EP-FXT follow-up of the X-ray transient EP260531a

ATel #17833; 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 5 Jun 2026; 08:37 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission performed a follow-up observation of the X-ray transient EP260531a (ATel #17828) on 2026-06-04 07:33:43 (UTC), with a net exposure of 6 ks. A bright X-ray source was detected within the WXT error circle at R.A. = 265.7118 and Dec. = -31.20025 (J2000). The source position has an uncertainty radius of 10 arcseconds (90% confidence level, including both statistical and systematic uncertainties).

The FXT spectrum can be fitted with an absorbed power-law plus blackbody model. The best-fitting parameters are a hydrogen column density of 5.4 (+0.4/-0.5)e22 cm-2, a photon index of 4.8 (+0.4/-0.5), and a blackbody temperature of 0.44 (+/-0.03) keV. The observed 0.5-10.0 keV flux is 1.84 (+/-0.04)e-10 erg/s/cm^2.

Additionally, we note that this source keeps brightening in the WXT monitoring, with the 0.5-4.0 keV observed flux rising from approximately 7.4 (+0.4/-0.2)e-11 erg/s/cm^2 at 2026-05-31 07:45:29 to about 2.3 (+0.6/-0.5)e-10 erg/s/cm^2 at 2026-06-04 18:58:37 (UTC).

Further follow-up observations are encouraged to identify the nature of this source.

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).

EP260531a: Einstein Probe detection of an X-ray transient