BL Lac identification for the ultraluminous X-ray source observed in the direction of NGC 4698

Forschini, L. et al. (14 authors including P. Martini), 2002, A&A, 396, 787

We report the identification of the optical and radio counterparts of the ultraluminous X-ray (ULX) source XMMU J124825.9+083020 (NGC 4698-ULX1). The optical spectrum taken with the VLT yields a redshift of z=0.43, which implies that the ULX is not associated with the nearby galaxy NGC 4698. The spectral energy distribution calculated from the available data indicates that the source is likely to be a BL Lac object. The possible synchrotron peak at X-ray energies suggests that this source may be a gamma-ray emitter.