Abstract
The nuclear region of M87 was observed with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) at six epochs, spanning 18 months, after the HST image quality was improved with the deployment of the corrective optics (COSTAR) in 1993 December. From the FOS target acquisition data, we have established that the flux from the optical nucleus of M87 varies by a factor of ~2 on timescales of ~2.5 months and by as much as 25% over 3 weeks and remains unchanged (≲2.5%) on timescales of ~1 day. The changes occur in an unresolved central region ≲5 pc in diameter, with the physical size of the emitting region limited by the observed timescales to a few hundred gravitational radii. The featureless continuum spectrum becomes bluer as it brightens while emission lines remain unchanged. This variability, combined with the observations of the continuum spectral shape, strong relativistic boosting, and the detection of significant superluminal motions in the jet, strongly suggest that M87 belongs to the class of BL Lac objects but is viewed at an angle too large to reveal the classical BL Lac properties.