TITLE:   GCN CIRCULAR
NUMBER:  29622
SUBJECT: GRB 210306A: Swift-XRT refined Analysis
DATE:    21/03/08 16:08:27 GMT
FROM:    Valerio D'Elia at ASDC  <valerio.delia@ssdc.asi.it>

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

We have analysed 10 ks of XRT data for GRB 210306A (D'Elia et al. GCN
Circ. 29597), from 65 s to 182.6 ks after the  BAT trigger. The data
comprise 576 s in Windowed Timing (WT) mode (the first 7 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. Using 3375 s of PC mode data and 9 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 = 129.95774, +60.20523
which is equivalent to:

RA (J2000): 08h 39m 49.86s
Dec(J2000): +60d 12' 18.8"

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

The light curve can be modelled with an initial power-law decay with an
index of alpha=0.435 (+/-0.027), followed by a break at T+1548 s to an
alpha of 1.58 (+/-0.06).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.89 (+0.05, -0.04). The
best-fitting absorption column is  1.53 (+0.15, -0.14) x 10^21 cm^-2,
in excess of the Galactic value of 4.2 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.98 (+0.14, -0.13)
and a best-fitting absorption column of 1.7 (+/-0.4) x 10^21 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.5 x 10^-11 (4.7 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.7 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 4.2 x 10^20 cm^-2
Excess significance: 5.2 sigma
Photon index:	     1.98 (+0.14, -0.13)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01035994.

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