X-ray timing of the enigmatic neutron star RX J0720.4-3125

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) over dot\ < 3.6 x 10(-13) s s(-1) (3 σ). While our upper limit on (P) over 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 ≲10(14) 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) over 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) over 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.