A multiwavelength study of the extreme AGN J2310-437

We present new X-ray, radio, and optical data for the unusual galaxy/cluste r system J2310-437. Our results confirm the presence of an active nucleus a nd suggest an interpretation as an anomalous BL Lac object of bulk relativi stic Doppler factor <2, with an optically deficient radio to X-ray spectrum . The radio, optical, and soft X-ray flux densities could lie along a singl e power-law function, lacking the curvature typical of BL Lac objects. Comp ared with other known sources that may have comparable multifrequency spect ra, J2310-437 is the most extreme. Its low isotropic optical/UV radiation i s consistent with the intensity of external photons governing the electron spectral break through Compton cooling; in this source the external photon density would be too low to produce a spectral break below the X-ray.