A Coherent Timing Solution for the Nearby Isolated Neutron Star

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 five years, we demonstrate that the 10.31-sec pulsations are slowing down steadily at a rate of Ṗ = 1.120(3)× 10 s s. 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 3.4×10 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. Subject headings: pulsars: individual (RX J1308.6+2127) — stars: neutron — X-rays: stars