AGN OBSCURATION, UNIFICATION AND THE COSMIC X-RAY BACKGROUND
Abstract
The CXB spectrum differs from the integration of the spectra of individual AGN, calling for a large population, undetected so far, of strongly obscured Compton thick AGN. Such objects are predicted by unified models, which attribute most of the AGN diversity to their inclination on the line of sight, and play an important role for the understanding of the growth of black holes in the early Universe. The fraction of obscured AGN at low redshift can be derived from the observed CXB spectrum assuming AGN spectral templates and luminosity functions. We show that high signal-to-noise average hard X-ray spectra, derived from more than a billion seconds of effective exposure time with Swift/BAT and NuStar, imply that mildly obscured Compton thin AGN feature a strong reflection and contribute massively to the CXB. A population of Compton thick AGN larger than that effectively detected is not required, as no more than 6% of the CXB flux can be attributed to them. The stronger reflection observed in mildly obscured AGN suggests that the covering fraction of the gas and dust surrounding their central engines is a key factor in shaping their appearance. NuStar observations of AGNs in addition indicate clearly that reflection is correlated to the spectral slope in unobscured sources, these are the objects were soft lags are observed and where reflection is dominated by disk reprocessing. Instead obscured objects feature a correlation between reflection and column density characteristics of a clumpy reprocessing region located far away.
- Publication:
-
AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1710103W