GCN Circular 39820

E. Ambrosi  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC &
INAF-OAR), M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU),
J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore
(U. Leicester) and P.A. Evans (U. Leicester) reports on behalf of the
Swift-XRT team:

We have analysed 6.8 ks of XRT data for the Swift-BAT-detected burst
GRB 250321A, from 108 s to 39.4 ks after the  Swift-BAT trigger. The
data comprise 9 s in Windowed Timing (WT) mode (taken while Swift was
slewing), with the remainder in Photon Counting (PC) mode. Using 1662 s
of PC mode data and 3 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 = 295.09844, +21.04038 which is equivalent
to:

RA (J2000): 19h 40m 23.62s
Dec(J2000): +21d 02' 25.4"

with an uncertainty of 2.6 arcsec (radius, 90% confidence). This
position is 72 arcsec from the Swift-BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.13 (+0.11, -0.10).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+1.3, -1.0). The
best-fitting absorption column is  3.7 (+3.4, -2.2) x 10^22 cm^-2, in
excess of the Galactic value of 1.1 x 10^22 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 7.3 x 10^-11 (2.4 x 10^-10) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.7 (+3.4, -2.2) x 10^22 cm^-2
Galactic foreground: 1.1 x 10^22 cm^-2
Excess significance: 2.0 sigma
Photon index:	     2.2 (+1.3, -1.0)

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 6.9 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.0 x
10^-14 (1.6 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/01297508.

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