J an 2 00 3 Phase Coherent Timing of PSR J 1811 – 1925 : The Pulsar at the Heart of G 11 . 2 – 0 . 3

The X-ray Pulsar PSR J1811–1925 in the historic supernova remnant G11.2–0.3 has a characteristic age over 10 times the age of the remnant. This likely implies that its current spin period, 65 ms, is close to its birth spin period. Alternatively, the pulsar may have an unusually high braking index. We report here on regular Rossi X-ray Timing Explorer / Proportional Counting Array (RXTE /PCA) timing observations of the pulsar that were designed to measure its braking index. We provide a preliminary phase-coherent timing solution which includes a significant¨ν. The braking index we measure is ≫ 3, likely a result of conventional timing noise. We also report on a preliminary analysis of the pulsar's unusually hard spectrum: we determine a photon index of Γ = 1.78 ± 0.74, for the pulsed component of the spectra, consistent within the uncertainties with previous ASCA and Chandra observations. The pulsed emission of PSR J1811–1925 is seen beyond 30 keV, and the pulsations remain sinusoidal up to and beyond this energy. INTRODUCTION The determination of the ages of pulsars is of crucial importance to neutron star astrophysics. Neutron star population synthesis, birthrate, supernova remnant associations, and even cooling timescales (which serve as probes of the neutron star equation of state), all depend on accurate determinations of pulsar ages. In general, a pulsar's age is estimated by assuming a spin-down evolution of the form