TITLE: GCN CIRCULAR NUMBER: 39869 SUBJECT: Swift follow-up observations of FRB250316A and EP J120944.2+585060 DATE: 25/03/25 17:26:37 GMT FROM: Eleonora nora.gsfc@gmail.com
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.
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