GCN Circular 45962

SVOM/GRM team: Yan-Qiu Zhang, Ce Cai, Chen-Wei Wang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

EP team: Maria Edvige Ravasio (ICE-CSIC and Radboud University)

For EP241111a and EP250125a, SVOM/GRM was observing normally and covered the sky regions of these two sources. The peak times and durations are taken from the first catalogue of EP sources (Wu et al. 2026, in prep.). For EP241111a, T0 = 2024-11-11T07:31:37.950 (UTC), T100 = 2024-11-11T07:35:23.000 (UTC), and Tpeak = 2024-11-11T07:33:24.974 (UTC). For EP250125a, T0 = 2025-01-25T02:36:19.650 (UTC), T100 = 2025-01-25T02:43:48.050 (UTC), and Tpeak = 2025-01-25T02:36:47.144 (UTC).

There was no SVOM/GRM trigger around the event times of EP241111a and EP250125a. The targeted search was performed over the two time intervals listed above for the respective sources, and no counterpart candidates were identified in either time interval.

We therefore set upper limits on impulsive gamma-ray emission for EP241111a and EP250125a. Three spectral templates were used: a soft spectrum (Band function with Epeak = 70 keV, alpha = -1.9, beta = -3.7), a normal spectrum (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3), and a hard spectrum (Comptonized function with Epeak = 1500 keV, alpha = -0.5). All pivot energies (Epiv) in the functions are fixed at 100 keV. We set the following 3 sigma flux upper limits over 10-1000 keV:

EP241111a:
Full time interval (T0 ~ T100, in units of 10^-8 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
---------------------------------- 
225.05 s:   7.40    5.68    6.75

Peak time (Tpeak, in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
----------------------------------
 0.1 s:    35.1    26.9     31.8
 1.0 s:    11.1     8.5     10.1
10.0 s:     3.5     2.7      3.2

EP250125a:
Full time interval (T0 ~ T100, in units of 10^-8 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
----------------------------------
448.4 s:   0.44    0.70    1.39

Peak time (Tpeak, in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
----------------------------------
 0.1 s:    2.87    4.38    8.63
 1.0 s:    0.91    1.39    2.73
10.0 s:    0.29    0.44    0.86

We note that these results are preliminary, and that the full-time-interval upper limits are subject to larger systematic uncertainties in the background estimation; a more careful treatment of the background is needed. Refined analysis will be reported later.

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 these events is: Yan-Qiu Zhang (IHEP) (yqzhang@ihep.ac.cn)