GCN Circular 43179

K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), S. Campana
(INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara
(PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 4.3 ks of XRT data for GRB 251221A, from 122 s to 56.3
ks after the   trigger. The data comprise 8 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.13 (+0.17, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.49 (+0.19, -0.18). The
best-fitting absorption column is  1.11 (+0.24, -0.22) x 10^22 cm^-2,
in excess of the Galactic value of 7.3 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 6.8 x 10^-11 (9.5 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.11 (+0.24, -0.22) x 10^22 cm^-2
Galactic foreground: 7.3 x 10^21 cm^-2
Excess significance: 2.8 sigma
Photon index:	     1.49 (+0.19, -0.18)

If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 1.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-13 (1.8 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01426088.

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