X-ray spectral variations of synchrotron peak in BL Lacs

TL;DR

This study investigates the spectral energy distribution variations in BL Lac objects, revealing diverse correlations between peak energy, luminosity, and curvature, and their dependence on source luminosity and black hole mass.

Contribution

It systematically analyzes spectral parameter relations in BL Lacs, highlighting variability patterns and their potential physical implications, which were previously not comprehensively studied.

Findings

Most BL Lacs show positive $E_p$-$L_p$ correlations.

The $E_p$-(1/$b$) relation varies significantly among sources.

Spectral variation causes differ between luminous and faint sources.

Abstract

The spectral energy distribution of blazars around the synchrotron peak can be well described by the log-parabolic model that has three parameters: peak energy ($E_\textrm{p}$), peak luminosity ($L_\textrm{p}$) and the curvature parameter ($b$). It has been suggested that $E_\textrm{p}$ shows relations with $L_\textrm{p}$ and $b$ in several sources, which can be used to constrain the physical properties of the emitting region and/or acceleration processes of the emitting particles. We systematically study the $E_\textrm{p}$-$L_\textrm{p}$ and $E_\textrm{p}$-(1$/b$) relations for 14 BL Lac objects using the 3-25~keV $RXTE$/PCA and 0.3-10~keV $Swift$/XRT data. Most objects (9/14) exhibit positive $E_\textrm{p}$-$L_\textrm{p}$ correlations, three sources show no correlation, and two sources display negative correlations. In addition, most targets (7/14) present no correlation between $E_\textrm{p}$ and 1$/b$, five sources pose negative correlations, and two sources demonstrate positive correlations. 1ES~1959+650 displays two different $E_\textrm{p}$-$L_\textrm{p}$ relations in 2002 and 2016. We also analyze $E_\textrm{p}$-$L_\textrm{p}$ and $E_\textrm{p}$-(1$/b$) relations during flares lasting for several days. The $E_\textrm{p}$-$L_\textrm{p}$ relation does not exhibit significant differences between flares, while the $E_\textrm{p}$-(1$/b$) relation varies from flare to flare. For the total sample, when $L_\textrm{p}$ < $\textrm{10}^\textrm{45}\ \textrm{erg}\ \textrm{s}^\textrm{-1}$, there seems to be a positive $E_\textrm{p}$-$L_\textrm{p}$ correlation. $L_\textrm{p}$ and the slope of $E_\textrm{p}$-$L_\textrm{p}$ relation present an anti-correlation, which indicates that the causes of spectral variations might be different between luminous and faint sources. $E_\textrm{p}$ shows a positive correlation with the black hole mass. We discuss the implications of these results.