Bepposax Observations of the Young Pulsar in the Kes 75 Supernova Remnant

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 ±ph=2.16+/-0.15, NH=(4.7+/-0.8)×1022 cm-2, and unabsorbed flux ~3.9×10-11 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 Ä=P/2P&d2;=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.