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An Extreme Scattering Event of the Crab Pulsar

Submitter: Laura Driessen
Description: The Crab Pulsar is one of the few pulsars still surrounded by the supernova remnant (SNR) from the explosion that created it. The SNR is composed of filaments of gas and dust which scatter the observed pulse profile properties of the pulsar.

Generally, scattering effects on pulse profiles are attributed to the interstellar medium (ISM) and are seen as an extra exponential tail in the pulse profile. In this summer student project, we measure this effect by convolving an intrinsic (unscattered) profile, with one-sided exponentials. In the top panel we show the intrinsic profile of the Crab Pulsar at 350MHz (red), as well as the expected profile including scattering (blue).

The Crab Pulsar is known to experience 'extreme scattering events' where the normal scattering is enhanced during a period of weeks to months. One of these scattering events was monitored in a high-cadence campaign using WSRT at 350MHz in late 2012 to early 2013, with regular monitoring observations continuing afterwards. One of the pulse profiles from December 2012 is shown in the lower plot (black). Surprisingly, we can see that there is an extra peak preceding both the main pulse and interpulse which is not visible in the upper plot.

Since the scattering is normally quite small at 350MHz, we can only see this during the extreme scattering event of the Crab Pulsar, at favourable radio frequencies where the effect is large enough to be measurable but not too large to smear the profile completely.

We model this effect by using two exponential tails, shifted relative to each other and with different scattering values. Our best fit model is shown in the lower plot (blue), as well as the two components (intrinsic profile scattered by the two different scattering tails; red, green) that combined make up the best-fit model.

We attribute the extra scattering components to changes in the filaments of the SNR still surrounding the Crab Pulsar. With further analysis and more frequent observations during a new extreme scattering event we will investigate the filamentary structure of the Crab Nebula and confirm the scattering effect dependence on frequency.
Copyright: Laura Driessen
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