TITLE: GCN CIRCULAR NUMBER: 44362 SUBJECT: GRB 260421B: SVOM detection of a long burst DATE: 26/04/21 05:20:23 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> D.Zhao (NAOC), H.Goto (RIKEN), W. L. Zhang (PMO), Z. Y. Liu (GZNU) report on behalf of the SVOM mission team: At 2026-04-21T04:13:25 UTC (T0), SVOM/ECLAIRs triggered and located the gamma-ray burst GRB 260421B (SVOM burst-id sb26042105). The Fermi GBM also detected this burst (GCN 44360 & 44361). 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 13 alerts. IMT provided the alert with the best signal-to-noise-ratio in the image (SNR) of 30.71 in the [8-120] keV energy band over a time window of 20.48 seconds starting at 2026-04-21T04:13:17. The localization of the best alert is R.A., Dec. 189.9811, -17.7676 degrees (J2000) with a 90% confidence level (C.L.) radius of 3.21 arcmin (including systematic error of 2 arcmin added in quadrature). The SVOM/ECLAIRs light curve showed a multiple peak structure with a T90 duration of about 52.824 (-0.172 +0.169). This burst also triggered SVOM/GRM at 2026-04-21T04:13:21 on a timescale of 1 seconds with an SNR of 7.90. The SVOM/GRM light curve showed a multiple peak structure with a T90 duration of about 20.04 (-0.93 +3.07). SVOM slewed to the burst. SVOM/MXT began observing the field at 2026-04-21T04:18:40 UTC, 315 seconds after T0. Using onboard processed data we found an uncatalogued X-ray source located at R.A., Dec. 190.0318, -17.7566 degrees: R.A. (J2000) = 12h40m07.62s Dec. (J2000) = -17d45m23.69s with a 90% C.L. radius of 97.27 arcseconds. This location is 2.97 arcminutes from the ECLAIRs onboard position. This position may be improved as more data is received. VT began observing the field after the slew. The analysis of the data will be published in a future circular. 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 W. L. Zhang: wlzhang@pmo.ac.cn Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44362. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...