On the Fraction of X-Ray-weak Quasars from the Sloan Digital Sky Survey

Abstract

We investigate systematically the X-ray emission from type 1 quasars using a sample of 1825 Sloan Digital Sky Survey non-broad absorption line (non-BAL) quasars with Chandra archival observations. A significant correlation is found between the X-ray-to-optical power-law slope parameter (α OX) and the 2500 Å monochromatic luminosity (L 2500Å), and the X-ray weakness of a quasar is assessed via the deviation of its α OX value from that expected from this relation. We demonstrate the existence of a population of non-BAL X-ray-weak quasars, and the fractions of quasars that are X-ray weak by factors of ≥6 and ≥10 are 5.8% ± 0.7% and 2.7% ± 0.5%, respectively. We classify X-ray-weak quasars (X-ray weak by factors of ≥6) into three categories based on their optical spectral features: weak emission-line quasars (WLQs; C iv rest-frame equivalent width < 16 Å), red quasars (Δ(g − i) > 0.2), and unclassified X-ray-weak quasars. The X-ray-weak fraction of within the WLQ population is significantly higher than that within non-WLQs, confirming previous findings that WLQs represent one population of X-ray-weak quasars. The X-ray-weak fraction of within the red quasar population is also considerably higher than that within the normal quasar population. The unclassified X-ray-weak quasars do not have unusual optical spectral features, and their X-ray weakness may be mainly related to quasar X-ray variability.

X-ray spectroscopic survey of highly-accreting AGN

“…Specifically, the X-ray weakness factors f weak measured for J1103+41, J1127+64, J1245+33 and J0300-08 span from ∼ 11 to ∼ 75, with ∆α ox comprised between −0.7 and −0.4. It is worth noting that the fraction of QSOs with f weak ≥ 6 in our sample is 29 +22 −14 % (with errors estimated according to Gehrels 1986), which is much larger (and inconsistent at the ∼ 99% confidence level) than that derived by Pu et al (2020) for a sample of 1825 type-1 QSOs from SDSS, i.e. 5.8 ± 0.7%.…”

“…Bearing in mind the uncertainties due to the limited number of sources, the fraction of QSOs with f weak ≥ 6 in our sample is 29 +22 −14 %, which is much larger than that derived for the AGN general population (e.g. Pu et al 2020), suggesting a possible change in the properties of the accretion disc-corona system in some objects radiating close (or above) to the Eddington limit, possibly related to the increas- ing importance of radiation pressure in shaping the efficiency of the accretion process (e.g. Proga & Kallman 2004;S ądowski & Narayan 2016;Zappacosta et al 2020;Nomura et al 2020).…”

“…Indeed, on average, the resulting effective photon index is usually Γ eff ∼ 1.2. Pu et al (2020), for instance, indicated Γ eff = 1.26 as the threshold below which a quasar is potentially obscured by cold absorption. However, by modelling the 0.3 − 10 keV X-ray continuum with a power law, we find none of our four high-λ Edd X-ray weak sources to have flat photon indices compatible with those typical of WLQs and other absorbed quasars.…”

“…Red solid line marks the reference value for X-ray weakness, i.e. ∆α ox ≤ −0.3(Pu et al 2020). Right panel: distribution of the same sources described in the adjacent panel, divided in two regimes of bolometric luminosity, in the α ox − log λ Edd plane.…”

Surveys of the Cosmic X-ray Background

“…The weak X-ray emission appears to be intrinsic, as spectral analyses suggest these particular quasars are X-ray unobscured. However, such X-ray weak objects appear rare, consisting of only a few percent of SDSS quasars (e.g., Gibson et al 2008;Pu et al 2020). Considering this rarity, our general understanding of AGN demographics appears largely unaffected by the intrinsically X-ray weak population.…”