Accreting white dwarfs, a growing number of new high energy sources
The high energy galactic survey of INTEGRAL has uncovered more than
400 sources emitting at energy above 20 keV. Among them, most are
galactic X-ray binaries containing a neutron star or extragalactic
active nuclei. But nearly one fourth of these sources are still
unidentified. A recent study (Bonnet-Bidaud et al.,
A&A 473, 185-189 (2007);
arXiv:0706.1433 )
now confirms that systems containing white dwarfs also contribute
significantly to the galactic high energy emission.
The image shows the location of IGR J00234+6141, one of the
faintest sources discovered by INTEGRAL with the corresponding mosaic
from the ISGRI camera (lower left). A detailed study of the field
(lower right) and associated optical photometry and spectroscopy has
led to an identification with a cataclysmic variable, a low mass
binary system containing a highly magnetized white dwarf, also called
'Intermediate Polars systems' (IPs). The INTEGRAL spectrum is well
decribed by a thermal emission at a temperature of 30 keV. Such high
temperatures are achieved at the basis of an accretion column above the
white dwarf as depicted in the artist's view above.
About 5% of the INTEGRAL sources have already been identified with
IPs and IGR00234+6141 shows that more such faint systems probably exist.
As the white dwarfs are much more numerous than the neutron stars,
it is anticipated that these systems represent a significant fraction
of the unresolved high energy background.
(Credits: Service d'Astrophysique(SAp), CEA France; artist view: Mark Garlick)
