TITLE:   GCN CIRCULAR
NUMBER:  37443
SUBJECT: EP240908a: Updates on the WXT detection result around the trigger time
DATE:    24/09/10 09:09:30 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

X. Mao, J. W. Hu (NAOC, CAS), J. Q. Peng (IHEP, CAS), S. K. Yang, H. He (WHU), W. Yuan (NAOC, CAS) on behalf of the Einstein Probe team

We have reported the detection of the fast X-ray transient EP240908a by the Einstein Probe (EP) mission (EP Team et al. GCN 37432). Based on the newly received EP-WXT telemetry data of this source, the transient event started at 2024-09-08T17:28:27 (UTC). The WXT position of the source is R.A. = 13.992 deg, DEC = 8.089 deg (J2000) with an uncertainty of 2.7 arcmin in radius (90% C.L. statistical and systematic), which is 1.1 arcmin away from the FXT position (EP Team et al. GCN 37432). The lightcurve of the transient observed by the WXT lasted around 950 seconds and has a peak flux of ~1 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.6(-0.5, +0.4) (with the column density fixed at the Galactic one of 7 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 7.4(-1.9, +2.4) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

An optical counterpart candidate was detected within the FXT error circle with an AB magnitude r ~ 24 (Quirola-Vasquez et al. GCN 37438). Further follow-up observations are encouraged to identify the nature of this X-ray transient.