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
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 http://www.swift.ac.uk/xrt_products/TILED_GRB00045/index_2.php.
The results of the full analysis of the tiled XRT observations are
available at http://www.swift.ac.uk/xrt_products/TILED_GRB00045.
This circular is an official product of the Swift-XRT team.