Abstract
We have measured variations in scattering time scales in the Crab Pulsar over a 30-year period, using observations made at 610 MHz with the 42-ft telescope at Jodrell Bank Observatory. Over more recent years, where regular Lovell Telescope observations at frequencies around 1400 MHz were available, we have also determined the dispersion measure variations, after disentangling the scattering delay from the dispersive delay. We demonstrate a relationship between scattering and dispersion measure variations, with a correlation coefficient of $0.56\pm0.01$. The short time scales over which these quantities vary, the size of the variations, and the close correlation between scattering and dispersion measure all suggest that the effects are due to discrete structures within the Crab Nebula, with size scales of $\sim6$ AU (corresponding to an angular size of $\sim2$ mas at an assumed distance of 2200 pc). We mitigate the effects of scattering on the observed pulse shape by using the measured scattering information to modify the template used for generating the pulse arrival times, thus improving the precision to which the pulsar can be timed. We test this on timing data taken during periods of high scattering, and obtain a factor of two improvement in the root mean square of the timing residuals.
Highlights
The Crab Pulsar (PSR B0531+21, known by the J2000 name PSR J0534+2200) was discovered in 1968 at the centre of the Crab Nebula, the remnant of its progenitor supernova SN 1054 (Staelin & Reifenstein 1968)
This preferred time scale corresponds to the typical length of the DM ‘events’ seen in the time series, which was confirmed by computing the structure function of simulated DM time series exhibiting similar DM events, using a model where DM variations are drawn from a normal distribution defined by the magnitude of variations we observe, and injecting larger variations of similar size to those observed in our data set
We have described a method for disentangling the effect of scattering from the profile of the Crab Pulsar, and used the scatter-corrected timing data to precisely measure DM variations in the Crab Pulsar timing data
Summary
The Crab Pulsar (PSR B0531+21, known by the J2000 name PSR J0534+2200) was discovered in 1968 at the centre of the Crab Nebula, the remnant of its progenitor supernova SN 1054 (Staelin & Reifenstein 1968). The nebula contains density ripples (some of which are seen as wisps in optical observations), clouds, and irregular regions of turbulence, which reveal a dynamic internal structure (Crossley et al 2007, Hester 2008). As the nebula contains a large number of free electrons, radio pulses propagating through it are scattered and dispersed in a manner similar to that caused by the interstellar medium (ISM). Due to a combination of effects including the pulsar proper motion, the Earth’s orbit, and the dynamics of the nebula, observed variations in electron number density are larger. Jan. 1984 - Oct. 2012 (MJD 45700 - 56201) Nov. 2011 - Apr. 2014 (MJD 55866 - 56756)
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