ROSAT/ASCA Observations of a Serendipitous BL Lac 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 serendipitous BL Lac object PKS 2316-423. Because of its featureless non-thermal radio/optical continuum, PKS 2316-423 has been called as 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 broad-band spectrum: a steep and downward curving spectrum between 0.1--10 keV, typical of high-energy peaked BL Lacs (HBL). The spectral energy distribution (SED) through radio-to-X-ray yields the synchrotron radiation peak at frequency Vp=7.3 10^{15} Hz, with integrated luminosity of Lsyn=2.1 10^{44} ergs s^{-1}. The averaged SED properties of PKS 2316-423 are very similar to those ``intermediate'' BL Lac objects (IBL) 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 a 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.