A coherent timing solution for the nearby isolated neutron star RX J0720.4-3125

We present the results of a dedicated effort to measure the spin-down rate of the nearby isolated neutron star RX J0720.4-3125. Comparing arrival times of the 8.39 s pulsations for data from Chandra, we derive an unambiguous timing solution for RX J0720.4-3125 that is accurate to < 0.1 cycles over 15 years. Adding data from XMM and ROSAT, the final solution yields P = (6.98 +/- 0.02) x 10(-14) s s(-1); for dipole spin-down, this implies a characteristic age of 2 Myr and a magnetic field strength of 2.4 x 10(13) G. The phase residuals are somewhat larger than those for purely regular spin-down, but they do not show conclusive evidence for higher order terms or a glitch. From our timing solution as well as recent X-ray spectroscopy, we concur with recent suggestions that RX J0720.4-3125 is most likely an off-beam radio pulsar with a moderately high magnetic field.