4U1722 + 11: the discovery of an X-ray selected BL Lac object

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Abstract
The Uhuru X-ray source 401722 + 11 was observed using the microchannel-plate detector (High Resolution Imager) on the Einstein Observatory, and its coordinates measured to a precision of ~ 5 arcsec. A 16th magnitude stellar object within the error circle was observed spectroscopically at the Cerro-Tololo Interamerican Observatory, and at the Anglo–Australian Telescope, and found to have a featureless continuum. Subsequent radio observations at the Very Large Array have established that the object is a radio source at the level of 60 mJy, and optical polarization measurements have determined that the source exhibits variable polarization at the level of 10 per cent. On the basis of these observations, it is concluded that 4U1722 + 11 is a member of the class of objects known as X-ray selected blazars or BL Lac objects. A redshift for the object has not yet been determined – neither the optical image or the VLA map show any evidence of an extended component. This is the first BL Lac object to be discovered from the Uhuru X-ray catalogue. It is the eighth BL Lac object to be found in the combined all-sky survey catalogues from Uhuru, Ariel V and HEAO-1.

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