The Curvature of Spectral Energy Distribution and $\gamma$-ray Dominance of Fermi BL Lac Objects

TL;DR

This study investigates why some BL Lac objects are detected in gamma-rays by analyzing their spectral curvature, finding that gamma-ray detected sources have smaller curvature, which relates to emission region size and acceleration mechanisms.

Contribution

It reveals that spectral curvature differences, independent of Doppler beaming, influence gamma-ray detection in BL Lac objects, supported by broadband SED modeling.

Findings

FBLs have smaller spectral curvature than NFBLs.

SED modeling shows curvature correlates with emission region size.

Differences in curvature and Compton dominance explain gamma-ray detection disparity.

Abstract

The extragalactic $\gamma$-ray sky is dominated by blazars and their study plays an important role in understanding jet physics, cosmic evolution history and origin of ultra high energy cosmic rays. In this work, we study a large sample of BL Lac objects to investigate why some sources are detected in $\gamma$-rays, while others not. We selected 170 BL Lac objects, with measured synchrotron spectral curvature and Doppler factor, and divided them into Fermi-LAT detected (FBLs) and non-detected (NFBLs) sources. We show that FBLs have smaller curvature than NFBLs, even after getting rid of Doppler beaming effect. The BL Lac objects PKS 0048-09 and S5 0716+714 have similar synchrotron peak frequency and luminosity but different $\gamma$-ray dominance and their quasi-simultaneous broadband spectral energy distributions (SEDs) can be well fitted by a log-parabolic synchrotron self-Compton (SSC) model with same jet parameters except for the curvature and source size, assuming curvature being proportional to the size of emission region. Our results imply that for a given synchrotron luminosity, the different SED curvature and Compton dominance may account for the discrepancy between FBLs and NFBLs. We discuss these results in context of stochastic particle acceleration and radiation mechanisms.