This project used rotating neutron stars called pulsars to study three main problems:  (1) gravitation in extreme situations; (2) the emission beams of  pulsars; and (3) the magnetic field in the plane of our Milky Way Galaxy.  Further details on the three principal scientific problems tackled in this project are given below, along with a final note on the broader impacts of this project. 

1. Gravitation in extreme situations: A binary pulsar is a pulsar orbiting another star.  Binary pulsars are sometimes found in extremely tight orbits, so small that they could fit inside the surface of our Sun.  In these situations, the gravity is so extreme that Newton's Theory of Gravitation fails to adequately describe the binary pulsars' orbital motions, and more advanced theories are required. Einstein's 100-year-old General Relativity Theory is a major advance over Newton's, and we have used our new observations of pulsar B1913+16, along with about 40 earlier years of measurements, to show that General Relativity indeed provides a correct description of the orbital system, including its rate of gravitational radiation emission.  These findings provide crucial structure and context for the recent stunning direct gravitational radiation measurements by the LIGO Observatories.  Our  timing measurements of another binary pulsar, PSR J1906+0746, determined the masses of both stars and indicated that the companion is either another neutron star or a white dwarf, while our distance measurements showed that the system is probablylocated  too far from Earth for the companion to be optically detectable.  All of these binary pulsar measurements and analyses have been reported in three refereed scientific papers during the term of this project.

2. The emission beams of  pulsars:  A pulsar concentrates its radio emissions into a narrow beam which is carried across us by the pulsar's spin, thereby causing an apparent "pulse"  to be seen, in much the same way that a rotating lighthouse appears to flash when its optical beam is pointed at us.  The cause and even the structure  of the emission beam is still rather poorly understood, even 50 years after the discovery of pulsars.  We conducted several experiments to further our understanding  of the pulsar emission mechanism and the structure of the beam.  In one study, we compared the structure of the beams of very quickly spinning pulsars ( tens or hundreds of rotations per second)  with those of normally spinning pulsars ( one or fewer revolutions per second), and found surprisingly that they share many more properties than would be otherwise expected. In another experiment, we explored  anomalous shifts in the reception time of sets of several pulses in two pulsars, which seem to occur at roughly repeating intervals.  No other pulsars are known to exhibit this phenomenon.  We advanced several possible explanations including orbital motion about a very low-mass companion, but none seems completely adequate without additional observations. In a third study, we showed how to observationally test whether the rotating lighthouse model of a pulsar is actually correct.  These three projects  resulted in three refereed, published papers.  We also made significant progress (but no publication yet) in a fourth related project, using the spin precession of binary pulsar B1913+16 (a phenomenon related to the wobbles of  a spinning toy top) to enable us to see the structure of the beam in both dimensions.

3. The magnetic field in the plane of our Milky Way Galaxy: Pulsar pulses undergo a rotation of the plane of linear polarization when they pass through the magnetized, ionized interstellar medium on their trip from the pulsar to the Earth.  This phenomenon, called Faraday rotation,  reveals the strength  of the galactic magnetic field along that path.  We have made new pulsar Faraday rotation measurements from Arecibo, using pulsars much farther from Earth than we were able to reach in a 2004 study.  We have finished determining the strength of the galactic magnetic field toward each pulsar and are now in the process of using these measurements to map out the magnetic field in the galactic spiral arms accessible from Arecibo.  This project  is in the final stages of analysis before preparing it  for publication.

Broader impacts of this project:  The broadest impacts of these projects, beyond their own intellectual merit in advancing our understanding of these problems, involved the scientific training of undergraduate students at Carleton College and with colleagues at observatories and research institutes in Puerto Rico and Australia.  These students experienced unique research opportunities while providing  important assistance in our studies;  meanwhile also developing  intellectual and creative capabilities useful in their future scientific careers.

Last Modified: 11/30/2018
Modified by: Joel M Weisberg