X-ray spectroscopy of PSR B1951+32 and its pulsar wind nebula

We present spatially resolved X-ray spectroscopy of PSR B1951+32 and its pulsar wind nebula (PWN) in supernova remnant (SNR) CTB 80 from a Chandra observation. The Chandra X-ray map clearly reveals various components of a ram-pressure-confined PWN embedded in the SNR ejecta: a point source representing the pulsar, X-ray emission from the bow shock, a luminous X-ray tail, a 3000 diameter plateau whose northwestern part is absent, and the more diffuse outer X-ray emission. The plateau is closely surrounded by the radio, [O III], [S II], and [N II] shells, and the outer diffuse emission is mostly within the H alpha shells. While the spectra of all the features are well fitted with power-law models, a power-law plus blackbody model can fit the spectrum of the pulsar significantly better than using a power-law model alone. Generally, the spectra of these components obey the trend of steepening from the inside to the outside. However, the edge of the plateau probably has a harder spectrum than the central region of the plateau. The cause of the apparent hard spectrum of the plateau edge is unclear, but we speculate that it might be due to a shock between the PWN and the SNR ejecta. The possible blackbody radiation component from the pulsar has a temperature of 0.13 +/- 0.02 keV and an equivalent emitting radius of 2.2(-0.8)(+1.4) (d/2 kpc) km, and is thus probably from the hot spots on the pulsar. We also show in this paper that the blackbody temperature of the entire surface of PSR B1951+32 is much lower than those predicted by the standard neutron star cooling models.