A Seyfert-like nucleus concealed in BL Lacertae?

We present a series of new spectroscopic observations of BL Lacertae. The observations were obtained over a period of 30 months and sample the violent continuum flare of 1997 June–August. The equivalent width of the recently discovered broad Hα line varies approximately inversely with the optical continuum flux, implying that the broad-line-emitting gas does not respond significantly to large variations in the beamed synchrotron continuum which dominates the spectrum at optical wavelengths. The simplest and most plausible interpretation of this result is that the broad-line region in BL Lacertae is ‘unaware’ of the synchrotron beam and is photoionized by another source, a likely candidate being a hot accretion disc. Alternatively, similar behaviour would be observed if the broad-line region is indeed photoionized by the synchrotron beam but is predominantly matter-bounded so that it absorbs only a small fraction of the incident ionizing photon flux.