PAIR CASCADES IN EXTRAGALACTIC JETS .3. SYNCHROTRON EMISSION

The synchrotron emission produced by cascading pairs in a relativistic jet is considered, and the relation between the emergent synchrotron spectrum and the structure of the gamma-ray-emitting jet is carefully examined, using numerical simulations. Many blazars exhibit radio-to-u ltraviolet continuum spectral shape which can be well fitted by broken power laws, with breaks at millimeter to submillimeter wavelengths. T he spectrum below the break appears to be flat, and above it the slope is typically in the range -0.7 to -1.5. In some cases a second break at much shorter wavelengths is present. It is found that such spectra can be accounted for quite naturally by the pair-cascade model, provid ed that the product of pair injection rate and magnetic field declines sufficiently steeply with radius; the millimeter/submillimeter break is associated, in the model, with a radius (annihilation radius) below which the pair content of the jet and, hence, the emissivity are stro ngly limited by annihilation The slope of the spectrum above the break reflects essentially the distribution of cascading pairs near the ann ihilation radius, which also produce the hard X-ray emission, and the flat spectrum at low frequencies is due to self-absorption at the zone above the annihilation radius. The second break at higher frequencies is due to the maximum energy cutoff of the injected pair spectrum. Th e emission at radio frequencies originates, quite generally, from radi i well above the gamma-ray emission region, suggesting that variations of the radio flux should be slower or later than variations of the ga mma-ray flux. Further implications for temporal variations of the broa dband emission from blazars are also briefly discussed.