A coherent timing solution for the nearby isolated neutron star RX J1308.6+2127/RBS 1223

We present a phase-connected timing solution for the nearby isolated neutron star RX J1308.6 + 2127 (RBS 1223). From dedicated Chandra observations as well as archival Chandra and XMM-Newton data spanning a period of 5 years, we demonstrate that the 10.31 s pulsations are slowing down steadily at a rate of. P = 1.120(3) X 10(-13) s s(-1). Under the assumption that this is due to magnetic dipole torques, we infer a characteristic age of 1.5 Myr and a magnetic field strength of 34 x 10(13)G. As with RX J0720.4 - 3125, the only other radio-quiet thermally emitting isolated neutron star for which a timing solution has been derived, the field strength is roughly consistent with what was inferred earlier from the presence of a strong absorption feature in its X-ray spectrum. Furthermore, for both sources the characteristic age is in excess of the cooling age inferred from standard cooling models. The sources differ, however, in their timing noise: while RX J0720.4 - 3125 showed considerable timing noise, RX J1308.6 - 2127 appears relatively stable.