The appearance of broad H alpha in BL Lacertae

We present optical spectropolarimetry confirming Vermeulen et al.'s recent discovery of a broad H alpha emission line in BL Lacertae. This feature, which is present at about the same strength in spectra taken two days apart, is comparable in width (FWHM approximate to 4000 km s(-1)) and luminosity (L(H alpha) similar to 10(41) erg s(-1)) to the broad Ha lines of Seyfert type 1 galaxies. We also identify narrow lines of [N II] lambda 6583, [O I] lambda 6300 and [Fe VII] lambda 6087, The continuum polarization was at a relatively low level (approximate to 5 per cent) with no significant wavelength dependence. We estimate that the possible depolarizing effect of the line emission is too small to be clearly detected in our data. Simultaneous photometry shows that the source was not in an unusually faint state, although its V magnitudes were close to the lower limit of its typical variability range. Frequent monitoring during one night revealed a very rapid decrease in brightness by approximate to 0.1 mag in 30 min. We tentatively conclude that the broad Ha feature must have increased in luminosity by a factor similar to 4 since earlier spectroscopic observations. While the beamed synchrotron continuum associated with the radio source can account for the observed equivalent width, the fact that the line has not previously been detected suggests either a sudden injection of cold gas into the radiation field or the strengthening of an additional ionizing radiation source. We show that the Ha emission could be powered by thermal radiation from an accretion disc without significantly affecting the shape or polarization of the optical continuum. The presence of a broad emission line at the redshift of the host galaxy demonstrates that the prototype BL Lac object is not a microlensed high-redshift quasar.