GCN Circular 39869

E. Troja ( U Rome), Y.-H. Yang ( U Rome), S. Dichiara (PSU), M. E. Kabir (U Rome) and H. Sun (NAO, CAS)  report:


We reobserved the field of FRB 20250316A (ATel #17081) with Swift starting on 2025-03-23 (~7 days after the FRB) for a total exposure of 10 ks. 
 
A blind search does not identify any X-ray source consistent with the CHIME localization (ATel #17086). However, a targeted search at the position of EP J120944.2+585060 (Sun et al., ATel #17100) finds a weak X-ray source at RA, Dec (J2000) = 182.4369,  58.8471 deg with a 90% error radius of 6 arcsec (stat+sys), as derived by xrtcentroid. The detection is significant at the >99.7% confidence level (Kraft, Nousek, and Burrows, 1991). A finder chart can be found at:

https://nora.users.roma2.infn.it/FRB250316A.png

Based on a preliminary analysis, we infer a count rate of (7 +/- 3)E-4 cts/s which corresponds to an observed flux of (2.4 +/- 1.0)E-14 erg/cm2/s (0.3-10 keV) assuming an absorbed power-law spectrum with a photon index of 2 and NH = 1.4E+20 cm^-2. Our observations are consistent with the reported flux for EP J120944.2+585060 (Sun et al., ATel #17100) and do not show substantial variability. 

We performed an independent analysis of the first epoch of Swift observations, taken at 12 hrs after the burst (Yang et al., ATel #17101). Using the above XRT position, we derive a 90% upper limit of 7E-4 cts/s,  consistent with our reported value. 
 
Within the XRT localization, no optical counterpart is visible in the Swift UVOT White images down to >22.3 AB mag
 
We thank the Swift team for scheduling these observations.