GCN Circular 20025

L.M. McCauley (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), B. Mingo (U. Leicester), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), B. Sbarufatti
(INAF-OAB/PSU) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT team:

Swift-XRT has performed follow-up observations of the INTEGRAL-detected
burst GRB 161010A (Mereghetti et al. GCN Circ. 20021
Not found
 
 
), collecting 5.0
ks of Photon Counting (PC) mode data between T0+12.0 ks and T0+30.3 ks.


An uncatalogued X-ray source is detected consistent with being within
148 arcsec of the INTEGRAL position and is above the RASS limit and
fading with 3-sigma significance, and is therefore likely the GRB
afterglow. Using 123 s of PC mode data and 1 UVOT image, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 275.21435, -28.78515
which is equivalent to:

RA (J2000): 18h 20m 51.44s
Dec(J2000): -28d 47' 06.6"

with an uncertainty of 2.9 arcsec (radius, 90% confidence). This
position is 12 arcsec from the INTEGRAL position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=1.36 (+0.22, -0.21).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+0.18, -0.17). The
best-fitting absorption column is  3.1 (+0.9, -0.6) x 10^21 cm^-2,
consistent with the Galactic value of 2.5 x 10^21 cm^-2 (Willingale et
al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux
conversion factor deduced from this spectrum  is 4.2 x 10^-11 (5.7 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     3.1 (+0.9, -0.6) x 10^21 cm^-2
Galactic foreground: 2.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.78 (+0.18, -0.17)

If the light curve continues to decay with a power-law decay index of
1.36, the count rate at T+24 hours will be 0.023 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.9 x
10^-13 (1.3 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis of the likely afterglow
are at http://www.swift.ac.uk/ToO_GRBs/00020702/index_18.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00020702.

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