GCN Circular 29625

M. Perri (SSDC & INAF-OAR), V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti
(PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U.
Leicester), K.L. Page (U. Leicester) and A. D'Ai (INAF-IASFPA) report
on behalf of the Swift-XRT team:

We have analysed 11 ks of XRT data for GRB 210305A (D'Ai et al. GCN
Circ. 29595
Not found
 
 
), from 67 s to 178.1 ks after the  BAT trigger. The data
comprise 184 s in Windowed Timing (WT) mode (the first 9 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 2502 s of PC mode data and 4 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 = 319.81054, +34.55403
which is equivalent to:

RA (J2000): 21h 19m 14.53s
Dec(J2000): +34d 33' 14.5"

with an uncertainty of 1.4 arcsec (radius, 90% confidence).

The late-time light curve (from T0+5.3 ks) can be modelled with a
power-law decay with a decay index of alpha=0.91 (+0.10, -0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.44 (+/-0.08). The
best-fitting absorption column is  3.2 (+0.5, -0.4) x 10^21 cm^-2, in
excess of the Galactic value of 2.0 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.84 (+/-0.16) and a
best-fitting absorption column of 4.0 (+0.9, -0.8) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.3 x 10^-11 (6.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     4.0 (+0.9, -0.8) x 10^21 cm^-2
Galactic foreground: 2.0 x 10^21 cm^-2
Excess significance: 4.1 sigma
Photon index:	     1.84 (+/-0.16)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01035922.

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