IDENTIFICATION OF THE SIGMA SOURCE NEAR 3C 273 - A NEW CLASS OF AGN

The hard X-ray source detected with SIGMA at an offset of approximately 15' from 3C 273 may be identified with an uncatalogued Einstein X-ray source detected in the deep Einstein HRI exposure of 3C 273 and in subsequent short IPC exposures. We have identified this source, within the SIGMA error circle, with a faint (mag 19-20) QSO at z = 0.57. Comparison of the SIGMA and Einstein fluxes with this optical object suggest it is highly self-absorbed, and possibly highly variable. The discovery of such a class of AGN would have important consequences for the cosmic X-ray background.