GCN Circular 38073
Subject
EP241101a: EP-FXT observations update
Date
2024-11-04T16:27:13Z (a month ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q. Y. Wu (NAO, CAS), S. M. Jia, Y. Chen (IHEP, CAS), Y. F. Liang (PMO, CAS), Q. C. Liu (THU), D. H. Zhao, H. N. Yang, W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:
Following the on-board detection of the fast X-ray transient EP241101a by EP-WXT, an autonomous follow-up observation was conducted by EP-FXT approximately 45 minutes later, with no counterpart detected (Liang at al., GCN 38039). We further analyzed the data during the FXT's slew to the target before the observation commenced, an uncatalogued X-ray source was found at R.A. = 37.7526 deg, DEC = 22.7175 deg (J2000) with an uncertainty of about 10 arcsecs (radius, 90% C.L. statistical and systematic). The slew observation began from 2024-11-01T23:54:21 (UTC), and observed the source from 2024-11-01T23:57:02 to 2024-11-01T23:58:02, which is from T0+253s to T0+313s (T0 = 2024-11-01T23:52:49). The spectrum of the source can be fitted by an absorbed power law with a photon index of 2.4(+/-0.4) (with a column density fixed at the Galactic one of 1.46 x 10^21 cm^-2), giving an average unabsorbed flux of 4.4(+0.7/-0.6) x 10^-11 erg/s/cm^2 in the 0.5-10 keV band. The FXT source is about 3 arcmin away from the optical candidate reported by Busmann et al. (GCN 38064), indicating that the optical source may not be associated with EP241101a.
We have conducted another FXT observation of EP241101a from T0+31.7ks to T0+43.7 ks with an exposure time of 6.5 ks. No significant source was detected within the WXT error circle, putting a flux upper limit of 1.5 x 10^(-14) erg/s/cm^2 in the 0.5-10 keV.
Given the rapid X-ray decay and the absence of an optical counterpart down to ~24.5 magnitude (Perez-Garcia et al., GCN 38047; Lipunov et al., GCN 38049; Adami et al., GCN 38060; Aryan et al., GCN 38061; Busmann et al., GCN 38064), we encourage further observations to determine the nature of the source.
Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with on-board X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is an international collaborative mission led by the Chinese Academy of Sciences (CAS) and participated by the European Space Agency (ESA), the Max-Planck Institute for Extraterrestrial Physics (MPE) in Germany and the Centre National d’Etudes Spatiales (CNES) in France.