  We report the discovery of a pulsar with period P = 136 ms and dispersion
measure 308 pc/cc in a deep observation of the supernova remnant (SNR)
G54.1+0.3 with the Arecibo radio telescope. Timing measurements of the new
pulsar, J1930+1852, reveal a characteristic age of Pdt/2dP = 2900 yr and
spin-down luminosity dE/dt = 1.2e37 erg/s. We have subsequently searched
archival ASCA X-ray data of this SNR, and detect pulsations with a consistent
period. These findings ensure that PSR J1930+1852 is the pulsar that powers the
"Crab-like" SNR G54.1+.3. Together with existing Chandra observations of the
SNR, we derive an X-ray pulsed fraction (2-10 keV) of ~27%. We also find that
the cooling efficiency of the pulsar wind nebula (PWN) is intermediate between
those of the Vela and Crab PWNe: Lx (2-10 keV) ~0.002 dE/dt. PSR J1930+1852 is
a very weak radio source, with period-averaged flux density at 1180 MHz of 60
microJy. For a distance of 5 kpc, its luminosity, ~1 mJy kpc^2, is among the
lowest for known young pulsars.

Astrophysics