ray afterglow [1] is shown at: https://www.bursthub.cn//admin/static/svgrb260428A_xag_pre.png The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS. The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang(a)ihep.ac.cn) [1] Chen-Wei Wang et al. ApJ 997 353 https://doi.org/10.3847/1538-4357/ae2eae View this GCN Circular online at https://gcn.nasa.gov/circulars/44424. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0L…

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