Observations of the Crab Nebula and its pulsar in the far-ultraviolet and in the optical

We present far-UV observations of the Crab Nebula and its pulsar made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescop e. Broad, blueshifted absorption arising in the nebula is seen in C IV lamb da 1550, reaching a blueward velocity of similar to 2500 km s(-1). This can be interpreted as evidence for a fast outer shell surrounding the Crab Neb ula, and we adopt a spherically symmetric model to constrain the properties of such a shell. From the line profile we find that the density appears to decrease outward in the shell. A likely lower limit to the shell mass is s imilar to 0.3 M-circle dot with an accompanying kinetic energy of similar t o 1.5 x 10(49) ergs. A fast massive shell with 10(51) ergs cannot be exclud ed but is less likely if the density profile is much steeper than rho(R) pr oportional to R-4 and the maximum velocity is less than or similar to 6000 km (-1). The observations cover the region 1140-1720 Angstrom, which is fur ther into the ultraviolet than has previously been obtained for the pulsar. With the time-tag mode of the spectrograph we obtain the pulse profile in this spectral regime. The profile is similar to that previously obtained by us in the near-UV, although the primary peak is marginally narrower. Toget her with the near-UV data, and new optical data from the Nordic Optical Tel escope, our spectrum of the Crab pulsar covers the entire region from 1140 to 9250 Angstrom. Dereddening the spectrum with a standard extinction curve we achieve a flat spectrum for the reddening parameters E(B - V) = 0.52, R = 3.1. This dereddened spectrum of the Crab pulsar can be fitted by a powe r law with spectral index alpha(nu) = 0.11 +/- 0.04. The main uncertainty i n determining the spectral index is the amount and characteristics of the i nterstellar reddening, and we have investigated the dependence of ct, on E( B-V) and R. In the extended emission covered by our 25 " x 0 ".5 slit in th e far-UV, we detect C Iv lambda 1550 and He II lambda 1640 emission lines f rom the Crab Nebula Several interstellar absorption lines are detected alon g the line of sight to the pulsar. The Ly alpha absorption indicates a colu mn density of (3.0 +/- 0.5) x 10(21) cm(-2) of neutral hydrogen, which agre es well with our estimate of E(B - V) = 0.52 mag. Other lines show no evide nce of severe depletion of metals in atomic gas.