GCN Circular 15085

A. Melandri (INAF/OAB), D.N. Burrows (PSU), P. Romano (INAF-IASFPA),  
H.A. Krimm (GSFC/USRA), J.P. Osborne, P.A. Evans (U. Leicester), N.  
Gehrels (GSFC) report on behalf of the Swift team.


We have analysed 14.4 ks of XRT data for GRB 130807A (Melandri et al.  
GCN Circ. 15082
Not found
 
 
), from 88 s to 80 ks after the  BAT trigger. The XRT  
position for this burst is RA, Dec = 269.8007, -27.6159 which is  
equivalent to:

RA (J2000): 17h 59m 12.16s
Dec (J2000): -27d 36' 57.5"

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

In the first orbit (WT mode) the source is initially detected at a  
rate of ~10 XRT count/s, rising to ~100 c/s at the end of the first  
orbit. During this period the spectrum can be fit by an absorbed power- 
law model with a photon index of 1.56 +/- 0.08. The source showed an  
absorption column of (8.6 +/- 1.0) x 10^21 cm^-2, slightly in excess  
with respect to the Galactic value of 6 x 10^21 cm^-2 (Kalberla et al.  
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion  
factor deduced from this spectrum  is 8.0 x 10^-11 (1.2 x 10^-10) erg  
cm^-2 count^-1.

In the second orbit (PC mode) the source had become much fainter  
(~0.03 c/s), and much softer, with a photon index of 2.3 (+0.5,  
-0.4).The source still showed an absorption column (9.0 +/- 5.0 x  
10^21 cm^-2) consistent with the Galactic value. The counts to  
observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from  
this spectrum  is 5.0 x 10^-11 (1.7 x 10^-10) erg cm^-2 count^-1.  
Starting with the second orbit, the light curve rose from a count rate  
of ~0.015 c/s after ~4 ks to a maximum of ~0.06 c/s at ~10 ks (with  
slope of -1.5 +/- 0.1). After that it decayed with a slope of 0.9 +/-  
0.1, reaching the count rate of ~0.012 c/s at ~75 ks.

The light curve behaviour (at late times) is unusual, but not  
unprecedented, for a GRB. The BAT and XRT spectral indices are similar  
to those observed for GRB 060124 (BAT spectrum gamma = 1.89 +/- 0.19,  
XRT-WT spectrum gamma = 1.40+/-0.01, XRT-PC spectrum gamma =  
2.1+/-0.1; Romano et al., 2006, A&A, 456, 917) and for the transient  
supermassive black hole Swift J1644+57 (Burrows et al., 2011, Nature,  
476, 421), but inconsistent with what generally observed for a  
Supergiant Fast X-ray Transient. The dynamical range of GRB 130807A  
(~3300) is similar to what observed for GRB 060124 or for a typical  
SFXT, but very different from what was observed for Swift J1644+57.

The source was not detected in the BAT transient monitor (15-50 keV)  
before 3:03 UT, August 6, 2013, the most recent monitor epoch  
currently available. Events like Swift J1644+57 or SFXTs are usually  
detected in X-rays for several days before the trigger, Swift J1644+57  
behaved similarly. It is therefore likely that BAT trigger 565651 was  
a somewhat unusual GRB rather than a Galactic or extragalactic  
transient, in spite of its proximity to the Galactic center.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/possible_GRBs/00565651/source1