H1722+119: A Highly Polarized X-Ray--selected BL Lacertae Object

Abstract

We report the discovery of a 15th magnitude X-ray-selected BL Lacertae object as part of a program to identify X-ray sources observed by the Scanning Modulation Collimator experiment during the HEAO I all-sky survey. The BL Lac object, H1722+119, is a strong and persistent X-ray source originally discovered by Uhuru and located by the HRI instrument on the Einstein Observatory, The Einstein HRI and MPC observations indicate a steep X-ray spectrum with a power-law energy index near 1.3. The optical spectrum shows no obvious emission or absorption features, and the U-B and B-V indices are typical of BL Lac objects. There is no evidence of a host galaxy in CCD images with broad-band R and I filters, implying that the X-ray and optical luminosities are both greater than 10^45^ ergs s^-1^. Radio measurements indicate a variable, compact source with a power-law energy index (1.4 to 4.9 GHz) of α = 0.04. Broad-band spectral indices from radio to UV, and UV to X-ray are consistent with other X-ray selected blazars, both being flatter than for radio-selected objects. We have difficulty connecting the optical, UV, and X-ray flux densities with a single power law, but this assessment is hampered by uncertainties due to source variability and the precise correction for interstellar absorption and reddening. The optical-infrared polarization of H1722+ 119 reaches a maximum of 17%, which is much higher than other X-ray-selected BL Lac objects. The degree of polarization is found to be wavelength-dependent, decreasing in a smooth and monotonic way toward the infrared. We find no significant variation in position angle with wavelength.