Characterizing Quasar C iv Emission-line Measurements from Time-resolved Spectroscopy

Abstract

We use multiepoch quasar spectroscopy to determine how accurately single-epoch spectroscopy can locate quasars in emission-line parameter space in order to inform investigations where time-resolved spectroscopy is not available. We explore the improvements in emission-line characterization that result from using nonparametric information from many lines as opposed to a small number of parameters for a single line, utilizing reconstructions based on an independent component analysis applied to the data from the Sloan Digital Sky Survey Reverberation Mapping project. We find that most of the quasars are well described by just two components, while more components signal a quasar likely to yield a successful reverberation mapping analysis. In single-epoch spectroscopy the apparent variability of equivalent width is exaggerated because it is dependent on the continuum. Multiepoch spectroscopy reveals that single-epoch results do not significantly change where quasars are located in C iv parameter space and do not have a significant impact on investigations of the global Baldwin effect. Quasars with emission-line properties indicative of higher L/L Edd are less variable, consistent with models with enhanced accretion disk density. Narrow absorption features at the systemic redshift may be indicative of orientation (including radio-quiet quasars) and may appear in as much as 20% of the quasar sample. Future work applying these techniques to lower-luminosity quasars will be important for understanding the nature of accretion disk winds.

A Novel Test of Quasar Orientation

“…Thus we have X-ray detections or upper limits for 96+ 19 + 3 + 4 + 4 = 126 of the sources analyzed in Rivera et al (2020), including 18 of the 20 AAL0s.…”

“…During the course of an analysis of repeat spectroscopy of quasars from the SDSS Reverberation Mapping (SDSS-RM; Shen et al 2015) campaign, Rivera et al (2020) discovered many examples of quasars with AAL0s (10-20%), which potentially represent a larger fraction of quasars than current estimates of orientation allow. For reference, a symmetric conical distribution of absorbing gas with a 10-20% covering fraction would correspond to a opening angle of 6-12 degrees with respect to the plane of the accretion disk (in the absence of toroidal obscuration).…”

“…This experiment was designed to determine the time delay between continuum and emission-line variations in order to estimate black hole masses using the reverberation mapping technique. Rivera et al (2020) analyzed the spectra of 133 of these 849 quasars that have 30 or more epochs, with a mean S/N> 6 per pixel over the wavelength interval 3650 < λ < 9300 Å. These data enabled an investigation of how much limited spectrum S/N and intrinsic spectral variability can affect the measurement of C IV equivalent width (EW) and C IV blueshift (see also Sun et al 2018), which are thought to be indicators of accretion disk winds (Richards et al 2011) 2020) define a hybrid metric, the C IV "distance", as a more robust wind indicator.…”

“…The Rivera et al (2020) analysis was based on independent component analysis (ICA) reconstructions of the SDSS-RM spectra using the spectral components determined from Rankine et al (2020). As a result of those spectral reconstructions, these 133 quasars have high-S/N composite spectra and accurately determined continua, including in the region of the broad emission line.…”

“…square degrees of X-ray coverage from XMM-Newton to an effective depth of ≈ 15 ks (Liu et al 2020), with detection of 584 of the 849 quasars in the full SDSS-RM sample and 96 of the 133 quasars from Rivera et al (2020). In addition, Liu et al (2020) performed forced photometry at the location of the full SDSS-RM sample, which provides 2σ upper limits for another 19 sources from Rivera et al (2020).…”