[vsnet-grb-info 21235] GRB 181013A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  23334
SUBJECT: GRB 181013A: Swift-XRT afterglow detection
DATE:    18/10/13 07:02:02 GMT
FROM:    Paolo D'Avanzo at INAF-OAB  <paolo.davanzo at brera.inaf.it>

J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU),
B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.P. Beardmore (U.
Leicester) and P. D'Avanzo (INAF-OAB) report on behalf of the Swift-XRT
team:

We have analysed 190 s of Photon Counting (PC) mode XRT data for the
Swift/BAT-detected burst GRB 181013A (D'Avanzo et al. GCN Circ. 23332),
collected between T0+115 s and T0+306 s. 

An uncatalogued X-ray source is detected consistent with being within
296 arcsec of the Swift/BAT position and is above the RASS limit and
fading with 3-sigma significance, and is therefore likely the GRB
afterglow.  The position of this source is RA, Dec=207.6518, +40.1082
which is equivalent to:

RA (J2000): 13:50:36.43
Dec(J2000): +40:06:29.5

with an uncertainty of 3.9 arcsec (radius, 90% confidence).  This
position is 4.8 arcmin from the Swift/BAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=7.6 (+0.4, -1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 0.98 (+0.30, -0.19). The
best-fitting absorption column is  consistent with the Galactic value
of 9.4 x 10^19 cm^-2 (Willingale et al. 2013). The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 6.2 x 10^-11 (6.2 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9 (+/-90) x 10^19 cm^-2
Galactic foreground: 9.4 x 10^19 cm^-2
Excess significance: <1.6 sigma
Photon index:	     0.98 (+0.30, -0.19)

If the light curve continues to decay with a power-law decay index of
7.6, the count rate at T+24 hours will be 5.9 x 10^-21 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x
10^-31 (3.6 x 10^-31) 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/00866783/Source1.php.
The results of the full analysis of the XRT observations are available
at http://www.swift.ac.uk/ToO_GRBs/00866783.

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