We present far-UV observations of the Crab nebula and its pulsar made with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope .
Broad , blueshifted absorption arising in the nebula is seen in C IV \lambda 1550 , reaching a blueward velocity of \sim 2500 ~ { } km~ { } s ^ { -1 } .
This can be interpreted as evidence for a fast outer shell surrounding the Crab nebula , 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 \sim 0.3 ~ { } M _ { \odot } with an accompanying kinetic energy of \sim 1.5 \times 10 ^ { 49 } ergs .
A fast massive shell with 10 ^ { 51 } ergs can not be excluded , but is less likely if the density profile is much steeper than \rho ( R ) \propto R ^ { -4 } and the maximum velocity is \lesssim 6000 ~ { } km~ { } s ^ { -1 } .
The observations cover the region 1140 - 1720 Å , which is further 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 .
Together with the near-UV data , and new optical data from the Nordic Optical Telescope , our spectrum of the Crab pulsar covers the entire region from 1140 - 9250 Å. 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 power law with spectral index \alpha _ { \nu } = 0.11 \pm 0.04 .
The main uncertainty in determining the spectral index is the amount and characteristics of the interstellar reddening , and we have investigated the dependence of \alpha _ { \nu } on E ( B - V ) and R .
In the extended emission covered by our 25 \arcsec \times 0 \farcs 5 slit in the far-UV , we detect C IV \lambda 1550 and He II \lambda 1640 emission lines from the Crab nebula .
Several interstellar absorption lines are detected along the line of sight to the pulsar .
The Ly \alpha absorption indicates a column density of ( 3.0 \pm 0.5 ) \times 10 ^ { 21 } cm ^ { -2 } of neutral hydrogen , which agrees well with our estimate of E ( B - V ) =0.52 mag .
Other lines show no evidence of severe depletion of metals in atomic gas .