2002ApJ...574..873M

We present the results of BeppoSAX observations of the young X-ray pulsar PSR J1846-0258, recently discovered at the center of the composite supernova remnant Kes 75. The pulsar (plus nebula) spectrum can be fitted by an absorbed power law with photon index α_ph=2.16±0.15, N_H=(4.7±0.8)x10²² cm^–2, and unabsorbed flux ∼3.9x10^–11 ergs.cm^–2.s^–1 (2-10 keV). By joining two observations taken at an interval of 2 weeks, we have been able to obtain a precise measurement of the spin period (P=324.818968±0.000006 ms). This value, when combined with previous measurements, cannot be fitted by a smooth frequency evolution with a canonical braking index n=3. With the hypothesis of no glitches and/or significant timing noise, the braking index would be n=1.86±0.08, and assuming a short initial period, the pulsar age would be ∼1700 yr, closer to that of the supernova remnant than the simple estimate τ=P/2P{dot}=723 yr. Other likely possibilities involve the presence of glitches and lead to a wide range of acceptable ages. For example, we obtain n in the range 1.8-2.5 if a glitch occurred near MJD 51500, while for a glitch between 1993 October and 1999 March, we can only get a lower limit of n>1.89.