The Spectroscopic Signature of Variability in High-Redshift Quasars

TL;DR

This study analyzes spectral variability in high-redshift quasars using a large SDSS dataset, confirming known effects and revealing how spectral changes depend on luminosity and wavelength, with implications for cosmology.

Contribution

It introduces a composite differential spectrum capturing wavelength-dependent quasar variability and links spectral behavior to accretion disk models, improving continuum predictions for Lyman-alpha forest studies.

Findings

Confirmed the intrinsic Baldwin Effect and brighter-means-bluer trends.

Created a wavelength-dependent differential spectrum of quasar variability.

Improved quasar continuum predictions, reducing Lyman-alpha forest uncertainty from 17.2% to 7.7%.

Abstract

Using 16,421 spectra from a sample of 340 quasars ($1.62<z<3.30$) from the SDSS Reverberation Mapping Project, we present an analysis of quasar spectral variability. We confirm the intrinsic Baldwin Effect and brighter-means-bluer trends in which emission line strength and color are associated with changes in luminosity. We then create a composite differential spectrum that captures the wavelength dependence of quasar variability as a function of change in luminosity. When using a bandpass around 1740 \AA\ to describe the luminosity, the differential spectrum follows a power law at longer wavelengths that breaks blueward of 1700 \AA. However, the shape of the differential spectrum, the location of the power law break, and the slope of the intrinsic Baldwin Effect all vary with the choice of bandpass used to define the change in quasar luminosity. We propose that the observed behavior can be explained by inhomogeneous accretion or slim accretion disk models where delays in the reprocessing of incident light on the accretion disk cause local deviations in temperature from the thin disk model. Finally, we quantify the effects on cosmology studies due to the variations in the quasar spectrum in the Lyman-$\alpha$ forest wavelength range. Using the observed spectroscopic signatures to predict the quasar continuum over the interval $1040 < \lambda < 1200$ \AA, we find that the derived spectral templates can reduce the uncertainty of the Lyman-$\alpha$ forest continuum level in individual epochs from 17.2% to 7.7%, near the level where systematic errors in SDSS flux calibration are expected to dominate.