Extreme Variability Quasars in Their Various States. I: The sample selection and composite SDSS spectra

TL;DR

This study constructs the largest sample of extremely variable quasars (EVQs) to analyze their spectral changes across different brightness states, revealing trends similar to normal quasars and suggesting EVQs are part of a broad quasar property distribution.

Contribution

It provides the first comprehensive spectral analysis of EVQs across various states using a large sample, highlighting their spectral variability and relation to normal quasars.

Findings

EVQs are bluer when brighter, consistent with known quasar trends.

Line equivalent widths decrease from dim to bright states, showing an intrinsic Baldwin effect.

EVQs have larger line EWs and subtle excess in very broad lines compared to controls.

Abstract

Extremely variable quasars (EVQs) are a population of sources showing large optical photometric variability revealed by time-domain surveys. The physical origin of such extreme variability is yet unclear. In this first paper of a series, we construct the largest-ever sample of 14,012 EVQs using photometric data spanning over $>$ 15 years from SDSS and Pan-STARRS1. We divide them into five sub-samples according to the relative brightness of an EVQ during SDSS spectroscopic observation compared to the mean brightness from photometric observations. Corresponding control samples of normal quasars are built with matched redshift, bolometric luminosity and supermassive black hole mass. We obtain the composite SDSS spectra of EVQs in various states and their corresponding control samples. We find EVQs exhibit clearly bluer (redder) SDSS spectra during bright (dim) states, consistent with the "bluer-when-brighter" trend widely seen in normal quasars. We further find the line EWs of broad MgII, CIV and OIII (but not broad Hb which is yet puzzling) gradually decrease from dim state to bright state, similar to the so-called intrinsic Baldwin effect commonly seen in normal AGNs. Meanwhile, EVQs have systematically larger line EWs compared with the control samples. We also see that EVQs exhibit subtle excess in the very broad line component compared with control samples. Possible explanations for the discoveries are discussed. Our findings support the hypothesis that EVQs are in the tail of a broad distribution of quasar properties, but not a distinct population.