X-ray spectral curvature of High Frequency Peaked BL Lacs: a predictor for the TeV flux

TL;DR

This study analyzes X-ray spectral curvature in high-frequency peaked BL Lacs to identify predictors of TeV flux, comparing detected and undetected sources to improve future TeV observation targeting.

Contribution

It introduces a new criterion based on X-ray spectral parameters to select promising HBL candidates for TeV detection, expanding previous correlations found in TeV-detected HBLs.

Findings

Spectral parameters differ between TeV-detected and undetected HBLs.

A new selection criterion for TeV candidate HBLs is proposed.

Correlations between spectral peak luminosity and energy are confirmed.

Abstract

Most of the extragalactic sources detected at TeV energies are BL Lac objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs) exhibiting spectral energy distributions with a lower energy peak in the X-ray band; this is widely interpreted as synchrotron emission from relativistic electrons. The X-ray spectra are generally curved, and well described in terms of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we found two correlations between their spectral parameters. (1) The synchrotron peak luminosity L_p increases with its peak energy E_p; (2) the curvature parameter b decreases as E_p increases. The first is consistent with the synchrotron scenario, while the second is expected from statistical/stochastic acceleration mechanisms for the emitting electrons. Here we present an extensive X-ray analysis of a sample of HBLs observed with XMM-Newton and SWIFT but undetected at TeV energies (UBLs), to compare their spectral behavior with that of TBLs. Investigating the distributions of their spectral parameters and comparing the TBL X-ray spectra with that of UBLs, we develop a criterion to select the best HBLs candidates for future TeV observations.