2002MNRAS.334..345Z

We present a combined analysis of XMM-Newton, Chandra and ROSAT observations of the isolated neutron star RX J0720.4-3125, spanning a total period of ∼7yr. We develop a maximum likelihood periodogram for our analysis based on the Δ C statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked a posteriori by folding a further BeppoSAX data set with the period predicted at the time of that observation: the phase is found to be consistent.

The study of the spin history and the measure of the spin-down rate are of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of P{dot}, here measured for the first time, is ~10^–14s.s^–1. This value cannot be explained in terms of torque from a fossil disc. When interpreted in terms of dipolar losses, it gives a magnetic field of B~10¹³G, making it also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find it unlikely that the field decayed from a much larger value (B ~10¹⁵G, as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of ~10⁴ yr, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is ~10⁶ yr old, and its magnetic field has not changed substantially over the lifetime.