Constraints on radiatively inefficient accretion history of active   galactic nuclei from hard cosmological X-ray background
  The transition of a standard thin disk to a radiatively inefficient accretion flow (RIAF) is expected to occur, when its dimensionless accretion rate is lower than the critical value. The RIAF is very hot, and it radiates mostly in the hard X-ray band (>100 keV). Assuming that the accretion disk in every bright active galactic nucleus (AGN) will finally undergo a RIAF phase while the accretion rate is lower than the critical value, we calculate the contribution of the RIAFs in AGNs to the cosmological X-ray background of 10-1000 keV. We find that the timescale of the RIAF accreting at slightly below the critical rate should be shorter than 0.01 of the bright AGN lifetime if the critical accretion rate is 0.01, i.e., the accretion rate declines from the critical value to a rate significantly lower than this critical rate within this RIAF timescale. The derived RIAF timescale is affected by the parameters adopted in the model calculations, which is also discussed in this Letter. 
Xinwu Cao