Simultaneous Multifrequency Observations of Markarian 421

Abstract

The highly variable BL Lacertae object Mrk 421 has been observed simultaneously in the radio, optical, ultraviolet, and X-ray bands over a period of 4 days in early 1984 December and once again in early 1985 January. Using the EXOSAT observatory, the authors found that during this time the 2 - 10 keV flux dropped by a factor of 8, whereas the 0.1 - 1 keV flux decreased by a factor of only 2. These changes were not reproduced at longer wavelengths during the period of simultaneous observations. The implications of these results are discussed in the context of (1) beaming models, (2) the connection between BL Lacertae objects and other classes of AGNs, and (3) the nature of the core region of active galaxies.