TITLE:   GCN CIRCULAR
NUMBER:  37909
SUBJECT: EP241026a (GRB 241026A): EP on-board trigger and follow-up observation
DATE:    24/10/27 14:34:15 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

D. Y. Li, T. Y. Lian, Y. L. Wang, S. X. Wen, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of an X-ray transient detected by EP-WXT, EP241026a, which triggered the on-board processing unit (trigger ID: 01709112485). A preliminary analysis of the EP-WXT data shows that the transient began at 2024-10-26T22:41:28 (UTC), and lasted for around 150 seconds, with the peak flux of around 8 x 10^-9 erg/s/cm^2 in 0.5-4 keV. The averaged spectrum can be fitted with an absorbed powerlaw（NH fixed at the Galactic value 9.2 x 10^20 cm^-2）with a photon index of 0.8 +/- 0.3. The average unabsorbed flux is (1.6 +/- 0.3) x 10^-9 erg/s/cm2 in 0.5-4 keV. An EP-FXT follow-up observation was triggered about 8 hours later, which detected an uncatalogued X-ray source at R.A. = 293.4008 deg, DEC = 57.9857 deg (J2000) with an uncertainty of about 10 arcsecs (radius, 90% C.L. statistical and systematic), consistent with the position of the WXT transient within the uncertainties. We note that the trigger time and position of this X-ray transient are consistent with those of GRB 241026A (Fermi GBM team, GCN #37894, Melandri et al. GCN #37896). The averaged EP-FXT spectrum can be fitted with an absorbed powerlaw (NH fixed at the Galactic value), with a photon index of 1.81 +/- 0.08. The derived average unabsorbed flux is (6.4 +/- 0.3) x 10^-12 and (1.06 +/- 0.07) x 10^-11 erg/s/cm2 in 0.5-4 keV and 0.5-10 keV, respectively. All the errors of the parameters quoted are at the 90% C.L.

Please note that the temporal and spectral analyses presented above are preliminary. Final results will be presented in future publications. EP-FXT will also keep monitoring this transient in the following days.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).