A Strong, Broad Absorption Feature in the X-ray Spectrum of the Nearby Neutron Star RX J1605.3+3249

We present X-ray spectra taken with XMM-Newton of RX J1605.3+3249, the third brightest in the class of nearby, thermally emitting neutron stars. In contrast to what is the case for the brightest object, RX J1856.5-3754, we find that the spectrum of RX J1605.3+3249 cannot be described well by a pure black body, but shows a broad absorption feature at 27\AA (0.45 keV). With this, it joins the handful of isolated neutron stars for which spectral features arising from the surface have been detected. We discuss possible mechanisms that might lead to the features, as well as the overall optical to X-ray spectral energy distribution, and compare the spectrum with what is observed for the other nearby, thermally emitting neutron stars. We conclude that we may be observing absorption due to the proton cyclotron line, as was suggested for the other sources, but weakened due to the strong-field quantum electrodynamics effect of vacuum resonance mode conversion.