GCN 44075: EP260321a: refined analysis of the EP-WXT and EP-FXT observations, implying a possible supernova shock breakout candidate

Q. J. Huang (PMO, CAS), Z.-C. Zou (NJU), D. Y. Li, X. Mao, H. W. Pan (NAO, CAS) on behalf of the Einstein Probe (EP) team:

The fast X-ray transient EP260321a was detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission (Huang et al., GCN 44068). The transient triggered WXT at T0=2026-03-21T12:23:07 (UTC). The WXT observation lasted for approximately 432 seconds, and was interrupted due to the autonomous follow-up observation. The WXT 0.5-4 keV spectrum was soft, and can be fitted with an absorbed black body with the absorption fixed at the Galactic value of 2.7 × 10^(20) cm^-2 and a temperature of 164 (-29/+40) eV. The derived average unabsorbed 0.5-4 keV flux is 8.0 (-1.9/+2.2) × 10^(-11) erg/s/cm^2.

The Follow-up X-ray Telescope (FXT) on board EP observed this source autonomously at 2026-03-21T12:35:02 (UTC, T0+12min). The exposure time of this observation is 4266s. On-ground analysis of the FXT data found an uncatalogued fading source within the WXT error circle at R.A., Dec. = 149.9287, 0.4177 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). A significant decay was detected in the FXT light curve during the initial phase of FXT observation. The average 0.5-10 keV spectrum can be fitted with an absorbed black body with Galactic absorption and a temperature of 121 (-3/+3) eV. The derived average unabsorbed 0.5-10 keV flux is 9.8 (-0.3/+0.3) × 10^(-12) erg/s/cm^2. All quoted errors are at the 90% confidence level. We note that within the FXT error circle, there is a galaxy 2dFGRS TGN352Z077, at a redshift of 0.0345. If the transient is associated with this galaxy, the corresponding luminosity of the WXT detection would be approximately 2.2 × 10^(44) erg/s. Given its very soft spectrum, rapid decay and the corresponding luminosity at the assumed redshift, we tend to consider EP260321a as possibly a supernova shock breakout candidate.

No optical counterpart of EP260321a has been detected so far (Lipunov et al., GCN 44069; Lee et al., GCN 44070; Ma et al., GCN 44074) , and further mutil-band follow-up observations are encouraged to explore the nature of EP260321a.

A follow-up observation with the EP-FXT was planned, 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).