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 Telescope. Broad, blueshifted absorption arising in the nebula is seen in C IV λ1550, reaching a blueward velocity of ~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 ~0.3 M☉ with an accompanying kinetic energy of ~1.5 × 1049 ergs. A fast massive shell with 1051 ergs cannot be excluded but is less likely if the density profile is much steeper than ρ(R) ∝ R-4 and the maximum velocity is ≲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 to 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 αν = 0.11 ± 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 αν on E(B-V) and R. In the extended emission covered by our 25′′ × 0.″5 slit in the far-UV, we detect C IV λ1550 and He II λ1640 emission lines from the Crab Nebula. Several interstellar absorption lines are detected along the line of sight to the pulsar. The Lyα absorption indicates a column density of (3.0 ± 0.5) × 1021 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.