Gamma-ray Burst 250321B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN Circular #39818
  S. Schanne (CEA), M. Brunet (IRAP), D. Turpin (CEA), T. Maiolino, F. Piron (LUPM), O. Godet, J-L. Atteia (IRAP)
  report on behalf of the SVOM mission team
  The SVOM/ECLAIRs telescope detected a transient source appearing below the onboard trigger thresholds, labelled GRB 250321B starting at 2025-03-21T03:36:28 UTC (Tb).
  The information of this sub-threshold source was received on the ground with low-latency by the SVOM VHF Alert Network. After arrival of the X-band data the source detection is confirmed.
  The burst was detected below the thresholds applied onboard by both the Count-Rate Trigger (CRT) and Image Trigger (IMT). It has been reported in the log messages of the onboard trigger downloaded via X-band. The best detection is found by IMT with a signal-to-noise ratio of 6.45 in the 8-120 keV energy band over a time window of 20.48 s starting at Tb. After reprocessing the photon by photon data in the onboard trigger replay with lower thresholds, as well as the on-ground offline trigger, a point-like source is confirmed in the reconstructed sky images.
  The localization of the source is RA, Dec = 169.687, 39.560 (J2000). The uncertainty on this position is about 12 arcminutes at 90% C.L.
  We notice the presence of several quasars in this error box, among which the closest is SDSS J111850.26+393258.6 located at 1.2 arcmin.
  We encourage follow-up observation of this source candidate.
  The Space 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.
  The point of contact for this event is Stéphane Schanne (s.schanne AT cea.fr) by email if you require additional information.
• GCN Circular #39828
  P. A. Evans (U. Leicester) reports on behalf of the Swift team:
  Swift has initiated a ToO observation of the SVOM/ECLAIRs GRB 250321B. Automated analysis of the XRT data will be presented online at https://www.swift.ac.uk/ToO_GRBs/00021824
  Any uncatalogued X-ray sources detected in this analysis will be reported on this website and via GCN COUNTERPART notices. These are not necessarily related to the SVOM/ECLAIRs event. Any X-ray source considered to be a probable afterglow candidate will be reported via a GCN Circular after manual consideration.
  Details of the XRT automated analysis methods are detailed in Evans et al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
  This circular is an official product of the Swift-XRT team.
• GCN Circular #39840
  Jean-Grégoire Ducoin (CPPM), Ny Avo Rakotondrainibe (LAM), Ramandeep Gill (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM), Alan M. Watson (UNAM), and Stéphane Schanne (CEA) report:
  We imaged the field of the SVOM/ECLAIRs GRB 250321B (Schanne et al., GCN Circ. 39818) with the DDRAGO imager on the SVOM/COLIBRÍ (formerly SVOM/F-GFT) telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.
  We observed from 2025-03-22 08:07 to 10:20 UTC (28.52 to 30.74 hours after the trigger) and obtained 100 minutes of exposure in the i band. The data were coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2021), with photometric calibration against Pan-STARRS DR1. Our photometry is in the AB system and is not corrected for Galactic extinction.
  Within the ECLAIRs localization of the source (Schanne et al., GCN Circ. 39818) we do not detect any uncatalogued sources to a 10-sigma limiting magnitude of :
  i > 22.68
  We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.
• GCN Circular #39856
  M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (SSDC & INAF-OAR) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
  Swift-XRT has performed follow-up observations of the SVOM/ECLAIRs-detected burst GRB 250321B, collecting 2.7 ks of Photon Counting (PC) mode data between T0+81.0 ks and T0+92.8 ks.
  No X-ray sources have been detected. The 3-sigma upper limit in the field ranges from ~0.005 to ~0.006 ct s^-1, corresponding to a 0.3-10 keV observed flux of 2.1e-13 to 2.4e-13 erg cm^-2 s^-1 (assuming a typical GRB spectrum).
  The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/ToO_GRBs/00021824.
  This circular is an official product of the Swift-XRT team.
• GCN Circular #39865
  N. Pankov (HSE, IKI), A. Schmalz (KIAM), A. Pozanenko (IKI) report on behalf of IKI-GRB-FuN:
  We performed optical observations of the field of GRB 250321B (Schanne et. al, GCN 39818; Capalbi et. al, GCN 39856) in the Clear light with the 0.36-meter RC-36 telescope of the ISON-Kitab observatory. The observations started on (UT) 2025-03-21 17:31:49, i.e. about 0.59 days since trigger. We do not detect an optical afterglow candidate in the stacked image. The preliminary photometry is as follows:

  Date       UT start  t-T0         Exp.    Filter OT     Err   UL(3sigma)
                       (mid, days)  (s)            
  2025-03-21 17:31:49  0.59330      38*60   Clear  n/d    n/d   17.0
  

  Ref. stars

  RA Dec r
  169.5809 +39.4176 13.063
  169.7213 +39.4809 13.341
  

  The photometry is based on nearby stars of SDSS-DR12 (r-magnitudes) and has not been corrected for the Galactic extinction. Our result is consistent with non-detections of an optical (Ducoin et. al, GCN 39840) and an X-Ray (Capalbi et. al, GCN 39856) counterpart.