Double-peaked Balmer line emission in the radio-quiet AGN RX J1042+1212

We present optical and X-ray spectra of a radio-quiet, X-ray-selected AGN, RX J1042+1212 (z=0.271). The Hα and Hβ emission lines are very broad (with full widths at half maximum of ~10000 km s^-1) and have double-peaked profiles. Such features are rarely observed in AGN in general, but are even more unusual in radio-quiet objects. The analysis of the ROSAT PSPC data reveals a non-varying, unabsorbed spectrum with an energy spectral index alpha_x=1.2 and little or no emission from a soft X-ray excess. The slope of the optical spectrum is similar, alpha_opt=1.0, and is consistent with an extrapolation of the X-ray spectrum, suggesting that the same power-law continuum may dominate throughout, and that the big blue bump component is relatively weak. We look for a link between these various properties, and investigate models of double-peaked Balmer line emission in AGN. An accretion disc origin is unlikely in RX J1042+1212, as this model predicts that lines emitted by a disc should have a net gravitational redshift (both Hα and Hβ have a net blueshift). Emission from two broad-line regions, each gravitationally bound to one component of a supermassive black hole binary, is a possibility if the two components are similar in size and nature. Alternatively, the lines (or at least the narrow peaks of the lines) may be produced by a double-sided jet or bipolar flow.