High Time Resolution Infrared Observations of the Crab Nebula Pulsar and the Pulsar Emission Mechanism

Abstract

We present new, high signal-to-noise near-infrared observations of the Crab Nebula pulsar using the Solid State Photomultiplier instrument on the Multiple Mirror Telescope. Our observations cover the J (1.25 μm), H (1.65 μm), and K (2.2 μm) infrared wavebands and have 20 μs time resolution. Together with visible and UV observations made by the Hubble Space Telescope High-Speed Photometer, we have high time resolution observations covering over a decade in wavelength. We present the pulse profiles over this wavelength range, and we analyze the pulse shape as a function of wavelength, including the peak-to-peak phase separation, the peak full width half-maxima (FWHM), and the peak half-width half-maxima (HWHM). We also create both phase-averaged and phase-resolved color spectra of the pulsar emission. We find that the peak-to-peak phase separation shows a significant trend for an increase with wavelength, in rough agreement with models of the pulsar emission mechanism. The FWHM for peaks 1 and 2 also show a trend for increase with wavelength, again in qualitative agreement with the models. However, the HWHM for peaks 1 and 2 show significant differences in their wavelength dependences from the leading to trailing edges. This behavior is not predicted by current pulsar emission models, and the different wavelength dependences of the component HWHM values call into question the usefulness of FWHM measurements. Our spectral analyses show that the IR-UV dereddened phase-averaged color spectrum is essentially flat over more than a decade in frequency. This is in clear contrast to the X-ray and γ-ray regimes, where the spectrum is falling steeply. The color spectra of peaks 1 and 2 are also essentially flat, but the ratio of the two shows statistically significant variations from a constant value. Finally, the color spectra of peaks 1 and 2 show significant differences from the leading to trailing edges. As with the HWHM, this behavior is not predicted by current pulsar emission models.