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

TL;DR

This study applies Non-Negative Matrix Factorization to analyze optical spectral variability in gamma-ray blazars, revealing distinct physical components and variability patterns across different blazar types over a decade.

Contribution

It introduces a novel application of NMF to disentangle physical processes in blazar spectra, enabling modeling of long-term variability with few components.

Findings

Host-galaxy dominated blazars show less variability and bluer-when-brighter trends.

FSRQs exhibit complex color-flux behaviors, including redder-when-brighter and saturation effects.

Spectral reconstructions identify components related to jets, accretion disks, and broad line regions.

Abstract

Blazars optical emission is generally dominated by relativistic jets, although the host galaxy, accretion disk and broad line region (BLR) may also contribute significantly. Disentangling their contributions has been challenging for years due to the dominance of the jet. To quantify the contributions to the spectral variability, we use the statistical technique for dimensionality reduction Non-Negative Matrix Factorization on a spectroscopic data set of 26 $\gamma$-ray blazars. This technique allows to model large numbers of spectra in terms of a reduced number of components.We use a priori knowledge to obtain components associated to meaningful physical processes. The sources are classified according to their optical spectrum as host-galaxy dominated BL Lac objects (BL Lacs), BL Lacs, or Flat Spectrum Radio Quasars (FSRQs). Host-galaxy sources show less variability, as expected, and bluer-when-brighter trends, as the other BL Lacs. For FSRQs, more complicated colour-flux behaviours are observed: redder-when-brighter for low states saturating above a certain level and, in some cases, turning to bluer-when-brighter. We are able to reproduce the variability observed during 10 years using only 2 to 4 components, depending on the type. The simplest scenario corresponds to host-galaxy blazars, whose spectra are reconstructed using the stellar population and a power law for the jet. BL Lac spectra are reproduced using from 2 to 4 power laws. Different components can be associated to acceleration/cooling processes taking place in the jet. The reconstruction of FSRQs also incorporates a QSO-like component to account for the BLR, plus a very steep power law, associated to the accretion disk.