GCN Circular 40472

A.P. Beardmore (U. Leicester), E. Ambrosi  (INAF-IASFPA) , M. Capalbi
(INAF-IASFPA), M. Perri (SSDC & INAF-OAR), J.A. Kennea (PSU), D.N.
Burrows (PSU), M.A. Williams (PSU), J.P. Osborne (U. Leicester) and
P.A. Evans report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 250516A, from 108 s to 23.8
ks after the   trigger. The data comprise 151 s in Windowed Timing (WT)
mode with the remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+5.9 ks) can be modelled with a
power-law decay with a decay index of alpha=1.45 (+0.18, -0.17).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.44 (+0.13, -0.12). The
best-fitting absorption column is  3.5 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 5.8 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.98 (+/-0.12) and a
best-fitting absorption column of 3.0 (+/-0.5) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.8 x 10^-11 (5.6 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.0 (+/-0.5) x 10^21 cm^-2
Galactic foreground: 5.8 x 10^20 cm^-2
Excess significance: 8.2 sigma
Photon index:	     1.98 (+/-0.12)

If the light curve continues to decay with a power-law decay index of
1.45, the count rate at T+24 hours will be 5.8 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.2 x
10^-13 (3.3 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01314210.

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