Optical Observations of the Isolated Neutron Star RX J0720.4−3125

RX J0720.4-3125 is an unidentified bright soft X-ray source which showspulsations at a 8.39 s period and has a thermal spectrum. We present deep B andR band images of its X-ray localization. We find one possible counterpart inthe X-ray error box, with magnitudes B=26.6+/-0.2 and R=26.9+/-0.3. The veryhigh X-ray to optical flux ratio confirms that this object is an isolatedneutron star. We discuss possible models and conclude that only two areconsistent with the data and at the same time are able to draw from a largeenough population to make finding one nearby likely. In our opinion the secondcriterion provides a stringent constraint but appears to have been ignored sofar. The first model, suggested earlier, is that RX J0720.4-3125 is a weaklymagnetized neutron star accreting from the interstellar medium. The second isthat it is a relatively young, highly magnetized neutron star, a ``magnetar'',which is kept hot by magnetic field decay.Comment: 8 pages, 1 Figure. Accepted for publication in ApJ (Letters