We report the discovery of a 206 ms pulsar associated with the TeV \gamma -ray source HESS J1640 - 465 using the Nuclear Spectroscopic Telescope Array ( NuSTAR ) X-ray observatory .
PSR J1640 - 4631 lies within the shell-type supernova remnant ( SNR ) G338.3 - 0.0 , and coincides with an X-ray point source and putative pulsar wind nebula ( PWN ) previously identified in XMM-Newton and Chandra images .
It is spinning down rapidly with period derivative \dot { P } = 9.758 ( 44 ) \times 10 ^ { -13 } , yielding a spin-down luminosity \dot { E } = 4.4 \times 10 ^ { 36 } erg s ^ { -1 } , characteristic age \tau _ { c } \equiv P / 2 \dot { P } = 3350 yr , and surface dipole magnetic field strength B _ { s } = 1.4 \times 10 ^ { 13 } G. For the measured distance of 12 kpc to G338.3 - 0.0 , the 0.2 - 10 TeV luminosity of HESS J1640 - 465 is 6 % of the pulsar ’ s present \dot { E } .
The Fermi source 1FHL J1640.5 - 4634 is marginally coincident with PSR J1640 - 4631 , but we find no \gamma -ray pulsations in a search using 5 years of Fermi Large Area Telescope ( LAT ) data .
The pulsar energetics support an evolutionary PWN model for the broad-band spectrum of HESS J1640 - 465 , provided that the pulsar ’ s braking index is n \approx 2 , and that its initial spin period was P _ { 0 } \sim 15 ms .