GCN Circular 45011

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

SVOM/ECLAIRs team: Ulysse Jacob (LUPM), Olivier GODET, Jean-Luc Atteia (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 260616B/EP260616a (SVOM trigger reference: sb26061603) at 2026-06-16T07:05:16.900 UTC (T0), which is also detected by Fermi/GBM (C. Malacaria et al., GCN #44953
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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 a main episode with two peaks a T90 of 13.30 +0.8/-0.4 s in the 15-5000 keV band. 

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260616B.png

In addition, the position of this burst, as determined by EP/WXT (RA= 256.059 deg, DEC= -32.305 deg, ERR=2.2 arcmin, GCN#44956
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 ), is located at about 67 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view, and ECLAIRs was totally blocked by the Earth. Nevertheless, ECLAIRs detected this burst clearly over the 4-120 keV via atmospheric reflection. 

With this localization, the time-averaged spectrum from T0-2 to T0+14 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.11 +/-0.04 and the cutoff energy, parameterized as Epeak, is 622 +80/-66 keV. The event fluence (10-1000 keV) in this time interval is (2.29 +/-0.06)E-05 erg/cm^2. 


The 1s peak spectrum, measured from T0+9.8 to T0+10.8 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -0.90 +0.05/-0.06 and the cutoff energy, parameterized as Epeak, is 472 +52/-44 keV. The flux (10-1000 keV) in this time interval is (4.85 +/-0.17)E-06 erg/cm^2/s.

The localization of GRB 260616B in the 'Amati' relation diagram is shown at: https://www.bursthub.cn//admin/static/svgrb260616B_amati.png
The localization of GRB 260616B in the 'Yonetoku' relation diagram is shown at: 
https://www.bursthub.cn//admin/static/svgrb260616B_yonetoku.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: Zheng-Hang Yu(IHEP)(zhyu@ihep.ac.cn)