Open AccessThe contribution of faint active galactic nuclei to the hard X‐ray background
Author(s) -
Fiore F.,
La Franca F.,
Giommi P.,
Elvis M.,
Matt G.,
Comastri A.,
Molendi S.,
Gioia I.
Publication year - 1999
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1999.02795.x
Subject(s) - physics , rosat , astrophysics , x ray background , active galactic nucleus , sky , starlight , astronomy , flux (metallurgy) , accretion (finance) , galaxy , stars , materials science , metallurgy
Hard X‐ray selection is the most efficient way to discriminate between accretion‐powered sources, such as active galactic nuclei (AGN), and sources dominated by starlight. Hard X‐rays are also less affected than other bands by obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5–10 keV band, finding 180 sources in ∼50 deg 2 of sky with flux≳5×10 −14 erg cm −2 s −1 . After correction for the non‐uniform sky coverage this corresponds to resolving about 30 per cent of the hard cosmic X‐ray background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X‐ray error boxes studied. Seven AGN show evidence for obscuration in X‐ray and optical bands, a fraction higher than in previous ROSAT or ASCA – ROSAT surveys (at 95–99 and 90 per cent confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN.