  The X-ray binary LSI +61 303, consisting of a main sequence Be star and a
compact object has been detected in the TeV range with MAGIC and VERITAS, and
showed a clear intensity modulation as a function of the orbital phase. We
describe a gamma-ray attenuation model and apply it to this system. Our first
result is that interaction of high energy photons with the background radiation
produced by the main sequence star alone does not account for the observed
modulation. We then include interactions between very high energy radiation and
matter, and are able to constrain fundamental parameters of the system such as
the mass of the compact object and the density of circumstellar matter around
the Be star. In our analysis of the TeV data, we find that the compact object
has mass $M_2>2.5M_{\odot}$ at the 99% confidence level, implying it is most
likely a black hole. However, we find a column density which conflicts with
results from X-ray observations, suggesting that attenuation may not play an
important role in the modulation.
