Hard X-ray emission from the Galactic ridge (GRXE) mapped with INTEGRAL
The nature of the GRXE is under scientific debate since its discovery
in 1970's. It is observed as extended emission along the Galactic disk
with a bulge near the center. The question was: is the GRXE truly diffuse
or is it composed from a large number of unresolved point sources?
Recently it was shown that there are good reasons to believe that the
GRXE (at least a dominant fraction of it) is due to a large number of
very weak X-ray sources - accreting white dwarf binaries and coronally
active stars. The implications of this conclusion are: (i) a prediction
of the spectral shape of the ridge emission and (ii) the spatial
distribution of the GRXE. Regarding (i) the spectrum of the ridge
emission should have an exponential cutoff at energies higher than ~20
keV, which approximately corresponds to the proton binding energy of the
white dwarf, and (ii), the spatial distribution of the GRXE should trace
the density distribution of Galactic stars.
These predictions have now been verified by analysing INTEGRAL observational
data obtained during its four years of operation. It has been possible
to map the weak hard X-ray glow of the Galaxy and measure its
spectrum. The map does not correlate well with the Gamma-ray map of
the Milky Way, strongly suggesting that X-ray emission is not
generated by the cosmic rays interaction with ISM. Instead, the
intensity of the GRXE in hard X-rays traces well the stellar mass
density distribution, thus providing strong support to the idea that
the bulk of GRXE is provided by weak compact sources. In particular,
for the considered (17-100) keV energy band, the dominant contribution
to the GRXE should come from accreting white dwarf binaries. This
result was further strengthened by the detection of a high energy
cutoff in the ridge spectrum.
Top figure:
The GRXE map (galactic co-ordinates) in the 17-60 keV energy band obtained by IBIS telescope
onboard INTEGRAL. The contours show the stellar mass distribution
tracer - Galaxy emission in near-infrared spectral domain at 4.9
micron (convolved with IBIS collimator response). NIR data are from DIRBE/COBE (
LAMBDA project).
Bottom figure:
Schematic broad band luminosity spectrum of the Galaxy. RXTE and
INTEGRAL data of the GRXE are shown in blue. The spectrum
has an energy cut-off at 60 keV. On the
other hand, the Galactic background emission caused by interactions of
cosmic ray particles with the interstellar medium starts to dominate in the
gamma-ray domain. That creates new questions about the change of origin for the
Galactic X-ray background from point sources to truly diffuse
emission of the interstellar gas.
Credit: R. Krivonos et al., MPI f Astrophysik, Garching, and IKI, Moscow, astro-ph/0605420
