GCN Circular 43213

A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAR), T. Sbarrato
(INAF-OAB), P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), D.N. Burrows
(PSU), S. Lanava (PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the
SVOM/ECLAIRs-detected burst GRB 251222A. We searched for X-ray sources
in  2.0 ks of Photon Counting (PC) mode data. The total exposure at the
position of the afterglow (see below) is 2.0 ks, obtained between
T0+10.7 ks and T0+17.4 ks.

An uncatalogued X-ray source is detected within the estimated 3-sigma
SVOM/ECLAIRs error region (49 arcsec) and is above the RASS 3-sigma
upper limit at this position, and is therefore likely the GRB
afterglow. Using 1130 s of PC mode data and 2 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 77.19502, -7.21354
which is equivalent to:

RA (J2000): 05h 08m 46.81s
Dec(J2000): -07d 12' 48.7"

with an uncertainty of 2.5 arcsec (radius, 90% confidence). This
position is 36 arcsec from the SVOM/ECLAIRs position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.9 (+0.8, -0.7).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.92 (+0.31, -0.20). The
best-fitting absorption column is  1.05 (+0.91, -0.12) x 10^21 cm^-2,
consistent with the Galactic value of 9.3 x 10^20 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 3.6 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.05 (+0.91, -0.12) x 10^21 cm^-2
Galactic foreground: 9.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.92 (+0.31, -0.20)

If the light curve continues to decay with a power-law decay index of
1.9, the count rate at T+24 hours will be 4.7 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x
10^-13 (2.0 x 10^-13) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021895/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021895.

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