A HELICAL JET IN THE ORTHOGONALLY MISALIGNED BL-LACERTAE OBJECT MARKARIAN-501 (B1652+398)

The BL Lacertae object Mrk 501 belongs to a class of core-dominated ra dio sources with orthogonal misalignments between the apparent positio n angles of their radio jets on parsec and kiloparsec scales. New VLBA and MERLIN images of Mrk 501 are analyzed in terms of the geometrical models for such orthogonal jets proposed by Conway & Murphy (1993). R ecent hydrodynamical studies by Hardee, Cooper, & Clarke (1994) of the effects of driving helical distortions into a jet by motion of the ce ntral engine provide a physical mechanism for generating helically dis torted jets with just the properties required by the geometrical model . Applying these hydrodynamical models to Mrk 501 we find that driving periods of a few times 10(4) yr are required to explain the observed morphology. This driving period and the precise jet morphology are con sistent with the helical structure being excited by the orbital motion of a binary black hole system. This result, combined with the fact th at orthogonally misaligned radio sources comprise approximately half o f the total population of core-dominated sources, suggests that binary black holes may exist within 50% of core-dominated sources. For Mrk 5 01, analysis of the observed jet path, of the VLBA intensity distribut ion, and of synchrotron/ inverse Compton models for the integrated rad io to gamma-ray spectrum provides strong constraints on the physical p roperties of the jet. Most importantly, the Lorentz factor of the bulk flow must satisfy gamma > 2.1, and the half-opening angle of the heli x cone zeta < 12 degrees. These properties and others derived for Mrk 501 are consistent with this BL Lacertae object being a radio galaxy o f Fanaroff & Riley (1974) type I (F-R I), whose innermost VLBI regions are viewed at a small angle theta(c) < zeta/(3)(1/2) to the line of s ight. For the gamma and zeta limits mentioned above, the simplest rela tivistic beaming models, with their assumption of a single bulk flow v elocity, are inconsistent with the presence of a putative counterjet f eature seen on VLBA scales. Such difficulties are avoided by a model, such as that proposed by Laing (1993) for F-R I radio galaxies, which invokes a slower moving shear layer on the surface of the jet.