Space Telescope Imaging Spectrograph Near-Ultraviolet Time-tagged Spectra of the Crab Pulsar

Abstract

We present the spectrum and the pulse profile of the Crab Pulsar in the near-ultraviolet (1600-3200 A) observed with the Space Telescope Imaging Spectrograph (STIS) during the Hubble Space Telescope (HST) second Servicing Mission Orbital Verification (SMOV) period. The two-dimensional near-ultraviolet Multianode Microchannel Array (NUV MAMA) was used in time-tag mode with a 2''×2'' aperture and the low-dispersion grating, G230L, to obtain a cube with axes of slit position, wavelength, and time. The observation-derived pulse period is consistent with radio measurements, and the pulse profile agrees well with previous NUV broadband measurements by the High Speed Photometer. The pulsar spectrum includes the 2200 A dust absorption feature, plus several interstellar absorption lines. Dereddening the spectrum using the Savage-Mathis model with E(B-V)=0.55±0.05 leads to a good fit to a power law with slope αν = -0.3±0.2. The spectra of the main pulse, the interpulse, and the individual rising and falling edges are similar to the total spectrum within the limits of photon statistics. The four pulse profiles produced by breaking the spectrum into 400 A bins show the pulse profile to be stable across the NUV spectral range. Histogram analysis reveals no evidence for the superpulses seen at radio wavelengths. The interstellar absorption-line equivalent widths of Mg I, Mg II, and Fe II are lower than expected based on the implied H I column density from E(B-V)=0.5. While several explanations are possible, additional studies will be necessary to narrow the options.