Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
; Tsinghua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
; Univ Alabama, Dept Phys, Huntsville, AL 35899 USA
; NASA, George C Marshall Space Flight Ctr, Space Sci Lab, Huntsville, AL 35812 USA
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
We critically examine the dependence of spectral index on luminosity in optically selected AGN samples. An analysis of optically selected high-z quasars showed an anticorrelation of alpha(OX), the spectral index between the rest-frame 2500 angstrom and 2 keV, with optical luminosity. We examine this relationship by means of Monte Carlo simulations and conclude that a constant alpha(OX) independent of optical luminosity is still consistent with this high-z sample. We further find that contributions of large dispersions and narrow range of optical luminosity are most important for the apparent, yet artificial, alpha(OX)-l(o) correlation reported. We also examine another, but more complete, low-z optical selected AGN sub-sample from Steffen et al., and our analysis shows that a constant alpha(OX) independent of optical luminosity is also consistent with the data. By comparing X-ray and optical luminosity functions, we find that a luminosity-independent alpha(OX) is in fact more preferred than the luminosity-dependent alpha(OX) model. We also discuss the selection effects caused by flux limits, which might systematically bias the l(X)-l(o) relation and cause discrepancy in optically selected and X-ray selected AGN samples. To correctly establish a dependence of alpha(OX) of AGNs on their luminosity, a larger and more complete sample is needed and consequences of luminosity dispersions and selection effects in flux-limited samples must be taken into account properly.
Tang, SM,Zhang SN,Zhang, SN,et al. Is the dependence of spectral index on luminosity real in optically selected AGN samples ?[J]. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY,2007,377(3):1113-1121.
Tang, SM,张双南,Zhang, SN,&Hopkins, PF.(2007).Is the dependence of spectral index on luminosity real in optically selected AGN samples ?.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY,377(3),1113-1121.
Tang, SM,et al."Is the dependence of spectral index on luminosity real in optically selected AGN samples ?".MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 377.3(2007):1113-1121.