Polarization Characteristics of Some Newly Discovered Pulsars

Posted May 11th, 2010 at 3:08 pm.

Shirley Chen

Teyvonia Thomas

Mentor: Professor Nice

A pulsar (pulsating star) is a rapidly rotating neutron star that radiates beams of electromagnetic radiation as it spins on its axis. These beams are detected on earth as pulses of radiation as they sweep across earth’s view, much like the beacon of a lighthouse. Neutron stars are the end point of evolution of extremely massive stars; they consist primarily of neutrons. The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge, UK. Since then, over 1500 pulsars have been detected with periods ranging from milliseconds to seconds. Pulsars have been observed in various bands of radiation including gamma rays, x-rays, visible light and radio waves. In this research, we will use data collected at the Arecibo Observatory to study the polarization characteristics of radio waves emitted by some recently discovered pulsars. Pulsar polarization measurements turn out to provide a means of mapping out our Galaxy’s magnetic field. We now know that this field is somewhat circular or spiral in shape and a few micro-gauss strong, but no one knows much more detail about it. Our ultimate goal, therefore, is to gain a deeper understanding about the nature of our galaxy’s magnetic field by conducting more research on pulsar polarization.

Polarization is a description of the direction of the electric field in electromagnetic waves like light, radio waves and x-rays. Radiation can be thought of as a combination of linearly and circularly polarized waves. The focus of our research is to quantify the angle that linearly polarized radiation rotates as it travels through space towards the radio receivers at Arecibo Observatory. As this radiation travels through space, the orientation of its polarization is affected by magnetic fields, a process known as Faraday rotation. Quantitatively, the rotation is proportional to the integral from the pulsar to the Earth, of charged particle density in the medium ne and the magnetic field along the line of sight, BcosΘ. It is also proportional to the square of the radiation’s wavelength. Hence the rotation angle is Δφ µ λ2 ∫ ne BcosΘ dl. Pulsars are a good source for testing the magnetic field because they are highly polarized and distributed widely through the Galaxy. Our study will ultimately help us understand how polarized waves are produced by pulsars, how pulsars generate magnetic fields, and what shape the galactic magnetic field takes.

The algorithm of processing raw data involves sending it through a pipeline that analyzes linear polarizations as well as left and right circularly polarized waves separately. The program then folds the data to give a stronger signal (it does this by adding together many pulses), calibrates the relative strengths of polarization, removes the dispersive effects of the interstellar medium and finally creates integrated pulse profiles which display the radiation pattern of the beam as it sweeps across the earth. This is all done remotely on machines at Arecibo from computers at Bryn Mawr. This data contains information about the frequency and period at which the pulses are traveling, their flux densities, their polarization, and other characteristics. Throughout this study, we will analyze data from three dozen pulsars, measuring their polarization characteristics. We will then use this information to measure the amount by which the pulses from each specific pulsar have been rotated. The results of our investigations will be used to make inferences about the nature of the magnetic field between the pulsar and us.

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