Unconventional quasars from the variability and proper motion survey

The bias introduced by the criteria for the selection of quasar candidates for follow-up spectroscopy remains a serious issue in quasar search, even though the problems of the detection of significant numbers of quasars have long been overcome. For all large optical surveys, candidate selection relies on characteristic properties of a presumed spectral energy distribution (SED) or on differences of the quasar SED from those of stars. However, the standard unified AGN scheme, in combination with scenarios of quasar evolution, predicts a broader variety of spectral features. In fact, the universe contains very unconventional AGN, but the sample of known objects of these types is still quite small. Hence, every single detection of odd quasars is important since it may hold clues to open questions of quasar structure and evolution. Quasars with unconventional spectra are obviously overlooked in classical optical search techniques based upon optical/UV colour or objective prism selection. We performed an alternative search programme based upon flux variations and positional stationarity measured in two Schmidt fields. This variability and proper motion survey (VPMS) provides a unique tool to select odd quasars with unusual spectra because it combines a high survey efficiency with the advantage of candidate selection not explicitly relying on a presumed quasar SED. The overwhelming majority of the VPMS quasars exhibit normal spectra, but we find also a couple of quasars with very unusual properties. Though higher resolution spectra have enabled to ascertain the redshifts, the physics behind the spectra is not yet understood. The most peculiar object, the radio-loud quasar VPMS J1342+2840, shows a strong compression of the continuum over a wide wavelength range in the UV, without the typical structures of broad absorption line (BAL) troughs. If this downturn at short wavelengths is due to dust, the extinction curve must be very different from the standard SMC curve. Unusual BAL quasars show the tendency to be reddened. In order to improve the sensitivity of our survey for reddened objects, we make the effort to improve the limiting magnitude by using stacked Schmidt plates instead of single plates. Preliminary results indicate a gain of about 1 mag, i.e. a survey limit of Blim > 21.5 for this deeper VPMS. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.