M87: A Misaligned BL Lacertae Object?

The nuclear region of M87 was observed with the Faint Object Spectrograph(FOS) on the Hubble Space Telescope (HST) at 6 epochs, spanning 18 months,after the HST image quality was improved with the deployment of the correctiveoptics (COSTAR) in December 1993. From the FOS target acquisition data, we haveestablished that the flux from the optical nucleus of M87 varies by a factor ~2on time scales of ~2.5 months and by as much as 25% over 3 weeks, and remainsunchanged (<= 2.5%) on time scales of ~1 day. The changes occur in anunresolved central region <= 5 pc in diameter, with the physical size of theemitting region limited by the observed time scales to a few hundredgravitational radii. The featureless continuum spectrum becomes bluer as itbrightens while emission lines remain unchanged. This variability combined withthe observations of the continuum spectral shape, strong relativistic boostingand the detection of significant superluminal motions in the jet, stronglysuggest that M87 belongs to the class of BL Lac objects but is viewed at anangle too large to reveal the classical BL Lac properties.Comment: 12 pages, 3 Postscript figure