The optical spectral features of 27 Fermi blazars

TL;DR

This study analyzes long-term optical data of 27 blazars, revealing that their spectral indices vary with brightness, showing consistent behaviors where spectra become bluer or redder as brightness increases, explained by two-component emission models.

Contribution

It provides the first comprehensive analysis of optical spectral variations in a sizable sample of blazars, identifying universal spectral behaviors and validating a two-component emission model.

Findings

Blazars exhibit two main spectral behaviors: bluer-stable-when-brighter (BSWB) and redder-stable-when-brighter (RSWB).

Most BL Lacs show BSWB behavior, while most FSRQs show RSWB behavior.

A two constant-spectral-index component model explains the spectral features qualitatively and quantitatively.

Abstract

A spectral variation accompanied with flux variability is a commonly-observed phenomenon for blazars. In order to further investigate the optical spectral feature of blazars, we have collected the long-term optical V and R band data of 27 blazars (14 BL Lacs and 13 FSRQs), and calculated their optical spectral indices. The results show that the spectral indices vary with respect to the brightness for all of these blazars. In general, the optical spectrum progressively becomes flatter (or steeper), when the brightness increases. However the spectrum changes more and more slowly, until it tends to be stable. In other words, the source becomes bluer (or redder) and then gradually stabilizes when it brightens, which are briefly named the bluer-stable-when-brighter (BSWB) and redder-stable-when-brighter (RSWB) behaviors, respectively. Thirteen of the 14 BL Lacs show the BSWB behavior, with an exception of AO 0235+164. On the contrary, most of FSRQs (10 out of 13) exhibit the RSWB trend. It is confirmed that blazars follow the two universal optical spectral behaviors, namely, BSWB and RSWB. The model of two constant-spectral-index components can well explain the optical spectral features qualitatively and quantitatively. The results illustrate that the optical emission are mainly composed of two stable-color components, i.e., less variable thermal emission and high variable synchrotron radiation. And in most cases, the thermal component of BL Lacs is redder than that of synchrotron radiation, whereas FSRQs are the opposite.