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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 pc/cc, is very faint, with pulse-averaged flux density of approximately 70 microJy 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 dP/dt = 2.02e-13. These spin parameters imply a characteristic age tau_c = 4.8 kyr and a spin-down luminosity dE/dt = 3.3e37 ergs/s, 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 ~3.0, and a narrow single-peaked pulse profile with full-width at half maximum of 0.04P. We have analyzed 350 ks of archival Chandra X-ray Observatory High Resolution Camera (HRC) data, and find a point-like source of luminosity approximately 3e-5dE/dt, offset from the center of an elliptical region of size ~7"x5" and luminosity approximately 1e-3dE/dt 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 HI 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.