EP251003a

G. Corcoran (UCD), J. A. Quirola-Vasquez (Radboud), R. A. J. Eyles-Ferris (U. Leicester), P. G. Jonker (Radboud), D. B. Malesani (DAWN/NBI and Radboud), F. E. Bauer (SSI and UTA), and J. A. Chacón (PUC) report on behalf of a larger collaboration:

We observed the location of EP251003a (Liu et al., GCN 42107
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 ) with the Sinistro instrument mounted on the 1-m telescopes at Cerro Tololo Inter-American Observatory, available via the 
LCO network. Six 200 s exposures were obtained in the SDSS i filter starting at 2025-10-05 UT 07:16:29 (~35.1 hr post trigger).

Within the WXT and FXT error circles (Liu et al., GCN 42107
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 ), we identify a new source. Calibrated to nearby stars from the Pan-STARRS catalog, we measure a magnitude i = 21.6 +/- 0.2 (AB), not corrected for Galactic extinction.

No corresponding object is visible in the Legacy Survey down to an AB magnitude of i > 24.2.

We obtained an additional observation with the VLT Survey Telescope (VST). Six exposures of 300 s each were obtained in the SDSS i filter as well ten 200 s exposures in the SDSS z filter, starting at 2025-10-06 UT 06:47:12 (~58.6 hr post trigger). The counterpart identified in the LCO image is again detected, and we measure a magnitude of i = 22.87 +/- 0.07 and z = 23.23 +/- 0.14 (AB). Given the significant fading, the consistency with the X-ray localization, and the lack of a bright archival counterpart, we consider this source as the likely counterpart of EP251003a.

The most accurate coordinates of the counterpart as measured from the VST image are:

RA(J2000): 03:47:40.99
DEC(J2000): -10:55:55.0

We estimate an uncertainty of 0.2" in both RA and Dec.

Q. C. Liu (THU), Y. Q. Zhao (USTC), C. Zhou (HUST), 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, designated EP251003a. The WXT position of the source is R.A. = 56.934 deg, DEC = -10.929 deg (J2000) with an uncertainty of 2.549 arcmin in radius (90% C.L. statistical and systematic). The transient was first detected by WXT in an observation that started at 2025-10-03T18:11:50 (UTC). The 0.5-4 keV spectrum can be fitted with an absorbed powerlaw model with hydrogen column density of 0.4(+3.2, -0.4)x10^21 cm^-2 and a photon index of 0.9(+1.1, -0.5). The average unabsorbed 0.5-4 keV flux is 3.8(+1.4, -1.1) x 10^-11 erg/s/cm^2. 

A follow-up observation with the Follow-up X-ray Telescope (FXT) was performed. An uncatalogued X-ray source was detected at R.A. = 56.9218 deg, DEC = -10.9321 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The 0.5-10 keV spectrum can be fitted with an absorbed powerlaw model with hydrogen column density of 0.0 (+1.7, -0.0)x10^21 cm^-2 and a photon index of 1.7(+0.6, -0.2). The average unabsorbed 0.5-10 keV flux is 8.3(+1.6, -1.9) x 10^-13 erg/s/cm^2. 


All the uncertainties of the parameters are 90% C.L. statistical. Further follow-up observations are encouraged.

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).