CONSTRAINTS ON THE RADIATIVELY INEFFICIENT ACCRETION HISTORY OF ACTIVE GALACTIC NUCLEI FROM THE HARD COSMOLOGICAL X-RAY BACKGROUND

摘要

The transition of a standard thin disk into a radiatively inefficient accretion flow (RIAF) is expected to occur when its accretion rate m is lower than a critical value m_(crit) (m = M/M_(Edd)). RIAFs are very hot, and they radiate mostly in the hard X-ray band (approx> 100 keV). Assuming that the accretion disk in every bright active galactic nucleus (AGN) will finally undergo an RIAF phase when m < m_(crit), we calculate the contribution of the RIAFs in AGNs to the cosmological X-ray background at 10-1000 keV. We find that the timescale t~(RIAF) of an RIAF accreting at ～m_(crit) should be shorter than ～10~(-2)t~b if m_(crit) = 0.01, where t~b is the lifetime of bright AGNs; that is, m declines from m_(crit) to a rate significantly lower than m_(crit) within t~(RIAF). The derived timescale t~(RIAF) is affected by the parameters adopted in the model calculations, which are also discussed.