BROAD Ne VIII λ774 EMISSION FROM QUASARS IN THE HST-FOS SNAPSHOT SURVEY (ABSNAP)

摘要

We discuss the strength and frequency of broad Ne VIII λ774 emission from quasars measured in the snapshot survey (Absnap). Five sources in the survey have suitable redshifts (0.86 ≤ z_(em) ≤ 1.31), signal-to-noise ratios and no Lyman limit absorptions. Three of the five sources have a strong broad emission line near 774 A (rest), and the remaining two sources have a less securely measured line near this wavelength. We identify these lines with Ne VIII λ774 based on the measured wavelengths and theoretical estimates of various line fluxes (Hamann et al. 1995a). Secure Ne VIII detections occur in both radio-loud and radio-quiet sources. We tentatively conclude that broad Ne VIII λ774 emission is common in quasars, with typical strengths between ～25% and ～200% of O VI λ1034. These Ne VIII λ774 measurements imply that the broad emission line regions have a much hotter and more highly ionized component than previously recognized. They also suggest that quasar continua have substantial ionizing flux out to energies greater than 207 eV ( > 15.2 ryd, λ < 60 A). Photoionization calculations using standard incident spectra indicate that the Ne vm emission requires ionization parameters U approx> 5, total column densities N_H approx> 10~(22) cm~(-2) and covering factors approx> 25%. The temperatures could be as high as ～10~5 K. If the gas is instead collisionally ionized, strong Ne VIII would imply equilibrium temperatures in the range ～400,000 approx< T_e approx< 10~6 K. In either case, the highly ionized Ne VIII emission regions would appear as X-ray "warm absorbers" if they lie along our line of sight to the X-ray continuum source.