6 Final remarks

We have presented a multiwavelength analysis of the ultraluminous X-ray source XMMU J124825.9+083020 observed in the direction of NGC 4698. The availability of X-ray, optical, and radio data makes it possible to construct the spectral energy distribution, but the crucial information to decipher the true nature of the source was provided by the optical spectrum. We show that the absorption features are unambiguously associated with a background source at z=0.43. We argue that the source is most likely a BL Lac object, most likely of the high-frequency peaked category, based on the absence of emission lines, the dilution of the absorption features, the slope of the optical continuum, and the overall shape of the spectral energy distribution.

Acknowledgements

We would like to thank A. Pizzella for useful discussion about NGC 4698, the Service Mode Observations personnel at ESO, and in particular M. Romaniello, for the help in the preparation of VLT observations. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). This research has made use of the NASA Astrophysics Data System Abstract Service and of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We acknowledge the partial support of the Italian Space Agency (ASI) to this research. L.C.H. is grateful for financial support from the Carnegie Institution of Washington and from NASA grants.