GCN Circular 38889
Subject
EP250109a / GRB 250109A: refined EP-WXT analysis and EP-FXT follow-up observation
Date
2025-01-10T09:43:01Z (a month ago)
Edited On
2025-01-10T21:02:26Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), X.-L. Chen (YNU), K. Chatterjee (YNU), Y.-L. Hua (PMO, CAS) and Y. Liu (NAO, CAS) report on behalf of the Einstein Probe team:
Since 2025-01-09T06:17:58 (UTC, Trigger time), EP250109a (Li et al., GCN 38864) has been detectable with high significance using the WXT onboard the EP, peaking about 60 seconds post-trigger. The observed peak flux in the 0.5–4 keV range is about 2.5 x 10^-9 erg/s/cm^2. The flux then rapidly decayed to background levels within 200 seconds post-trigger. The average 0.5-4 keV WXT spectrum can be fitted with an absorbed power law with a photon index of 2.9 +/- 1.0 (with a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2). The derived intrinsic column density is 5.0 (-0.3, +0.4) x 10^21 cm^-2 when the redshift is fixed at 0. The derived average unabsorbed 0.5-4 keV flux is 2.5 (-1.2, +4.6) x 10^-9 erg/s/cm^2.
We conducted a follow-up observation of EP250109a using the FXT onboard the EP. The observation began at 2025-01-09T08:06:40 (UTC, about 1.8 hours post-trigger), with an exposure time of approximately 4.2 ks. The average 0.5–4 keV FXT spectrum can be modeled with an absorbed power law, featuring a photon index of 1.9 (-0.7, +1.9) and a fixed Galactic equivalent hydrogen column density of 3.04 x 10^21 cm^-2. The derived intrinsic column density is about 3 x 10^20 cm^-2 when the redshift is fixed at 0. The estimated average unabsorbed 0.5–4 keV flux is about 4.9 (-1.2, +2.0) x 10^-13 erg/s/cm^2.
The detection of EP-WXT/FXT (GCN 38864) and Fermi-GBM (GCN 38873 and GCN 38887), combined with the non-detection by Swift-XRT (GCN 38867), indicates that the flux of this transient has decreased very rapidly.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).