Two X-Ray--selected BL Lacertae Objects Observed with the HEAO 1 Scanning Modulation Collimator

Abstract

We have discovered two 16th magnitude BL Lac objects, H1101-232 and H1426+428, during an ongoing program to identify the optical counterparts of X-ray sources from the HEAO 1 survey. Each is a persistent and relatively bright X-ray source that was also detected by the Uhuru and Ariel 5 surveys. Our classifications are based on the combined observations of featureless optical continua, compact radio emission, and images of the underlying host galaxies. In addition, we have measured 2.7% linear polarization in the optical emission of H1101-232. We have confirmed the X-ray identification of H1426+428 in an observation with EXOSAT. The EXOSAT X-ray spectrum is well modeled by a power law with an energy index of 1.19. Both BL Lac objects exhibit high f_x_/f_opt_ ratios, and they show a distinct resemblance to other X-ray-selected BL Lac objects. The host galaxies are ~ 18th mag with angular diameters ~ 20", and each may be the dominant member of a cluster of galaxies. Additional wide-slit spectroscopy reveals faint absorption features with redshifts of 0.129 for H1426+428 and 0.186 for H1101-232. The implied X-ray and optical luminosities exceed 10^45^ ergs s^-1^ for both BL Lac nuclei, assuming isotropic emission. The optical brightness of H1101-232 faded by 0.3-0.6 mag during most of 1987. The HEAO 1 survey continues to identify BL Lac objects at a rate ~ 15% of the total number of active galactic nuclei, in sharp contrast with the statistics of the Einstein serendipitous sources at lower X-ray flux. Since the percentage of measurable redshifts is high among HEAO 1 BL Lacs (14 of 24, currently), the flux-limited subset of HEAO 1 LASS-MC identifications may clarify this problem by constraining the luminosity function of nearby, X-ray-selected BL Lac objects.