GCN Circular 29624

A. D'Ai (INAF-IASFPA), J. D. Gropp (PSU), J.A. Kennea (PSU), A.
Tohuvavohu (U. Toronto), K.L. Page (U. Leicester), A.P. Beardmore (U.
Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB) and E.
Ambrosi (INAF-IASFPA) report on behalf of the Swift-XRT team:

We have analysed 7.0 ks of XRT data for GRB 210307A (Ambrosi et al. GCN
Circ. 29610
Not found
 
 
), from 97 s to 40.0 ks after the  BAT trigger. The data
comprise 149 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. Using 2981 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 =
119.23831, -66.97092 which is equivalent to:

RA (J2000): 07h 56m 57.19s
Dec(J2000): -66d 58' 15.3"

with an uncertainty of 1.5 arcsec (radius, 90% confidence). 

The light curve can be modelled with a series of power-law decays
followed 
by a plateau. The initial decay index is alpha= : 2.11 (+0.10, -0.11).
At T= T0+242.1 s the decay index is alpha=7.88 (+0.12, -2.58) before
breaking again at T=T0+280.5 s with an index alpha= 3.2 (+0.6, -0.4).
After T>T0+10^4 s the lightcurve is in a long lasting plateau phase.

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.520 (+0.035, -0.026). The
best-fitting absorption column is  consistent with the Galactic value
of 1.4 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.75 (+0.16, -0.14) and a best-fitting absorption
column of 1.75 (+0.61, -0.30) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 4.0 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.75 (+0.61, -0.30) x 10^21 cm^-2
Galactic foreground: 1.4 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.75 (+0.16, -0.14)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01036139.

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