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 alpha(ph) = 2.16 +/- 0.15, N-H = (4.7 +/- 0.8) x 10(22) cm(-2), and unabsorbed flux similar to3.9 x 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 similar to1700 yr, closer to that of the supernova remnant than the simple estimate tau = P/2P = 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.