TITLE:   GCN CIRCULAR
NUMBER:  44389
SUBJECT: GRB 260421B: SVOM/GRM observation
DATE:    26/04/22 08:31:00 GMT
FROM:    guohx@ihep.ac.cn

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

SVOM/ECLAIRs team: Olivier GODET (IRAP), Frédéric Daigne (IAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260421B (SVOM trigger reference: sb26042105) at 2026-04-21T04:13:22.000 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN #44360) and SVOM/ECLAIRs (D.Zhao et al., GCN #44362).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multiple pulses with a T90 of 21.5 +3.5/-2.5 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb260421B.png

In addition, the position of this burst, as determined by SVOM/ECLAIRs (RA = 190.0318, Dec = -17.7566, GCN #44362), is located at about 44 degrees from the SVOM optical axis, which is inside the ECLAIRs field of view.

With this localization, the time-averaged spectrum from T0-14.5 to T0+21.5 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.26 +0.07/-0.06 and the cutoff energy, parameterized as Epeak, is 162 +18/-14 keV. The event fluence (10-1000 keV) in this time interval is (1.19 +/-0.05)E-05 erg/cm^2.

The 1s peak spectrum, measured from T0+14.0 to T0+15.0 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -0.72 +/-0.07 and the cutoff energy, parameterized as Epeak, is 200 +13/-12 keV. The flux (10-1000 keV) in this time interval is (2.27 +0.09/-0.08)E-06 erg/cm^2/s.

With the measured redshift z=2.115 (B. Schneider et al., GCN 44368), we calculate the isotropic energy Eiso is about 1.5E53 erg. Thus GRB 260421B is well consistent with Type II GRBs in the 'Amati' relation diagram, as shown at:
https://www.bursthub.cn//admin/static/svgrb260421B_amati.png
The localization of GRB 260421B in the 'Yonetoku' relation diagram is shown at:
https://www.bursthub.cn//admin/static/svgrb260421B_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: Hao-Xuan Guo (IHEP)(guohx@ihep.ac.cn)