Host Galaxies and the Spectral Variability of Quasars

Abstract

Variability of the spectral energy distribution (SED) of active galactic nuclei (AGNs) can provide clues for understanding both the main emission processes and the origin of their variations. So far, multi-wavelength monitoring has been possible for a limited number of bright objects, thanks to large international cooperations (see , and refs. therein). The “ensemble” analysis of the light curves was obtained in the past only from single-band observations of statistical samples of QSOs (, , , ). , , and , found a hardening of the spectrum in the bright phase in small QSO samples. A positive correlation of the spectral index α (f ν ∝ν α) with brightness variations Δ log f ν has been found by . The recent publication, by Giveon et al. 1999, of two-band monitoring of 42 PG QSOs, allows a statistical study of the spectral variability of the entire set of light curves (). They find a positive correlation of the B - R color with the brightness variations δB. These results confirm the interpretation of the variability-redshift (v-z) correlation suggested by Giallongo, and . An independent analysis of the same light curves, performed by implies the existence of an underlying spectral component, redder than the variable one, which could be identified either with the host galaxy, considered by , or with a non-flaring part of the QSO spectrum.