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GCN Circular 17917

GRB 150608A: Swift-XRT afterglow detection
2015-06-08T22:33:22Z (9 years ago)
Maria Grazia Bernardini at INAF/Brera <>
M.G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB), A. Melandri
(INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), A.
Amaral-Rogers (U. Leicester), J.P. Osborne (U. Leicester), A. D'ai
(INAF-IASFPA), A. Maselli  (INAF-IASFPA), D.N. Burrows (PSU) and P.A.
Evans (U. Leicester) report on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the MAXI-detected
burst GRB 150608A (Arimoto et al. GCN Circ. 17912) in a series of
observations tiled on the sky. The total exposure time is 3.1 ks,
distributed over 7 tiles; the maximum exposure at a single sky location
was 955 s. The observations started at T0+8.0 ks, and are entirely in
Photon Counting (PC) mode. 

An uncatalogued X-ray source is detected in the data collected between
T0+9.1 ks and T0+14.7 ks. This source is above the RASS limit and is
therefore likely the GRB afterglow. The position of this source is RA,
Dec=13.1663, -0.6442 which is equivalent to:

RA (J2000): 00:52:39.90
Dec(J2000): -00:38:39.0

with an uncertainty of 3.6 arcsec (radius, 90% confidence).  This
position is 29.1 arcmin from the MAXI position.  The light curve is
consistent with a constant source of mean count rate 8.3e-01 ct/sec. 

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.78 (+0.28, -0.26). The
best-fitting absorption column is  9.8 (+8.3, -6.0) x 10^20 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 (4.0 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     9.8 (+8.3, -6.0) x 10^20 cm^-2
Galactic foreground: 3.8 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.78 (+0.28, -0.26)

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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