Predictions of the High‐Energy Emission from BL Lacertae Objects: The Case of W Comae

Abstract

Spectral fitting of the radio through hard X-ray emission of BL Lac objects has previously been used to predict their level of high-energy (GeV-TeV) emission. In this paper, we point out that such spectral fitting can have very large uncertainties with respect to predictions of the very high energy (VHE) emission, in particular if no reliable, contemporaneous measurement of the GeV flux is available and the νFν peak (flux and frequency) of the synchrotron component is not very precisely known. This is demonstrated with the example of the radio-selected BL Lac object W Comae, which is currently on the source list of the STACEE and CELESTE experiments, based on extrapolations of the EGRET flux measured from this source, and on model predictions from hadronic blazar jet models. We show that the best currently available contemporaneous optical-X-ray spectrum of W Comae, which shows clear evidence for the onset of the high-energy emission component beyond ~4 keV and thus provides a very accurate guideline for the level of hard X-ray synchrotron self-Compton (SSC) emission in the framework of leptonic jet models, still allows for a large range of possible parameters, resulting in drastically different greater than 40 GeV fluxes. We find that all acceptable leptonic-model fits to the optical-X-ray emission of W Comae predict a cutoff of the high-energy emission around ~100 GeV. We suggest that detailed measurements and analysis of the soft X-ray variability of W Comae may be used to break the degeneracy in the choice of possible fit parameters and thus allow a more reliable prediction of the VHE emission from this object. Using the available soft X-ray variability measured by BeppoSAX, we predict a greater than 40 GeV flux from W Comae of ~(0.4-1) × 10-10 photons cm-2 s-1 with no significant emission at E 100 GeV for a leptonic jet model. We compare our results concerning leptonic jet models with detailed predictions of the hadronic synchrotron-proton blazar model. This hadronic model predicts greater than 40 GeV fluxes very similar to those found for the leptonic models, but results in greater than 100 GeV emission that should be clearly detectable with future high-sensitivity instruments like VERITAS. Thus, we suggest this object as a promising target for VHE γ-ray and coordinated broadband observations to distinguish between leptonic and hadronic jet models for blazars.