Discovery of heavily-obscured AGN among seven INTEGRAL hard X-ray sources observed by Chandra

Abstract

Aims: We identify hard X-ray sources discovered by the INTEGRAL all-sky survey. We complete identification of a unique sample of active galactic nuclei (AGN) selected in the hard X-ray band (17-60 keV) with minimal effects from absorption. Subsequently, we determine the fraction of obscured AGN in the local Universe.
Methods: We observed 7 INTEGRAL sources with the Chandra X-ray Observatory to refine their localization to ~2 arcsec and to study their X-ray spectra.
Results: Two sources are inferred to have a Galactic origin: IGR J08390-4833 is most likely a magnetic cataclysmic variable with a white dwarf spin period ~1450 s; and IGR J21343+4738 is a high-mass X-ray binary. Five sources (IGR J02466-4222, IGR J09522-6231, IGR J14493-5534, IGR J14561-3738, and IGR J23523+5844) prove to be AGN with significant intrinsic X-ray absorption along the line of sight. Their redshifts and hard X-ray (17-60 keV) luminosities range from 0.025 to 0.25 and from ~ 2× 10⁴³ to ~2× 10⁴⁵ erg s^-1, respectively, with the distance to IGR J14493-5534 remaining unknown. The sources IGR J02466-4222 and IGR J14561-3738 are likely Compton-thick AGN with absorption column densities N_H>10²⁴ cm^-2, and the former further appears to be one of the nearest X-ray bright, optically-normal galaxies.
Conclusions: With the newly-identified sources, the number of heavily-obscured (N_H⪆ 10²⁴ cm^-2) AGN detected by INTEGRAL has increased to ~10. Therefore, such objects constitute 10-15% of hard X-ray bright, non-blazar AGN in the local Universe. The small ratio (≪1%) of soft (0.5-8.0 keV) to hard (17-60 keV) band fluxes (Chandra to INTEGRAL) and the non-detection of optical narrow-line emission in some of the Compton-thick AGN in our sample suggests that there is a new class of objects in which the central massive black hole may be surrounded by a geometrically-thick dusty torus with a narrow ionization cone.