  We report the discovery of a 38.5 ms X-ray pulsar in observations of the soft
gamma-ray source IGR J18490-0000 with the Rossi X-ray Timing Explorer (RXTE).
PSR J1849-0001 is spinning down rapidly with period derivative 1.42E-14 s/s,
yielding a spin-down luminosity 9.8E36 erg/s, characteristic age 42.9 kyr, and
surface dipole magnetic field strength 7.5E11 G. Within the INTEGRAL/IBIS error
circle lies a point-like XMM-Newton and Chandra X-ray source that shows
evidence of faint extended emission consistent with a pulsar wind nebula (PWN).
The XMM-Newton spectrum of the point source is well fitted by an absorbed
power-law model with photon index Gamma(PSR) = 1.1 +/- 0.2, N_H =
(4.3+/-0.6)E22 cm^-2, and F(PSR;2-10keV) = (3.8+/-0.3)E-12 erg/s/cm^2, while
the spectral parameters of the extended emission are Gamma(PWN) = 2.1 and
F(PWN;2-10 keV) = 9E-13 erg/s/cm^2. IGR J18490-0000 is also coincident with the
compact TeV source HESS J1849-000. For an assumed distance of 7 kpc in the
Scutum arm tangent region, the 0.35-10 TeV luminosity of HESS J1849-000 is
0.13% of the pulsar's spin down energy, while the ratio F(0.35-10 TeV)/F(PWN;
2-10 keV) of approx. 2. These properties are consistent with leptonic models of
TeV emission from PWNe, with PSR J1849-0001 in a stage of transition from a
synchrotron X-ray source to an inverse Compton gamma-ray source.
