The Properties of X-Ray Selected BL Lac Objects

Abstract

The optical and radio properties of the X-ray selected BL Lacs in the Einstein Medium Sensitivity Survey are presented and compared with those of the HEAO 1 A-2 sample and with those of radio selected BL Lacs. The X-ray selected BL Lacs possess smaller polarized fractions and less violent optical variability than radio selected BL Lacs. These properties are consistent with the substantial starlight fraction seen in the optical spectra of a majority of these objects. This starlight allows a determination of definite redshifts for two of four MSS BL Lacs and a probable redshift for a third. These redshifts are 0.19, 0.297 and 0.59.

As a class, X-ray selected BL Lacs have the largest ratio of X-ray to optical flux of any Active Galactic Nuclei (AGN) yet discovered. We have also found that the number-flux relation for BL Lacs shows a drastic flattening below fluxes of the order of 10^-12 ergs cm^-2 s^-1 and we have interpreted this as evidence that BL Lacs do not share the evolutionary properties of quasars.

The results presented here are summarized from two papers, Maccacaro et al. 1984 and Stocke et al. 1985, to which we refer the reader seeking a detailed discussion of the available radio, optical and X-ray data. For a description of the Einstein Medium Sensitivity Survey see Maccacaro et al. 1982, Stocke et al. 1983, and Gioia et al. 1984.