Steep Hard-X-Ray Spectra Indicate Extremely High Accretion Rates in Weak Emission-line Quasars

Publication Title

Astrophysical Journal

Document Type

Article

Department or Program

Physics and Astronomy

Publication Date

9-2018

Keywords

galaxies: active, galaxies: nuclei, quasars: emission lines, quasars: general, X-rays: galaxies

Abstract

We present XMM-Newton imaging spectroscopy of 10 weak emission-line quasars (WLQs) at , six of which are radio-quiet, and four that are radio-intermediate. The new X-ray data enabled us to measure the power-law photon index, at rest-frame energies >2 keV, in each source with relatively high accuracy. These measurements allowed us to confirm previous reports that WLQs have steeper X-ray spectra, suggesting higher accretion rates with respect to "typical" quasars. A comparison between the photon indices of our radio-quiet WLQs and those of a control sample of 85 sources shows that the first are significantly higher, at the 3σ level. Collectively, the four radio-intermediate WLQs have lower photon indices with respect to the six radio-quiet WLQs, as may be expected if the spectra of the first group are contaminated by X-ray emission from a jet. Therefore, in the absence of significant jet emission along our line of sight, these results are in agreement with the idea that WLQs constitute the extreme high end of the accretion-rate distribution in quasars. We detect soft excess emission in our lowest-redshift radio-quiet WLQ, in agreement with previous findings suggesting that the prominence of this feature is associated with a high accretion rate. We have not detected signatures of Compton reflection, Fe Kα lines, or strong variability between two X-ray epochs in any of our WLQs, which can be attributed to their relatively high luminosity.

Comments

Original version is available from the publisher at: https://doi.org/10.3847/1538-4357/aad812

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