The Parameters of the Gamma-Ray Emission Region in Mrk 501

We determine the physical parameters magnetic field and Doppler factor of the homogeneous synchrotron self-Compton model allowed by the observed X ray to gamma-ray spectra and variability of Markarian 501 during the 15 16 April 1997 aring activity. We find that, for 20 minute variability, a magnetic field of 0.2 G and a Doppler factor of 20 could t these observations. We take account of photon-photon pair production interactions of gamma-ray photons occurring both inside the emission region and during propagation to Earth and find these to be extremely important in correctly determining the allowed model parameters.