During a single Arecibo observation in 1981 of the Crab pulsar, a profile at 4.7 GHz was recorded which appeared to contain additional components and an interpulse (IP) shifted to earlier phase. The experiment was continued at the VLA, taking advantage of its phased array mode to form a synthesized beam, which resolves out the bright Crab Nebula background. Observations were conducted between February 9 and May 27, 1994, at 0.33, 1.4, 4.9, and 8.4 GHz. Additional radio profiles presented here were recorded at Arecibo (0.43, 0.6, and 4.7GHz) and Effelsberg (2.7GHz) by Hankins & Fowler (unpublished).
In Figure 1 we have plotted a summary of normalized profiles from several radio frequencies and infrared. The VLA profiles are time aligned, while the rest are aligned to the main pulse (MP). A new component (labeled LFC) appears 36° ahead of the MP between 0.6 and 4.9 GHz, not coincident with the position of the precursor, and with a spectral index similar to that of the MP. The MP disappears at 8.4 GHz, probably due to spectral effects. The IP appears to undergo a transition in phase and flux, disappearing at 2.7 GHz and reappearing 10° earlier at 4.7 GHz with a radically different spectral index. Two high radio frequency components (labeled HFC1 and HFC2) appear at 4.9 and 8.4 GHz, and possibly at the noise level at 1.4 GHz. They have flatter spectral indices than the MP and IP and their centroid phase changes with respect to the MP – moving to later phase with increasing frequency. The infrared profile exhibits a “bump”, or third component near the same phase as HFC1 and HFC2.