A Plasma Prism Model for an Anomalous Dispersion Event in the Crab Pulsar

In 1997 October daily monitoring observations of the Crab pulsar at 327 MHzand 610 MHz with an 85ft telescope in Green Bank, WV showed a jump in thedispersion measure by 0.1 cm$^{-3}$ pc. Pulses were seen simultaneously at bothold and new dispersions over the course of several days. During the dispersionjump the pulsed flux diminished by an order of magnitude. In the months beforethis event the average pulse profiles contained faint ``ghost'' pulsecomponents offset in phase from the regular main pulse and interpulsecomponents by a nearly frequency independent time delay that quadraticallydiminished to zero over a month. After the dispersion event there was an orderof magnitude increase in the level of scattering, as measured by pulsebroadening at 327 MHz. There was also a curious shift in the rotational phase,a slowing down, at both frequencies at the time of the dispersion jump which weassociate with intrinsic timing noise. All of the observed phenomena exceptthis slowing down can be explained by the variable perturbing optics of atriangular plasma prism that is located in the filamentary interface betweenthe synchrotron nebula and the supernova ejecta and which crosses the line ofsight over a period of months. The required density, scale length and velocityare reasonable given previous observations and analysis of these filaments. Ourstudy thus provides a probe of the plasma column on scales of 30 microarcsecondto 3 milliarcsecond ($10^{12-14}$ cm) which complements scales accessible tooptical emission line studies with HST resolution ($10^{16-18}$ cm). Incombination both observations provide a detailed look at a sample of theinterface region that can be matched statistically to the results of numericalsimulations.Comment: 13 pr.pp., 9 fig.; scheduled for the v543n2p Nov 10, 2000 issue of Ap