Blazar Anti-Sequence of Spectral Variability for Individual TeV Blazars

TL;DR

This study analyzes the spectral variability of individual TeV blazars, revealing an anti-sequence phenomenon where spectral peak frequency and flux increase together, contrary to the traditional blazar sequence, and links these behaviors to accretion rate differences.

Contribution

It introduces the concept of blazar anti-sequence for individual sources and proposes a model connecting accretion rate variations to spectral changes.

Findings

Spectral energy distributions fit well with synchrotron and SSC models for BL Lacs.

FSRQs require external Compton scattering contributions.

The anti-sequence phenomenon shows increasing peak frequency with flux in individual blazars.

Abstract

We compile from literature the broadband SEDs of twelve TeV blazars observed simultaneously or quasi-simultaneously with Fermi/LAT and other instruments. Two SEDs are available for each of the objects and the state is identified as a low or high state according to its flux density at GeV/TeV band. The observed SEDs of BL Lac objects (BL Lacs) are fitted well with the synchrotron + synchrotron-self-Compton (syn+SSC) model, whereas the SEDs of the two flat spectrum radio quasars (FSRQs) need to include the contributions of external Compton scattering. In this scenario, it is found that the Doppler factor delta of FSRQs is smaller than that of BL Lacs, but the magnetic field strength B of FSRQs is larger than that of BL Lacs. The increase of the peak frequency of the SEDs is accompanied with the increase of the flux for the individual sources, which seems opposite to the observational phenomena of the blazar sequence. We refer this phenomenon to blazar anti-sequence of spectral variability for individual TeV blazars. However, both the blazar sequence from FSRQs to BL Lacs and blazar anti-sequence of the spectral variability from low state to high state are accompanied by an increase of the break Lorentz factor of the electron's spectrum gamma_b and a decrease of B. We propose a model in which the mass accretion rate is the driving force behind both the blazar sequence for ensembles of blazars and the blazar anti-sequence for individual blazars. Specifically we suggest that the differences in mass accretion rate of different blazars produce the observed blazar sequence, but \Delta M in each blazar results in the observed blazar anti-sequence.