GCN Circular 30172

B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L. Page
(U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB),
T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB) and P.A. Evans (U.
Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the Swift/BAT-GUANO
detected burst GRB 210606A (James DeLaunay et al. GCN Circ. 30134
Not found
 
 
),
collecting 7.3 ks of Photon Counting (PC) mode data between T0+62.3 ks
and T0+385.7 ks. 

Four uncatalogued X-ray sources are detected consistent with being
within 394 arcsec of the Swift/BAT position, of which one ("Source 1")
is fading with 4.6-sigma significance, and is therefore likely the GRB
afterglow. Using 4521 s of PC mode data and 5 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 = 170.94110, +0.81293
which is equivalent to:

RA (J2000): 11h 23m 45.86s
Dec(J2000): +00d 48' 46.5"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 6.1 arcmin from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.1 (+0.9, -0.6).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+0.5, -0.4). The
best-fitting absorption column is  1.4 (+1.6, -1.0) x 10^21 cm^-2,
consistent with the Galactic value of 3.8 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 3.5 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+1.6, -1.0) x 10^21 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.8 (+0.5, -0.4)

The results of the XRT-team automatic analysis of the likely afterglow
are at https://www.swift.ac.uk/ToO_GRBs/00021451/Source1.php.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021451.

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