V. Savchenko, V. Beckmann (APC), C. Ferrigno, E. Bozzo, M. Beck (ISDC),
J. Borkowski (CAMK/Torun), D. G��tz (CEA/Saclay), S. Mereghetti
(INAF/IASF-Milano), A. von Kienlin, A. Rau (MPE), and K. Hurley
The possible GRB or tidal disruption event (Burrows et al, GCN 15253,
15257) 130925A, detected by Swift/BAT (Lien et al. GCN 15246, Markwardt
et al GCN 15257) and Fermi/GBM (Fitzpatrick et al, GCN 15255), has been
independently detected by the SPI Anti-Coincidence System (ACS) on-board
The SPI-ACS light curve reveals rich multi-peak structure with a total
duration of about 5000 seconds and evolution on a time scale as small
as 2 seconds.
The initial burst, detected by Fermi/GBM at 2013-09-25T04:09:26, also
triggered SPI-ACS at 04:09:25 (T0 hereafter). The light curve displays
two fast-rise slow-decay pulses, typical for a GRB, lasting a total of
about 200 seconds and reaching a peak count rate of about 3500
counts/s (approximately corresponding to 3.5e-7 erg/cm2/s in the 75
keV-1 MeV range, Vigano' and Mereghetti 2009, arXiv:0912.5329, assuming
a GRB-like spectrum) over 1s at T0+13s.
The second episode is brighter. It starts at about T0+1800s,
(04:42:00) and lasts for ~1700 seconds with a peak count rate of about
5000 counts/s (approximately 5e-7 erg/cm2/s) over 1s at T0+2770s
We note that this episode was not observed by Swift.
The last, weaker, episode spans from ~T0+4000 to ~T0+4500 seconds with a
peak count rate of ~1000 count/s (1e-7 erg/cm2/s).
A possible precursor observed by Fermi/GBM at 03:56:23.29 (T0-900) is
not visible in the SPI-ACS data. (note, however, that Fermi/GBM is
more sensitive to photons below 100keV than SPI-ACS).
Since SPI-ACS has no imaging capabilities, it is not possible to
conclude with absolute certainty that all three episodes, spanning
over almost 5000 seconds, belong to the same source. However, a chance
temporal coincidence of these events in the SPI-ACS data is unlikely.
No strong solar activity was reported at the time of the event.
Rapid X-ray flaring detected by XRT (Evans et al, GCN 15251, Burrows
et al, GCN 15253) for at least 10000 seconds strengthens the hypothesis
that this source was active much longer than a typical GRB.
The SPI-ACS light curve of this burst, in 10s bins is available:
Zoomed on the first and the second peaks:
The original SPI-ACS light curve of this event is available at:
Arbitrary interval of the SPI-ACS light curve can be accessed with:
All SPI-ACS light curves are available (both as images and data files)
at http://isdc.unige.ch/Soft/ibas/ibas_acs_web.cgi. The light curves,
binned at 50 ms, are derived from 91 independent detectors with
different lower energy thresholds (mainly between 50 keV and 150 keV).
SPI-ACS has no upper energy threshold and can detect photons with
energies up to at least 100 MeV. The ACS response varies as a function
of the GRB incident angle. For these reasons we caution that the count
rates cannot be easily translated into physical flux units.