Investigations of Single Pulses from Radio Pulsars

My first ever research experience happened in the Department of Physics at the University of Puerto Rico, Mayagüez Campus, where I got my undergraduate degree. I went to university for physics because I wanted to be an astronomer, and I knew that having a solid foundation in physics would be helpful if I went to graduate school in astronomy. The department had a variety of research in different areas of physics, but there were only a couple of astronomers among them. This meant that I had little choice when deciding on a research project to work on.

When I asked Professor Leszek Nowakowski if he had any research projects for undergrads, he said that he studied pulsars and he always enjoyed having students work with him (in fact, his research group included two other undergrads when I joined). At that point, I only had a cursory knowledge of pulsars, and therefore had no idea what research on pulsars would involve, nor why pulsars would be so interesting to study. But then I worked with Dr. Nowakowski for two years and I ended up thinking that pulsars are pretty darn interesting.

Pulsars are rapidly rotating neutron stars, formed in the gravitational collapse of a very massive star that ends its life as a supernova. All pulsars are neutron stars, but not all neutron stars are pulsars. A neutron star is a pulsar when it is highly magnetized and it emits a beam of radiation along its magnetic axis (see the diagram on the right), which is not aligned with its rotation axis. As it spins, the radiation beam sweeps around. From our point of view on Earth, the radiation beam is then seen as a pulse, once per rotation (this is called the "lighthouse model" of pulsar emission). This pulsating behavior is what led to the discovery of pulsars in 1967 by Jocelyn Bell, who was a graduate student at the time. She noticed in her data a periodic radio signal, and the newly discovered objects were dubbed 'pulsars' as a portmanteau of 'pulsating star'.

[Image of the Lighthouse Model of Pulsar Emission]

Dr. Nowakowski's research was all about learning what are the physical processes that drive the emission of radiation in a pulsar's magnetosphere, and where exactly in the magnetosphere the pulses are emitted. The beam of radiation is emitted along the magnetic poles, in a trumpet-shaped region bounded by open magnetic field lines. By looking at the characteristics of the average profile of a pulsar (including its shape, components, subpulses, and intensity), you can infer where in the magnetosphere the pulse was emitted.

Our research group used the 430 MHz antenna of the Arecibo Radiotelescope (pictured on the left) to collect data on thousands of pulses from a handful of radio pulsars. When I first joined the group, we worked with data that had already been collected, but during my time participating in this project, Dr. Nowakowski took us all with him to Arecibo one night for an observing run. We (meaning, me and the other undergrads who worked with him) didn't get to actually do anything there but just hang around the control room, but it was still a pretty nifty experience, and afterwards we had sparkling new higher-resolution data just waiting to be analyzed.

The raw data (old or new, the procedure was the same) was prepared by performing dedispersion and baseline removal, then an average profile was produced for each pulsar. We then performed a visual inspection of the data by looking at stacked and phase sequences of single pulses, which allowed us to observe drifting subpulses (when consecutive subpulses arrive at a slightly earlier or later phase), nulling (when one or more single pulses are missing), and mode switching (when the shape of the average profile changes completely for a time and then changes back). For example, take a look at the image on the right. The left-side panel shows a stacked sequence of pulses, which allows us to see the shape and intensity of each pulse. The right-side panel is similar, but in this visualization we see the phase of each subpulse more clearly. Notice how the black blobs kinda zigzag a bit? Those are drifting subpulses.

After producing an average profile, we then integrated it in intensity bins, which allowed us to determine how many components made up the profile. If the components didn't overlap, they were easily modeled with Gaussian fits, but when there was overlap of components we would use a Lorentzian or a combination fit. Our analysis of pulse intensity and components showed that the position of the components was correlated to the intensity of the pulse -- weak profiles were wide (components spread out), and strong profiles were narrow (components closer together). Given that the emission region of the pulsar magnetosphere is shaped like a trumpet, with the narrow end near the surface of the neutron star and the opening widening as you go higher, our results hinted that the stronger, narrower pulses are being emitted from lower regions in the magnetosphere, and the wider and weaker pulses are produced higher up.

The exact physical process that caused the intensity of the pulses to vary depending on the height in the magnetosphere at which they were emitted remained a mystery to me back then. This project was more observational and phenomenological, as Dr. Nowakowski probably deduced, correctly, that the theory of pulsar emission was a bit over our heads as undergrads. But we still learned a lot about pulsars and about astronomy research. As I mentioned at the start of the page, this was my first ever research experience, and I consider it to have been very valuable. I realized that pulsars, and radioastronomy as a whole, are pretty interesting. I developed some computational skills, since our analysis was performed with Fortran and Mathcad programs. I learned what the process of astronomical data reduction and analysis involves, particularly for radioastronomy. I learned how to look at data critically, to extract astrophysical information from it. And I learned and developed presentation skills, something that has been of enormous significance in my academic career ever since.

Before participating in this research I had never made a scientific presentation. During the course of my participation in this project, I presented two talks at scientific meetings:

The first one, my first ever presentation, was at the Puerto Rico Interdisciplinary Scientific Meeting (PRISM), on 10 March 2001 at the Pontifical Catholic University in Ponce, Puerto Rico. My talk was titled Investigations of Single Pulses from Radio Pulsar PSR 0540+23 (link goes to a PDF of the presentation; the difference in format between slides is due to the fact that the data slides were shown on transparencies).
The second time I presented results from my research was a year later, at the Puerto Rico Interdisciplinary Scientific Meeting (PRISM) on 16 March 2002, at the UPR Arecibo Campus. My talk then was titled Investigations of Single Pulses from Radio Pulsars (link to PDF; again, data slides are transparencies), since I presented results about more pulsars, though the main focus was PSR 0611+22.

I ended my participation in this research during my senior year, when I was getting ready to go to grad school. And even though in grad school I didn't work on pulsar research, this project remains important in my mind as my introduction to scientific research, and as such I'll always have a soft spot for pulsars. In fact, when it was recently announced that a pulsar had been discovered that exhibited simultaneous mode switching in radio and x-ray wavelengths, I had to immediately look for the paper and read it, even though it's been more than a decade since my involvement in pulsar research.

Pulsar Model image credit: NRAO.

Arecibo Radiotelescope image credit: Wikipedia.

Pulsar Data image comes from my presentation on 10 March 2001.

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