ROSAT/ASCA observations of a serendipitous BL Lacertae object PKS 2316-423: The variable high-energy tail of synchrotron radiation

We present the analysis of archival data from ROSAT and ASCA of a serendipi tous BL Lac object PKS 2316-423. Because of its featureless nonthermal radi o/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 ob servations; in particular, a variable high-energy tail of the synchrotron r adiation 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 L ac objects (HBLs). The spectral energy distribution (SED) through radio to X-ray yields the synchrotron radiation peak at frequency v(p) = 7.3 x 10(15 ) Hz, with integrated luminosity of L-syn = 2.1 x 10(44) ergs s(-1). The av eraged SED properties of PKS 2316-423 are very similar to those "intermedia te" BL Lac objects (IBLs) found recently in several deep surveys, such as D eep 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 f eatures 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 synchrotr on or inverse Compton characteristics in different variability states.