MeV - GeV emission from a Supergiant Fast X-ray Transient ?
AGILE/GRID and Fermi/LAT observations have recently indicated the
existence of a possible population of transient MeV - GeV sources
located on the Galactic plane and characterized by fast flares
lasting only a very few days. Notably, no blazar-like counterparts
are known within their error circles so they could represent a
completely new class of high-energy Galactic fast transients. The
task of identifying their counterparts at lower energies remains
very challenging, mainly because of their large error circles.
INTEGRAL/IBIS observations are particularly suited to search for
reliable best candidate counterparts. In particular, recent results
suggest that reliable best candidate counterparts could be found
among the members of Supergiant Fast X-ray Transients (SFXTs), merely
based on intriguing hints such as spatial correlations and common
transient behaviours on similar, though as yet not simultaneous, short
time scales. The so far proposed associations represent an important
first step towards obtaining reliable test cases on which to concentrate
further efforts to obtain quantitative proofs for a real physical
association. In this respect, so far, the best test case is represented
by the recently proposed association between IGR J17354-3255 and
AGL J1734-3310.
IGR J17354-3255 is an unidentified hard X-ray source whose possible
nature of intermediate SFXT was recently unveiled thanks to INTEGRAL's
long term monitoring in the hard X-ray band 18-60 keV. Several flares
have been detected with typical durations from a few hours to a few
days while the dynamic range is as high as 200. The flares are
mainly clustered around the periastron passage of the likely neutron
star compact object during its 8.4 days orbit around the companion
donor star. Interestingly, IGR J17354-3255 is the only hard X-ray
source located inside the error circle of the unidentified transient
MeV-GeV source AGL J1734-3310. AGILE detected several recurrent high
energy flares (E>100 MeV) with typical duration of 1-2 days, in
particular two of them were quasi-simultaneous with hard X-ray activity
detected by INTEGRAL/IBIS from IGR J17354-3255, providing a possible
quantitative proof for a real physical association. Further studies
of IGR J17354-3255/AGL J1734-3310 with AGILE and INTEGRAL are under
way. If confirmed, the implications of SFXTs producing MeV-GeV emission
are huge, both theoretically and observationally, and would add a
further extreme characteristic to this already extreme class of
transient sources.
The figure shows a significance map obtained with INTEGRAL/IBIS in
the 18-60 keV band in equatorial co-ordinates (J 2000). IGR J17354-3255
is located inside the error circle (radius 0.46 deg) determined for
AGL J1734-3310.
