Coordinated observations of Markarian 501 at ultraviolet, X-ray, optical, and radio wavelengths

The results of multifrequency observations of MrK 501 = B2 1652+39 are presented here. It is a BL Lac object embedded in a giant elliptical galaxy obtained during the period August 1980 - March 1981. It is argued that MrK 501 varies sufficiently slowly at ultraviolet, optical and radio wavelengths on time scales of months that the data are representative of a spectrum obtained at a single epoch. The importance of MrK 501 derives mostly from its association of a quasar-like object with a galaxy whose absorption-line redshift (z = 0.034) unambiguously determines its distance. This implies that physical conditions within this local quasar can be discussed without the confusion introduced by distance uncertainties.