TITLE:   GCN CIRCULAR
NUMBER:  21989
SUBJECT: GRB 171010A: Swift-XRT afterglow detection
DATE:    17/10/11 13:48:44 GMT
FROM:    Kim Page at U.of Leicester  <klp5@leicester.ac.uk>

A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S.L. Gibson
(U. Leicester), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester),
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
Fermi/LAT-detected burst GRB 171010A (Omodei et al. GCN Circ. 21985),
collecting 5.0 ks of Photon Counting (PC) mode data between T0+24.2 ks
and T0+41.7 ks. 

Four uncatalogued X-ray sources are detected, of which one ("Source 2")
is above the RASS limit, and is therefore likely the GRB afterglow.
Using 4537 s of PC mode data and 7 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 = 66.58092, -10.46325 which is
equivalent to:

RA (J2000): 04h 26m 19.42s
Dec(J2000): -10d 27' 47.7"

with an uncertainty of 1.4 arcsec (radius, 90% confidence). This
position is 10.5 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.2 (+/-0.3).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.01 (+/-0.15). The
best-fitting absorption column is  3.6 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 7.0 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 3.3 x 10^-11 (5.0 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.6 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 7.0 x 10^20 cm^-2
Excess significance: 7.4 sigma
Photon index:	     2.01 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
2.2, the count rate at T+24 hours will be 0.054 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x
10^-12 (2.7 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020778.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020778.

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