Monitoring the Ha Line in BL Lacertae - Evidence for an Accretion Disk

Abstract

We present new spectroscopic observations of the recently discovered broad Ha line in BL Lacertae. These data, obtained over a period of 30 months, indicate that the line equivalent width is anti-correlated with the continuum flux, implying that the broad line emission is not powered primarily by synchroton emission from the relativistic jet. The most probable explanation is that the broad line region in BL Lacertae is photoionized by continuum emission from an accretion disk which at optical wavelengths is dominated by the Doppler beamed synchroton continuum.