Discovery of an Energetic 38.5 ms Pulsar Powering the Gamma-ray Source IGR J18490-0000/HESS J1849-000

We report the discovery of a 38.5 ms X-ray pulsar in observations of the soft γ-ray source IGR J18490-0000 with the Rossi X-ray Timing Explorer (RXTE). PSR J1849-0001 is spinning down rapidly with period derivative 1.42 × 10^-14 s s^-1, yielding a spin-down luminosity \dot{E} = 9.8 × 10³⁶ erg s^-1, characteristic age τ_c ≡ P/2\dot{P} = 42.9 kyr, and surface dipole magnetic field strength B_s = 7.5 × 10¹¹ 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 Γ_PSR = 1.1 ± 0.2, N _H = (4.3 ± 0.6) × 10²² cm^-2, and F _PSR(2-10 keV) = (3.8 ± 0.3) × 10^-12 erg cm^-2 s^-1, while the spectral parameters of the extended emission are roughly Γ_PWN ≈ 2.1 and F _PWN(2-10 keV) ≈ 9 × 10^-13 erg cm^-2 s^-1. 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) ≈ 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 γ-ray source.