TITLE:   GCN CIRCULAR
NUMBER:  15280
SUBJECT: GRB 130925A: continuing Swift/XRT monitoring
DATE:    13/10/01 17:52:33 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani@dark-cosmology.dk>

D. Malesani (DARK/NBI), S. B. Cenko (NASA/GSFC), D. N. Burrows (PSU), A. 
Y. Lien (NASA/GSFC/ORAU), P. A. Evans (U. Leicester), and N. Gehrels 
(NASA/GSFC) report:

Swift has been continuously monitoring the X-ray counterpart of GRB 
130925A (Lien et al., GCN 15246), with data currently extending up to 
512 ks after the trigger. The intense X-ray flaring activity reported by 
Burrows et al. (GCN 15253) and Evans et al. (GCN 15254) ceased between 
12 and 28 ks after the trigger. The X-ray light curve from 27.9 to 512.4 
ks can be modelled with an initial power-law decay with an index of 
alpha = 0.84 +/- 0.03, followed by a possible break at 310 (+60, -50) ks 
to an alpha of 1.38 (+0.47, -0.23). The updated X-ray light curve is 
available at the following URL:

http://www.swift.ac.uk/xrt_curves/00571830/

While the duration of the prompt emission and the flaring activity are 
extraordinary among GRBs (Suzuki et al., GCN 15248; Fitzpatrick, GCN 
15255; Markwardt et al., GCN 15257; Savchenko et al., GCN 15259; 
Golenetskii et al., GCN 15260; Jenke, GCN 15261; Hurley et al., GCN 
15278), the late-time behaviour is typical of long-duration GRB 
afterglows, and quite different from that of the TDE Swift J1644+57, 
which showed continuous flares and dips, with no regular power-law decay 
(e.g. Burrows et al. 2011, Nat, 476, 421). There is also no prior 
detection of the source in BAT before the trigger (Markwardt et al., GCN 
15257).

The extremely long-lived high-energy emission coupled with the 
relatively steady power-law X-ray decay at t > 20 ks is reminiscent of 
the so-called "ultra-long" GRBs (Levan et al. 2013, arXiv:1302.2352), 
including GRB 101225A (Thoene et al. 2011, Nat, 480, 72; Campana et al. 
2011, Nat, 480, 69), GRB 111209A (Gendre et al. 2013, ApJ, 766, 30), and 
GRB 121027A. The origin of these events and their relation to 
traditional long-duration GRBs (i.e., the core-collapse of a massive 
star) remain controversial, so we encourage continued multi-wavelength 
follow-up of this object.