  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.
