EP250109b: EP-WXT detection of an X-ray transient and EP-FXT follow-up observations
ATel #16974; H. Q. Cheng (NAO, CAS), R.-Z. Li (YNAO, CAS), X.-L. Chen, K. Chatterjee (YNU) and Z. X. Ling (NAOC, CAS) report on behalf of the Einstein Probe team
on 12 Jan 2025; 12:09 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Transient
We report on the detection of an X-ray transient, designated EP250109b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event was detected during an observation starting at 2025-01-09T08:06:40 (UTC), with an exposure of 4.2 ks. The WXT position of the source is R.A. = 118.611 deg, DEC = -14.651 deg (J2000) with an uncertainty of 3.3 arcmin in radius (90% C.L. statistical and systematic). During the late-stage of the observation, the source count rate showed a significant increase, exhibiting a flare lasting about 200 seconds. The peak flux in the 0.5-4 keV is around 7 x 10^-10 erg/s/cm^2. The average 0.5-4 keV spectrum during the flare can be fitted with an absorbed power law with a photon index of 1.5 (+1.2/-1.1) (with a galactic column density fixed at 1.68 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 3.0 (+2.3/-1.2) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Two follow-up observations were performed with the Follow-up X-ray Telescope (FXT) onboard EP, one at 2025-01-10T07:45:19 (UTC), with an exposure of 5.4 ks, and the other at 2025-01-11T07:46:42 (UTC) with an exposure of 8.1 ks. An uncatalogued X-ray source was detected in both epochs within the WXT error circle. The position of the source is R.A. = 118.6255 deg, DEC = -14.6377 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). In the first epoch, the average 0.5-10 keV spectrum can be fitted with an absorbed power law with a photon index of 1.88 (+0.49/-0.47) (with a galactic column density fixed at 1.68 x 10^21 cm^-2), yielding an average unabsorbed 0.5-10 keV flux of 1.49 (+0.74/-0.43) x 10^-13 erg/s/cm^2. In the second epoch, the fitted photon index and unabsorbed flux are 1.79 (+0.71/-0.64) and 6.25 (+4.74/-2.38) x 10^-14 erg/s/cm^2, respectively. Despite of a large uncertainty, the source tends to exhibit a decaying trend in two FXT observations. We suggest it being associated with the WXT transient. The uncertainties are at the 90% confidence level for the above parameters.
We note that close to the WXT position (with a separation of 3.6 arcmin, slightly outside the error circle), there is a nearby (~1 kpc) eclipsing binary, MT Pup. The historical flux given by eRosita All Sky Survey is ~4.5e-13 erg/s/cm^2 (0.2-2.3 keV). This star is detected in both FXT epochs, with consistent 0.5-10 keV fluxes of ~1.5 x 10^-12 erg/s/cm^2. However, the flaring luminosity would be too high (> 10^34 erg/s) if it is associated with the WXT detection. We thus tend to suggest it is irrelevant to this transient.
Further observations are encouraged to explore the origin of EP250109b.
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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.