X-ray Selected BL Lacs and Blazars

Abstract

With their rapid, violent variability and broad featureless continuum emission, blazars have puzzled astronomers for over two decades. Today blazars represent the only extragalactic objects detected in high-energy gamma-rays. Their spectral energy distributions (SEDs) are characteristically double-humped, with lower-energy emission originating as synchrotron radiation in a relativistically beamed jet, and higher-energy emission due to inverse-Compton processes. This has accentuated the biases inherent in any survey to favor objects which are bright in the survey band, and should serve as a cautionary note both to those designing new surveys as well as theorists attempting to model blazar properties. The location of the synchrotron peak determines which blazar population is dominant at GeV and TeV energies. At GeV energies, low-energy peaked, high luminosity objects, which have high $L_C/L_S$ ratios, dominate, while at TeV energies, high-energy peaked objects are all that is seen. I review the differences between low-energy peaked and high-energy peaked blazars, and models to explain those differences. I also look at efforts to bridge the gap between these classes with new surveys. Two new surveys have detected a large population of high-energy peaked emission line blazars (FSRQ), with properties somewhat different from previously known objects. This discovery has the potential to revolutionize blazar physics in a way comparable to the discovery of X-ray selected BL Lacs ten years ago by {\it Einstein}. I cull from the new and existing surveys a list of $z<0.1$ high-energy peaked blazars which should be targets for new TeV telescopes. Among these are several high-energy peaked FSRQ.