Possible outburst from the X-ray source RX J0218.7+3854 observed by Einstein Probe

ATel #16937; C. Y. Wang (THU), Y.-H. I. Yin (NJU), D. H. Zhao (NAO, CAS), H. W. Pan, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team
on 8 Dec 2024; 14:25 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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Following the detection of the X-ray transient EP241206a (Yin et al., GCN 38457), we performed an observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation started at 2024-12-07T14:34:29 (UTC), about 22 hours after the EP-WXT detection, with an exposure time of 3.0 ks. Within the WXT error circle, an X-ray source is detected at RA = 34.6990 deg, DEC = 38.9136 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic).

The FXT detected source is spatially correlated with Gaia object 331926892386271872, offset by approximately 3 arcsec. This Gaia source is associated with a known X-ray source RX J0218.7+3854 (morxv2, Eric 2024, [1]) with a flux of ~ 4.7 x 10^-13 erg/s/cm^2 and has been detected in radio wavelengths.

The average FXT spectrum in 0.5-10 keV can be fitted by an absorbed power law with a photon index of 2.6(-0.2,+0.2) and a column density fixed at the Galactic value of 5.47 x 10^20 cm^-2. The derived average unabsorbed flux is 4.9(-0.4, +0.5) x 10^-13 erg/s/cm^2 in the 0.5-10 keV band, consistent with the historical measurement by ROSAT. We tend to consider this FXT source to be associated with EP241206a, suggesting the WXT detection to be a possible X-ray outburst from RX J0218.7+3854.

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).

Reference:

[1] Eric Wim Flesch, "The Millions of Optical - Radio/X-ray Associations (MORX) Catalogue, v2" OJAp, 21-Jan-2024.