Predicted extragalactic TeV gamma-ray sources

We suggest that low-redshift X-ray-selected BL Lacertae objects (XBLs) may be the only extragalactic gamma-ray sources observable at TeV energies. We use simple physical considerations involving synchrotron and Compton component spectra for blazars to suggest why the observed TeV sources are XBLs, whereas mostly radio-selected BL objects (RBLs) and flat spectrum radio quasars (FSRQs) are seen at GeV energies. These considerations indicate that the differences between XBLs and RBLs cannot be explained purely as relativistic jet orientation effects. We note that the only extragalactic TeV sources that have been observed are XBLs and that a nearby RBL with a very hard spectrum in the GeV range has not been seen at TeV energies. We also note that of the 14 BL Lac objects observed by EGRET, 12 are RBLs, whereas only two are XBLs. We give a list of nearby XBLs that we consider to be good candidate TeV sources and predict estimated TeV fluxes for these objects.