Spectral variability of FIRST bright QSOs with SDSS observations

TL;DR

This study investigates spectral variability in a large sample of bright QSOs over up to 10 years, revealing that many become redder when brighter, regardless of radio loudness, with different behaviors observed blueward and redward of 2500 Å.

Contribution

It provides the first large-scale analysis of spectral slope variability in bright QSOs over long timescales, comparing radio loud and quiet populations.

Findings

Half of the QSOs are redder when brighter.

No significant difference in slope variability between radio loud and quiet QSOs.

Spectral variability differs blueward and redward of 2500 Å.

Abstract

For some samples, it has been shown that spectra of QSOs with low redshift are bluer during their brighter phases. For the FIRST bright QSO sample, we assemble their spectra from SDSS DR7 to investigate variability between the spectra from White et al. (2000) and from the SDSS for a long rest-frame time-lag, up to 10 years. There are 312 radio loud QSOs and 232 radio quiet QSOs in this sample, up to $z \sim 3.5$. With two-epoch variation, it is found that spectra of half of the QSOs appear redder during their brighter phases. There is no obvious difference in slope variability between sub-samples of radio quiet and radio loud QSOs. This result implies that the presence of a radio jet does not affect the slope variability on 10-year timescales. The arithmetic composite difference spectrum for variable QSOs is steep at blueward of $\sim$ 2500\AA. The variability for the region blueward of 2500 \AA\ is different to that for the region redward of 2500 \AA.