INTEGRAL detection of GRB131224A: short GRB or Type-I X-ray burst?
The gamma-ray burst (GRB) 131224A was discovered by the Imager on Board
of the INTEGRAL Satellite (IBIS/ISGRI) on 2013 December 24 at 16:54:37
UT, with a fluence in the energy range 20 - 200 keV of about 3x10-8
erg/cm2/sec and a duration of about 0.8 sec
(S. Mereghetti
et al. 2013, GCN Circular #15607). The burst was also observed by
INTEGRAL's Joint European X-Ray Monitor, JEM-X (upper left panel). The
refined coordinates (J2000) are: R.A.= 296.821 deg, Dec.= +31.663 deg
with an uncertainty of 1 arcmin (90% c.l.). The source is located (in
projection) in the Galactic plane. The burst light curve consists of a
nearly symmetric single pulse in the hard IBIS/ISGRI channels (see
bottom, left panel).
No optical fading component was discovered, despite a fast MASTER-II
robotic telescope observation starting about 39 sec after the burst
(E. Gorbovskoy
et al. 2013, GCN Circular #15608), and a deep observation with the
10.4m Gran Telescopio Canarias starting about 1.11 days later (right
panel).
The nature of the event is discussed in a recent paper by Pandey et al..
Originally, the burst was classified as a short duration GRB (sGRB), a
special class of GRBs resulting from binary neutron stars merges. If so,
the emission visible up to 10 sec after the trigger in the soft channels
of JEM-X may be attributed to extended emission (see, e.g.,
P.Yu. Minaev
et al. 2010, AstL 36, 707; arXiv:1009.2685). However, the burst is
unusually soft for a short GRB and is not detected above 70 keV. sGRBs
are related to gravitational wave transient sources, such as the one
registered by the LIGO/Virgo experiments, i.e., GW170817
(B.P. Abbott
et al. 2017, ApJ 848, L12). GRB131224A shows similarities to
GRB170817A which accompanied GW170817.
There may be another plausible explanation. The burst came from
the direction of the Galactic plane, where the largest number of known
LMXBs is located. The event may, therefore, be a Type-I X-ray burst from
a source belonging to our Galaxy. Taking into account the absence of any
persistent X-ray emission in the follow-up Swift/XRT observation
(see B.P.
Gompertz et al. 2013, GCN Circular #15610), the source could be a
new member of the rare class of X-ray bursters with very low (<1e35
erg/sec) luminosity, the so-called "burst-only" sources (e.g.,
Cornelisse
et al. 2002, A&A 392, 885). Further investigations are needed to
discriminate the nature of the event.
