GCN Circular 44136
Subject
EP260327a: refined analysis of the EP-WXT and autonomous EP-FXT observations
Event
Date
2026-03-28T07:22:27Z (4 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. H. Yang(IHEP), C. Y. Dai (NJU), Q. C. Zhao(IHEP), W. F. Wen(SZTU) on behalf of the Einstein Probe (EP) team:
The fast X-ray transient EP260327a was detected by the Wide-field X-ray Telescople (WXT) on board the Einstein Probe (EP) mission (Dai et al, GCN 44126). No reliable optical counterpart has been identified, and only upper limits have been reported (GCNs 44126, 44127, 44129, 44131, 44132, and 44135). The refined analysis of the WXT data shows that the event started at T0=2026-03-27 05:54:41 (UTC), and lasted for approximately 100 seconds. The average WXT 0.5-4 keV spectrum can be fitted with an absorbed power law with a fixed Galactic hydrogen column density of 7.4×10^20 cm^-2, and a photon index of 1.2 (-0.4/+0.4). The derived average unabsorbed 0.5-4 keV flux is 1.9 (-0.4/+0.5) × 10^(-10) erg/s/cm^2.
The Follow-up X-ray Telescope (FXT) on board EP autonomously observed this source a few minutes after the on board trigger. The FXT observation started at 2026-03-27 05:56:22 (UTC, T0+109 s). The effective exposure time of the observation is around 2 ks. On-ground analysis of the FXT data found an uncatalogued source at R.A. = 215.3694, DEC = 42.1379 (J2000) with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average FXT 0.5-10 keV spectra can be fitted with an absorbed power-law with a fixed Galactic hydrogen column density of 7.4×10^20 cm^-2, and a photon index of 3.4 (-0.3/+0.3). The derived average unabsorbed 0.5-10 keV flux is 7.3 (-1.3/+1.3) × 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
Further FXT follow-up observations have been arranged.
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).