Host Galaxies and the Spectral Variability of Quasars

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 Ulrich, Maraschi, & Urry 1997, 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 (Giallongo, Trèvese, & Vagnetti 1991; Hook et al. 1994; Cristiani et al. 1996; Di Clemente et al. 1996). Cutri et al. (1985), Kinney et al. (1991), and Edelson et al. (1990), found a hardening of the spectrum in the bright phase in small QSO samples. A positive correlation of the spectral index α (fν ∝ ν) with brightness variations ∆ log fν has been found by Trèvese et al. (2000). 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 (Trèvese & Vagnetti 2001). 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, Trèvese, & Vagnetti (1991) and Di Clemente et al. (1996). An independent analysis of the same light curves, performed by Cid Fernades et al. (2000) implies the existence of an underlying spectral