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GCN Circular 19585

GRB 160625B: Swift-XRT afterglow detection
2016-06-26T06:54:13Z (8 years ago)
Antonino D'Ai at IASF-PA <>
A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), D.N.
Burrows (PSU), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), S.L. Gibson
(U. Leicester), J.P. Osborne (U. Leicester) 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 160625B (Dirirsa et al. GCN Circ. 19580)
in a series of observations tiled on the sky. The total exposure time
is 2.7 ks, distributed over 8 tiles; the maximum exposure at a single
sky location was 1.1 ks. The data were collected between T0+9.6 ks and
T0+10.0 ks, and are entirely in Photon Counting (PC) mode. 

An uncatalogued X-ray source is detected and is above the RASS limit,
and is therefore likely the GRB afterglow.  The position of this source
is RA, Dec=308.5969, +6.9196 which is equivalent to:

RA (J2000): 20:34:23.25
Dec(J2000): +06:55:10.5

with an uncertainty of 3.8 arcsec (radius, 90% confidence).  This
position is 19.7 arcmin from the Fermi/LAT position. 

The light curve can be modelled with a power-law decay with a decay
index of alpha=3.156 (+0.011, -2.764).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.09 (+0.20, -0.19). The
best-fitting absorption column is  1.6 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 9.8 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 3.1 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.6 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 9.8 x 10^20 cm^-2
Excess significance: 1.8 sigma
Photon index:	     2.09 (+0.20, -0.19)

If the light curve continues to decay with a power-law decay index of
3.156, the count rate at T+24 hours will be 6.1 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x
10^-13 (2.6 x 10^-13) erg cm^-2 s^-1.

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

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