Predictions of the high-energy emission
from BL Lac objects:
The case of W Comae
M. Böttcher, R. Mukherjee, and A. Reimer,
Astrophysical Journal, 581, 143 (2002)
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 VHE emission,
in particular if no reliable, contemporaneous measurement of
the GeV flux is available and the 
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
SSC emission in the framework of leptonic jet models, still allows
for a large range of possible parameters, resulting in drastically
different > 40 GeV fluxes. We find that all acceptable
leptonic-model fits to the optical - X-ray emission of W Comae predict
a cut-off 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 > 40 GeV
flux from W Comae of ~ (0.4 - 1) x 10-10 photons
cm-2 s-1 with no significant emission at
E > 100 GeV. We compare our results concerning
leptonic jet models with detailed predictions of the hadronic
Synchrotron-Proton Blazar model. This hadronic model predicts
> 40 GeV fluxes very similar to those found for the leptonic
models, but results in > 100 GeV emission which should be clearly
detectable with future high-sensitivity instruments like VERITAS.
Thus, we suggest this object as a promising target for VHE gamma-ray
and co-ordinated broadband observations to distinguish between leptonic
and hadronic jet models for blazars.
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