X-Ray Spectral Variations of Synchrotron Peak in BL Lacs

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 (Ep), peak luminosity (Lp), and the curvature parameter (b). It has been suggested that Ep shows relations with Lp 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 Ep–Lp and Ep–(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 Ep–Lp correlations, three sources show no correlation, and two sources display negative correlations. In addition, most targets (7/14) present no correlation between Ep and 1/b, five sources pose negative correlations, and two sources demonstrate positive correlations. 1ES 1959+650 displays two different Ep–Lp relations in 2002 and 2016. We also analyze Ep–Lp and Ep–(1/b) relations during flares lasting for several days. The Ep–Lp relation does not exhibit significant differences between flares, while the Ep–(1/b) relation varies from flare to flare. For the total sample, when Lp < 1045 erg s−1, there seems to be a positive Ep–Lp correlation. Lp and the slope of Ep–Lp relation present an anticorrelation, which indicates that the causes of spectral variations might be different between luminous and faint sources. Ep shows a positive correlation with the black hole mass. We discuss the implications of these results.