ROSAT/ASCAObservations of a Serendipitous BL Lacertae Object PKS 2316−423: The Variable High‐Energy Tail of Synchrotron Radiation

Abstract

We present the analysis of archival data from ROSAT and ASCA of a serendipitous BL Lac object PKS 2316-423. Because of its featureless nonthermal radio/optical continuum, PKS 2316-423 has been called a BL Lac candidate in the literature. PKS 2316-423 was evidently variable over the multiple X-ray observations; in particular, a variable high-energy tail of the synchrotron radiation is revealed. The X-ray spectral analysis provides further evidence of the synchrotron nature of its broadband spectrum: a steep and downward-curving spectrum between 0.1 and 10 keV, typical of high-energy peaked BL Lac objects (HBLs). The spectral energy distribution (SED) through radio to X-ray yields the synchrotron radiation peak at frequency νp = 7.3 × 1015 Hz, with integrated luminosity of Lsyn = 2.1 × 1044 ergs s-1. The averaged SED properties of PKS 2316-423 are very similar to those intermediate BL Lac objects (IBLs) found recently in several deep surveys, such as Deep X-ray Radio Blazar, Radio-Emitting X-ray, and ROSAT-Green Bank surveys. We suggest that PKS 2316-423 is an IBL though it also shows some general features of an HBL. Actually, this double attribute of PKS 2316-423 provides a good test of the prediction that an IBL object can show either synchrotron or inverse Compton characteristics in different variability states.