[vsnet-grb-info 16695] GRB 150627A: Swift-XRT afterglow detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  17976
SUBJECT: GRB 150627A: Swift-XRT afterglow detection
DATE:    15/06/28 01:43:15 GMT
FROM:    Marissa McCaule at PSU  <marissamc at swift.psu.edu>

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 team:

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
Dec(J2000): -51:29:23.9

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
cm^-2 count^-1. 

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
at http://www.swift.ac.uk/ToO_GRBs/00020539.



Paul Kuin (MSSL-UCL), Marissa McCauley (PSU) and Daniele Malesani
(DARK/NBI)
report on behalf of the Swift/UVOT team:

The Swift/UVOT and XRT began settled observations of the field of GRB
150627A
33622 s after the LAT trigger (H.-F. Yu et al., GCN Circ.17975).

An optical afterglow consistent with the XRT position

is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial
exposures
are:

Filter	       T_start(s)   T_stop(s)	   Exp(s)	  Mag

white		 33963	      34289	     335	 19.11+/-0.10
white		 39860	      40112	     248	 19.55+/-0.17
white		 45659	      45863	     204	 19.28+/-0.14
v		 34304	      34601	     298	 19.02+/-0.32
v		 40117	      40346	     225	 18.98+/-0.36
u		 33622	      38243	     577	 18.90+/-0.10
u		 43668	      45654	     966	 19.00+/-0.11

The magnitudes in the table are not corrected for the Galactic
extinction
due to the reddening of E(B-V) = 0.32 in the direction of the burst
(Schlegel et al. 1998).

An optical afterglow consistent with the XRT position is detected
at RA,Dec = 117.47108,-51.489417 deg (J2000), which is:
RA (J2000):  07:49:53.06
Dec(J2000): -51:29:21.9
With a 1" confidence.

The photometric decay is not so large that we can be sure this is the
GRB transient.

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