Optical classification of unidentified INTEGRAL sources
The all-sky surveys performed with IBIS onboard INTEGRAL detected
hundreds of sources in the hard X-rays between 20 and 100 keV.
Most of them are Galactic X-ray binaries, followed by Active Galactic
Nuclei (see INTEGRAL Picture of the Month of February 2007).
A quarter of those objects, however, still has no obvious counterpart at
other wavelengths and therefore cannot be associated with any known class
of high-energy emitting objects.
Comparison with catalogues or surveys at other wavelengths (especially
soft X-rays) is of invaluable help in pinpointing the putative optical
candidates, but only accurate optical spectroscopy can reveal the true
nature of the source.
Since 2004, a program of optical spectroscopy of candidate counterparts
of unidentified INTEGRAL sources, ongoing at several observatories
worldwide, has allowed the identifiaction and classification of nearly
50 of these unknown INTEGRAL hard X-ray sources.
The picture above shows the optical spectra of 4 unidentified INTEGRAL
sources. Each one corresponds to a class of objects which are found to
preferentially populate the high-energy sky above 20 keV.
