GCN Circular 31930

B. Sbarufatti (PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB), J. D.
Gropp (PSU) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the
Swift/BAT-GUANO-detected burst GRB 220418B, collecting 4.8 ks of Photon
Counting (PC) mode data between T0+113.8 ks and T0+148.8 ks. These
observations were delayed due to the Moon observing constraint.

Due to a high XRT temperature and consequent high background, the data
contain many low significance detections (ranked as "poor") which are
likely to be spurious, but no objects are detected within the BAT/GUANO
error region. We therefore conclude that no XRT afterglow to this
object has been found.

The 3-sigma upper limit is 2.3e-3 ct/sec, which, assuming a typical GRB
spectral, equates to an observed 0.3-10 keV flux of ~9e-14 erg/cm^2/s.

The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021486.

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