Thermal AGN Signatures in Blazars

Abstract

Long ignored in blazars because of the dominance of the beamed radiation from the jet, the topic of thermal emissions in these objects is just beginning to be explored. While this emission is weak in most blazars compared to the dominant nonthermal jet components, there is a growing body of evidence that suggests that thermal emission is observable even in the most highly beamed objects. The emitting regions, which can include the accretion disk as well as the torus, are key parts of the central engine which also powers the jets. They also may be of critical importance in helping us decide between unified scheme models. We will review the observational evidence for thermal emissions in blazars, with an emphasis on recent work, and the spectral and variability characteristics that have been observed. The majority of the evidence for thermal emission in blazars (now observed in several objects) has come as a result of multiwavelength campaigns, where the object showed a clear bump in the optical-UV in a faint state. However, evidence exists from other avenues as well, including both purely spectral and variability based arguments as well as statistical analyses of large samples of objects. We will also discuss the impact of thermal emission on an object's overall SED, including its Comptonized signatures. Finally, we will assess the current standing of unified scheme models as respects thermal signatures and the prospects for detecting thermal emission with new telescopes and missions, and further utilizing it as a probe of the central engine of blazars.