2002ApJ...570L..79K

RX J0720.4-3125 is the third brightest neutron star in the soft X-ray sky and has been a source of mystery since its discovery, as its long 8 s period separates it from the population of typical radio pulsars. Three models were proposed for this source: a neutron star accreting from the interstellar medium, an off-beam radio pulsar, or an old, cooling magnetar. Using data from Chandra, ROSAT, and BeppoSAX, we are able to place an upper limit to the period derivative, |P{dot}|<3.6x10^–13 s.s^–1 (3 σ). While our upper limit on P{dot} allows for the accretion model, this model is increasingly untenable for another similar but better studied neutron star, RX J1856.5-3754, and we therefore consider the accretion model unlikely for RX J0720.4-3125. We constrain the initial magnetic field of RX J0720.4-3125 to be ≲10¹⁴ G based on cooling models, suggesting that it is not and never was a magnetar but is instead a middle-aged neutron star. We propose that it is either a long-period high magnetic field pulsar with P{dot}∼10^–13 s.s^–1, similar to PSR J1814-1744, or a neutron star born with an initial period of ~8.3 s and P{dot}∼10^–15 s.s^–1. The proximity of RX J0720.4-3125 is strongly suggestive of a large population of such objects; if so, radio pulsar surveys must have missed many of these sources.