The Intrinsic UV/Soft X-Ray Spectrum of Quasars

Abstract

The detection of a Hell absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 Å require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the Hell reionization of the universe was completed well before z ~ 3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological “filtering” through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported Hell absorption, any thermal component responsible for the “soft X-ray excess” observed in the spectra of AGNs at z ≲ 0.3 must, at z ~ 3, have a typical temperature ≳ 80 eV, and a luminosity not exceeding ≈ 20% of that of the “UV bump”.