ROSAT/ASCAObservations 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 aserendipitous BL Lac object PKS 2316-423. Because of its featurelessnon-thermal radio/optical continuum, PKS 2316-423 has been called as a BL Laccandidate in the literature. PKS 2316-423 was evidently variable over themultiple X-ray observations, in particular, a variable high-energy tail of thesynchrotron radiation is revealed. The X-ray spectral analysis provides furtherevidence of the synchrotron nature of its broad-band spectrum: a steep anddownward curving spectrum between 0.1--10 keV, typical of high-energy peaked BLLacs (HBL). The spectral energy distribution (SED) through radio-to-X-rayyields the synchrotron radiation peak at frequency Vp=7.3 10^{15} Hz, withintegrated luminosity of Lsyn=2.1 10^{44} ergs s^{-1}. The averaged SEDproperties of PKS 2316-423 are very similar to those ``intermediate'' BL Lacobjects (IBL) found recently in several deep surveys, such as Deep X-ray RadioBlazar, Radio-Emitting X-ray, and ROSAT-Green Bank surveys. We suggest that PKS2316-423 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 theprediction that an IBL object can show either synchrotron or inverse-Comptoncharacteristics in different variability states.Comment: 12 pages, including 4 figures, to appear in ApJ, 2000 July 2