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

Abstract

RX J0720.4-3125 is an unidentified bright soft X-ray source which shows pulsations at a 8.39 s period and has a thermal spectrum. We present deep B and R band images of its X-ray localization. We find one possible counterpart in the X-ray error box, with magnitudes B=26.6+/-0.2 and R=26.9+/-0.3. The very high X-ray to optical flux ratio confirms that this object is an isolated neutron star. We discuss possible models and conclude that only two are consistent with the data and at the same time are able to draw from a large enough population to make finding one nearby likely. In our opinion the second criterion provides a stringent constraint but appears to have been ignored so far. The first model, suggested earlier, is that RX J0720.4-3125 is a weakly magnetized neutron star accreting from the interstellar medium. The second is that it is a relatively young, highly magnetized neutron star, a ``magnetar'', which is kept hot by magnetic field decay.

8 pages, 1 Figure. Accepted for publication in ApJ (Letters)

References in corpus (4)

• Neutron Star Population Dynamics.II: 3D Space Velocities of Young Pulsars
• The Discovery of an Anomalous X-ray Pulsar in the Supernova Remnant Kes 73
• Evidence for magnetic field decay in RX J0720.4-3125
• What is the nature of RX J0720.4-3125?

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