PSR J1833-1034: Discovery of the central young pulsar in the supernova remnant G21.5-0.9

We have discovered the pulsar associated with the supernova remnant G21.5-0.9. PSR J1833-1034, with spin period P = 61.8 ms and dispersion measure 169 cm(-3) pc, is very faint, with pulse-averaged flux density of approximate to 70 mu Jy at a frequency of 1.4 GHz, and was first detected in a deep search with the Parkes telescope. Subsequent observations with Parkes and the Green Bank Telescope have confirmed this detection and yield a period derivative. P 2.02 x 10(-13). These spin parameters imply a characteristic age tau(c) = 4.8 kyr and a spin-down luminosity E 3: 3; 10(37) ergs s(-1), the latter value exceeded only by the Crab pulsar among the rotation-powered pulsars known in our Galaxy. The pulsar has an unusually steep radio spectrum in the 0.8-2.0 GHz range, with power-law index approximate to 3.0, and a narrow single-peaked pulse profile with FWHM of 0.04P. We have analyzed 350 ks of archival Chandra X-Ray Observatory HRC data and find a pointlike source of luminosity approximate to 3 x 10(-5). E, offset from the center of an elliptical region of size approximate to 70"5" and luminosity approximate to 10(-3). E within which likely lies the pulsar wind termination shock. We have searched for X-ray pulsations in a 30 ks HRC observation without success, deriving a pulsed fraction upper limit for a sinusoidal pulse shape of about 70% of the pulsar flux. We revisit the distance to G21.5-0.9 based on H (I) and CO observations, arguing that it is 4.7 +/- 0.4 kpc. We use existing X-ray and radio observations of the pulsar wind nebula, along with the measured properties of its engine and a recent detection of the supernova remnant shell, to argue that G21.5-0.9 and PSR J1833-1034 are much younger than tau(c) and likely their true age is <= 1000 yr. In that case, the initial spin period of the pulsar was >= 55 ms.