In the last couple of years, INTEGRAL/IBIS has been joined in operation by the
Fermi gamma-ray satellite, providing a coverage of the sky over 10 orders of
magnitude in energy, from keV to GeV. Many of the detected Fermi sources are
expected to emit around 20 keV, i.e. in the IBIS regime. With the aim of
investigating such a link, the positions of the 205 Fermi/LAT bright sources
have been cross-correlated with the fourth INTEGRAL/IBIS catalog. The main result
is that only a minuscule part (14) of the more than 720 sources detected by INTEGRAL
and the population of 205 Fermi/LAT sources are detected in both spectral regimes.
Their great majority are optically identified Blazars (10) complemented by two
isolated pulsars (Crab and Vela) and two high-mass X-ray binaries (LS I +61 303 and
LS 5039).
Two more possible associations have been found: one is related to the massive
colliding wind binary system Eta Carinae, discovered to be a soft gamma ray emitter
by recent INTEGRAL observations, and the other one is related to the still
unidentified INTEGRAL source IGR J17459-2902 in the Galactic Center region. For the
remaining 189 Fermi/LAT sources no INTEGRAL counterpart was found, providing a
2 sigma upper limit.
The Figure above - which is our 100th INTEGRAL Picture of the Month - shows the
gamma-ray flux (100 MeV-1 GeV) of each Fermi source as a function of the corresponding
20-40 keV IBIS flux. Light blue points are blazars, green are pulsars, magenta are
HMXBs, dark blue is Eta Carinae, white is IGR J17459-2902 and finally yellow points
refer to sources emitting at high energy and not in the IBIS range (2 sigma upper
limits).
We note that no LAT sources (except for the peculiar object ETA Carinae) have been
detected by IBIS at a flux below 1 mCrab despite the fact that a large fraction of
the IBIS sky has a detection limit well below this value (typically about 0.1 mCrab)
and is well populated with hard X-ray sources.
This indicates that the typical LAT source is either very strong or very weak in
the IBIS energy range providing important information on the spectral energy
distribution of these objects.
