A statistical study of the optical spectral variability in gamma-ray blazars

ABSTRACT Blazars optical emission is generally dominated by relativistic jets, although the host galaxy, accretion disc, and broad-line region (BLR) may also contribute significantly. Disentangling their contributions has been challenging for years due to dominance of jet. To quantify spectral variability, we use statistical technique dimensionality reduction non-negative matrix factorization on a spectroscopic data set 26 ?-ray blazars. This allows model large numbers spectra in terms reduced number components. We priori knowledge obtain components associated with meaningful physical processes. The sources are classified according spectrum as host-galaxy BL Lac objects (BL Lacs), Lacs, or flat radio quasars (FSRQs). Host-galaxy show less expected, bluer-when-brighter (BWB) trends, other Lacs. For FSRQs, more complicated colour-flux behaviours observed: redder-when-brighter low states saturating above certain level and, some cases, turning BWB. able reproduce variability observed during 10 yr using only two four components, depending type. simplest scenario corresponds blazars, whose reconstructed stellar population power law (PL) reproduced from PLs. Different can be acceleration/cooling processes taking place reconstruction FSRQs incorporates QSO-like component account BLR, plus very steep PL, disc.