GCN 45216: EP260723a: further EP-FXT follow-up observation

K. R. Ni (CCNU), Y. J. Zhang (PKU), X. H. Tang (THU), and H. Sun (NAOC) report on behalf of the Einstein Probe (EP) team:

EP-FXT performed a second follow-up observation of EP260723a, which was detected by EP-WXT (Zhang et al., GCN 45201) and previously followed up by EP-FXT (Ni et al., GCN 45209). The observation started at 2026-07-25T11:52:47 UTC, about 40.9 hr after the WXT detection, with an exposure time of approximately 6.0 ks. On-ground analysis of the FXT data identified an uncatalogued X-ray source within the WXT error circle at R.A. (J2000) = 2.9759 deg and Dec. (J2000) = -12.1022 deg, with an uncertainty of 10 arcsec in radius (90% confidence level, including both statistical and systematic uncertainties).

The source position is consistent with the optical counterpart reported by the Liverpool Telescope (Eyles-Ferris et al., GCN 45202, 45212), the Nordic Optical Telescope (He et al., GCN 45203), COLIBRÍ (Basa et al., GCN 45204), and SVOM/VT (Ma et al., GCN 45205), and also with the weak X-ray excess reported in the first EP-FXT follow-up observation (Ni et al., GCN 45209).

The average FXT spectrum in the 0.5-10 keV band can be fitted with an absorbed power-law model, with the hydrogen column density fixed at the Galactic value of 2.0 x 10^20 cm^-2 and a photon index of 2.96 (+1.53, -1.21). The derived average unabsorbed 0.5-10 keV flux is 3.1 (+3.0, -1.4) x 10^(-14) erg cm^-2 s^-1. The uncertainties quoted above are at the 90% confidence level.

Further EP-FXT follow-up observations will be arranged. Continued multi-band follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory designed to monitor the dynamic soft X-ray sky and perform follow-up observations of newly discovered transients (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).