B.P. Gompertz (U. Leicester), 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), J.P. Osborne (U. Leicester), K.L. Page (U.
Leicester) and P.A. Evans (U. Leicester) report on behalf of the
Swift-XRT has performed follow-up observations of the
Fermi/LAT-detected burst GRB 150627A (Arimoto et al. GCN Circ. 17971),
collecting 4.9 ks of Photon Counting (PC) mode data between T0+33.6 ks
and T0+51.0 ks.
An uncatalogued X-ray source is detected inside or close to the
Fermi/LAT error region and is above the RASS limit, and is therefore
likely the GRB afterglow. The position of this source is RA,
Dec=117.4706, -51.4900 which is equivalent to:
RA (J2000): 07:49:52.94
with an uncertainty of 3.5 arcsec (radius, 90% confidence). This
position is 4.3 arcmin from the Fermi/LAT position.
The light curve can be modelled with a power-law decay with a decay
index of alpha=2.2 (+/-0.3).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.99 (+/-0.13). The
best-fitting absorption column is 3.5 (+/-0.6) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 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 3.8 x 10^-11 (5.8 x 10^-11) erg
A summary of the PC-mode spectrum is thus:
Total column: 3.5 (+/-0.6) x 10^21 cm^-2
Galactic foreground: 2.6 x 10^21 cm^-2
Excess significance: 2.8 sigma
Photon index: 1.99 (+/-0.13)
If the light curve continues to decay with a power-law decay index of
2.2, the count rate at T+24 hours will be 0.066 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 2.5 x
10^-12 (3.8 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/00020539/index_1.php.
The results of the full analysis of the XRT observations are available
[GCN OPS NOTE(28jun15): Per author's request, the copy of the
Swift-UVOT circular on this burst was deleted. See GCN 17977 for the
formal UVOT report on this burst.]