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GCN Circular 31072

GRB 211107B: Swift-XRT afterglow detection
2021-11-10T11:36:34Z (3 years ago)
Phil Evans at U of Leicester <>
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.
Melandri (INAF-OAB), T. Sbarrato (INAF-OAB) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has conducted further observations of the field of the
Swift/BAT-detected burst GRB 211107B (Swift/BAT did not successfully
trigger on GRB 211107B et al. GCN Circ. 31057). The observations now
extend from T0+124.3 ks to T0+147.5 ks. The source previously reported
by D'Ai et al. (GCN Circ. 31064), "Source 1", is fading with 1.8 sigma
significance and thus is believed to be the GRB afterglow. Using 2958 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 = 65.80812, -27.56084 which is equivalent

RA (J2000): 04h 23m 13.95s
Dec(J2000): -27d 33' 39.0"

with an uncertainty of 2.7 arcsec (radius, 90% confidence). This
position is 51 arcsec from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.84 (+0.04, -2.67).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.5 (+0.9, -0.7). The
best-fitting absorption column is  1.6 (+2.5, -1.2) x 10^21 cm^-2,
consistent with the Galactic value of 4.3 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 2.7 x 10^-11 (4.5 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.6 (+2.5, -1.2) x 10^21 cm^-2
Galactic foreground: 4.3 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     2.5 (+0.9, -0.7)

The results of the XRT-team automatic analysis are available at
The results of the full analysis of the XRT observations are available

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