EP260531a: Einstein Probe detection of an X-ray transient

ATel #17828; G. L. Huang, Z. X. Li, J. Y. Cao (IHEP, CAS), J. W. Hu, Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team
on 3 Jun 2026; 09:39 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of an X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission through ground data analysis, designated EP260531a. The WXT position of the source is R.A. = 265.737 deg, DEC = -31.194 deg (J2000) with an uncertainty of 2.6 arcmin in radius (90% C.L. statistical and systematic), located at low Galactic latitude (l = 357.772, b = -0.706).

A preliminary spectral analysis indicates that the 0.5-4 keV spectra obtained on 2026 May 31 and June 3 can be fitted with an absorbed power-law model. Fixing the hydrogen column density at the Galactic value of 1.06 x 10^22 cm^-2 yields photon indices of 0.8 (+/-0.5) and 0.7 (+0.5/-0.6), respectively. Allowing NH to vary yields NH values of 3.3 (+2.1/-2.9) x 10^22 cm^-2 and 3.7 (+1.8/-2.5) x 10^22 cm^-2, with corresponding photon indices of 2.8 (+2.4/-2.0) and 3.7 (+2.5/-1.9), respectively. The observed 0.5-4 keV fluxes are 3 x 10^-11 erg cm^-2 s^-1 and 6 x 10^-11 erg cm^-2 s^-1, respectively. The source has shown a steady increase in flux between 2026 May 31 and June 3.

The low Galactic latitude and hard X-ray spectrum suggest that the source may be a Galactic X-ray binary candidate, although its nature remains uncertain.A follow-up observation of EP260531a with the Follow-up X-ray Telescope (FXT) on board EP has been scheduled.

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).