TITLE: GCN CIRCULAR NUMBER: 39681 SUBJECT: GRB 250311A: Swift-XRT observations DATE: 25/03/12 09:58:00 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

J.A. Kennea (PSU), D.N. Burrows (PSU), M. A. Williams (PSU), A.P. Beardmore (U. Leicester), J.P. Osborne (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAR) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected burst GRB 250311A/MAXI J1457-026 (GCN Circ. 39660) in a series of observations tiled on the sky. The total exposure time is 1.4 ks, distributed over 4 tiles; the maximum exposure at a single sky location in the tiling was 731 s. The data were collected between T0+34.4 ks and T0+36.0 ks, and are entirely in Photon Counting (PC) mode.

Two uncatalogued X-ray sources have been detected, however none of them is above the RASS limit or shows definitive signs of fading. Therefore, at the present time we cannot identify which, if any, is the afterglow. Details of these sources are given below:

Source 1: RA (J2000.0): 224.4476 = 14:57:47.43 Dec (J2000.0): -2.7859 = -02:47:09.2 Error: 9.8 arcsec (radius, 90% conf.) Count-rate: (9.8 [+6.3, -4.5])e-3 ct s^-1 Distance: 777 arcsec from MAXI position.

Source 2: RA (J2000.0): 224.4381 = 14:57:45.15 Dec (J2000.0): -2.7813 = -02:46:52.7 Error: 8.7 arcsec (radius, 90% conf.) Count-rate: 0.0134 [+0.0074, -0.0055] ct s^-1 Distance: 744 arcsec from MAXI position.

We note that these observations do not cover the localisation of X-ray transient EP250311a (GCN 39664), which may be associated with the same trigger.

The results of the XRT-team automatic analysis of the tiled XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00135.

This circular is an official product of the Swift-XRT team.

View this GCN Circular online at https://gcn.nasa.gov/circulars/39681. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...