The emission from blazars is thought to arise from a relativistic jet which is oriented near the observer's line-of-sight. The near UV-optical through radio emissions are likely to be synchrotron radiation with, in some cases, an additional thermal UV-optical component arising from a hot central accretion disk. The gamma-ray emission is most likely explained as inverse Compton scattering of photons by relativistic electrons in the jet. However the origin of the seed photons is not clearly understood. Major flares in the radio-infrared-optical synchrotron component have been modelled as shocks propagating down the jet (e.g. Marscher and Gear 1985). Attempts have been made to extend these models to flares observed at shorter wavelengths, but with only limited success. At the highest energies (i.e. gamma-rays and hard X-rays), there seems to be two emission components: a flat spectrum hard X-ray-gamma-ray component, and a steep-spectrum soft X-ray component which is often assumed to be the tail of the radio-infrared-optical synchrotron component.
In the standard jet models (Blandford and Konigl 1979, Marscher 1993), there are two major causes of brightness variations: (1) the energy flow into the jet is increased dramatically for an extended time, or (2) a less dramatic or prolonged energy input can produce shocks in the jet which result in flares. Multifrequency variability studies can, in principle, allow one to test and compare these different jet models.
Marscher (1980) and Ghisellini, Maraschi, and Treves (1985) have shown that the shape of the blazar multifrequency spectra can be understood as synchrotron-self Compton (SSC) emission from a tapered relativistic jet that accelerates from its base to the site of the radio emission. The alternative jet model of Melia and Konigl (1989) starts the jet as an extremely relativistic particle beam which is decelerated through scattering of the particles by the ambient radiation.
In either jet model, as a shock propagates down the jet, the resulting flare is time-delayed different amounts at different frequencies. In general, variations occuring in shocks should exhibit approximately simultaneous variations at optical-UV frequencies and be time-delayed, and of smaller amplitude, at lower frequencies. There are, however, specific differences in the predictions of the two models. The {\bf decelerating jet model} predicts that a {\bf UV-optical flare should occur before a gamma-ray/X-ray flare} is observed, which should, in turn, be followed by a radio flare. In the {\bf accelerating jet model}, {\bf UV-optical and gamma-ray flares should occur simultaneously} and then be followed by an infrared and X-ray flare. In either case, the frequency-dependent time-delays will allow one to test and constrain particular jet and shock models.
The blazar, Mrk 501, is an optically bright, TeV blazar. An extensive monitoring program at optical and TeV frequencies, simultaneously, would allow one to provide a crucial observational test which would allow one to distinguish between the decelerating jet model and the accelerating jet model. In the case of the decelerating jet model described above, in the event of the detection of a flare at optical frequencies followed by a flare at TeV energies, one would also request support from groundbased radio observers (e.g., Margo Aller at the University of Michigan) in an effort to determine if this latter flare is followed by a flare at radio wavelengths. In the case of the accelerating jet model, simultaneous observations at optical/gamma-ray freqencies are required in order to detect a simultaneous event. If such an event is observed, then one would immediately request a ToO observation with RXTE in an effort to detect the predicted X-Ray flare. Thus simultaneous optical/gamma-ray observations are essential to distinguishing between these two models.
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