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 HSTimage quality was improved with the deployment of the corrective optics (COSTAR)in December 1993. From the FOS target acquisition data, we have established that theflux from the optical nucleus of M87 varies by a factor ∼2 on time scales of ∼2.5 monthsand by as much as 25% over 3 weeks, and remains unchanged (∼< 2.5%) on time scalesof ∼ 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 time scales to afew hundred gravitational radii. The featureless continuum spectrum becomes blueras it brightens while emission lines remain unchanged. This variability combined withthe observations of the continuum spectral shape, strong relativistic boosting and thedetection of significant superluminal motions in the jet, strongly suggest that M87belongs to the class of BL Lac objects but is viewed at an angle too large to reveal theclassical BL Lac properties.Subject headings: galaxies: individual (M87) — galaxies: active — galaxies: nuclei —BL Lacertae objetcs: general1. INTRODUCTIONVariability is a fundamental property of Active Galactic Nuclei (AGN). In fact, it was the rapidchanges of the observed optical continuum fluxof quasars that led early researchers to the conclusion