EP250916a

C. Y. Dai (NJU), Q. Y. Wu, D. Y. Li, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

EP-FXT performed a follow-up observation of EP250916a (GCN 41841
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 ) at 2025-09-17 11:34:24 (UTC), about 32 hours after the WXT detection, with an exposure time of 3.1 ks. FXT detected a bright source within the WXT error circle at R.A. = 268.2379 and Dec. = –35.3222 (J2000), with a positional uncertainty of 10 arcseconds (radius, 90% confidence level, including both statistical and systematic errors). The fit with a power-law spectrum yields Nh = 3.1 × 10^21 cm^-2 and a photon index of 1.66 ± 0.02, with a 0.5–10 keV flux of 5.4 × 10^-10 erg/s/cm^2.

Additionally, we note that this source keeps brightening in the WXT monitoring, with the flux rising from approximately 3 × 10^-11 erg/s/cm^2 at 2025-09-16 03:33:13 to about 2 × 10^-10 erg/s/cm^2 at 2025-09-17 17:54:10 (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).

Q. Y. Wu (NAO, CAS), C. Y. Dai (NJU), D. Y. Li, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

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 EP250916a. The WXT position of the source is R.A. = 268.279 deg, DEC = -35.317 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic), located at low Galactic latitude (l = 355.331, b = -4.634).

The source was detected in three consecutive observations: the first started at 2025-09-16T03:33:13 (UTC) with a flux of ~3e-11 erg/s/cm^2; two subsequent observations, approximately 2 h and 6 h later, also detected the source with slightly higher fluxes of ~5e-11 erg/s/cm^2. As of the latest observation, the transient has persisted for ~7 hours. A preliminary spectral analysis shows that the 0.5-4 keV spectrum can be fitted by an absorbed power-law model with Galactic hydrogen column density fixed at 3.76 x 10^21 cm^-2 and a photon index of 1.1 (-/+0.8). No known X-ray sources have been found previously within the EP-WXT error circle. A stellar flare origin cannot be excluded at this stage. 

Follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP have been scheduled, and further information will be updated when the telemetry data are received.

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).