Radio and X-ray emission from disc winds in radio-quiet quasars

TL;DR

This study tests the free-free emission model for radio-quiet quasars' radio spectra by comparing predicted and observed X-ray luminosities, finding that most quasars require high column densities incompatible with the model.

Contribution

The paper provides observational constraints on the disc wind free-free emission model by analyzing X-ray spectra of PG quasars, challenging its applicability.

Findings

Most PG quasars require high hydrogen column densities, inconsistent with the free-free emission model.

Two quasars may still be explained by wind-originated emission near the black hole.

The free-free emission model cannot explain the radio and X-ray luminosities for 20 out of 22 studied quasars.

Abstract

It has been proposed that the radio spectra of radio-quiet quasars is produced by free-free emission in the optically thin part of an accretion disc wind. An important observational constraint on this model is the observed X-ray luminosity. We investigate this constraint using a sample of PG radio-quiet quasars for which XMM-Newton EPIC spectra are available. Comparing the predicted and measured luminosities for 0.5, 2 and 5 keV, we conclude that all of the studied PG quasars require a large hydrogen column density absorber, requiring these quasars to be close to or Compton-thick. Such a large column density can be directly excluded for PG 0050+124, for which a high-resolution RGS spectrum exists. Further constraint on the column density for a further 19 out of the 21 studied PG quasars comes from the EPIC spectrum characteristics such as hard X-ray power-law photon index and the equivalent width of the Fe Kalpha line; and the small equivalent width of the C IV absorber present in UV spectra. For 2 sources: PG 1001+054 and PG 1411+442 we cannot exclude that they are indeed Compton-thick, and the radio and X-ray luminosity are due to a wind originating close to the super-massive black hole. We conclude that for 20 out of 22 PG quasars studied free-free emission from a wind emanating from the accretion disc cannot mutually explain the observed radio and X-ray luminosity.