The physical parameters of Markarian 501 during flaring activity

We determine the physical parameters (magnetic field and Doppler factor) of the homogeneous synchrotron self-Compton model allowed by the observed X-r ay to gamma-ray spectra and variability of Markarian 501 during the 1997 Ap ril 15-16 flaring activity. We find that magnetic fields between 0.07 and 0 .6 G and Doppler factors between 12 and 36 could fit (depending on observed variability time-scale) these observations. We take account of photon-phot on pair production interactions of gamma-ray photons occurring both inside the emission region and during propagation to Earth, and find these to be e xtremely important in correctly determining the allowed model parameters. P revious estimates of the allowed parameter space have neglected this effect . Future multiwavelength campaigns during strong flaring activity, includin g observations from optical to TeV gamma-rays, should enable the physical p arameters to be further constrained.