GRB 250317C

Yue Wang, Shao-Lin Xiong, Chen-Wei Wang report on behalf of the GECAM team:

GECAM-B was triggered both in-flight and on-ground by a long burst, GRB 250317C, at 2025-03-17T17:31:21.700 UTC (T0). 

According to the event-by-event data, the GECAM-B light curve shows a main pulse with a with a T90 of 23.0 +3.0/-4.0 s in the 40-6000 keV band.

The GECAM-B light curve could be found here: 
https://www.bursthub.cn//admin/static/gecamgrb250317C.png

Using the automatic on-ground localization pipeline, GECAM-B localized this burst to the following position (J2000): 
Ra: 320.42 deg 
Dec: -16.07 deg
Err: 8.59 deg (1-sigma, statistical only)

We note that these results are preliminary. Refined analysis will be reported later.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor(GECAM) mission originally consists of two microsatellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

S. Salunke (IUCAA), M. Tembhurnikar (IUCAA), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long duration GRB 250317C which was also detected by GECAM-C (Wang et al., GCN Circ. 39768
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 ).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-03-17 17:31:23.50 UTC. The measured peak count rate associated with the burst is 87 (+34, -21) counts/s above the background in the combined data of three quadrants (out of four), with a total of 356 (+153, -114) counts. The local mean background count rate was 278 (+3, -5) counts/s. Using cumulative rates, we measure a T90 of 8.1 (+1.62, -3.35) s.

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-03-17 17:31:25.21 UTC. The measured peak count rate associated with the burst is 225 (+62, -64) counts/s above the background in the combined data of all quadrants, with a total of 770 (+299, -346) counts. The local mean background count rate was 1266 (+9, -9) counts/s. We measure a T90 of 8.29 (+1.8, -4.7) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

SVOM/GRM team: Yue Wang, Chen-Wei Wang, Shi-Jie Zheng, Yue Huang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team:  Tais Maiolino, Frédéric Piron (LUPM), Stéphane Schanne (CEA), Jean-Luc Atteia (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 250317C (SVOM trigger reference: sb25031702) at 2025-03-17T17:31:28.000 (T0). This burst was also detected by GECAM-B and AstroSat CZTI (Wang et al., GCN 39768
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 ; S. Salunke et al., GCN 39771
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 ). 

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of single main pulse with a T90 of 23.4 +3.7/-2.0 s in the 15-5000 keV band.

In addition, ECLAlRs was taking data and did not trigger, since the position of this burst, as determined by GECAM-B (GCN 39768
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 , RA: 320.42 deg, DEC: -16.07 deg, Error: 8.59 deg), is located at about 108.3 degrees from the SVOM optical axis, and outside the ECLAIRs field of view.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250317C.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/GRM point of contact for this burst is: Yue Wang (IHEP) (yuewang@ihep.ac.cn)