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Mapping the magnetic field of the Milky Way

U of T astronomers uncover additional pulsars in the Northern Hemisphere
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The team mapped the galactic magnetic field using bursts of light from dense spinning stars called pulsars. DENIS DEGIOAN/UNSPLASH
The team mapped the galactic magnetic field using bursts of light from dense spinning stars called pulsars. DENIS DEGIOAN/UNSPLASH

How many times have we looked up and wondered how massive and complex our universe is? Our galaxy, the Milky Way, is a spiral galaxy with a central bulge and four spiral arms. It has a diameter spanning about 100,000 light years.

Large-scale mapping of the magnetic field around the galaxy is an active research field. Although the galactic magnetic field is much weaker than the magnetic field close to the Earth’s surface, astronomers believe mapping it could help us to understand astrophysical processes like star formation and cosmic rays dynamics.

A team of astronomers at the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics has recently mapped a portion of the Northern Hemisphere of the Milky Way Galaxy’s magnetic field using data acquired from the novel Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope. The team’s results were published in the journal Monthly Notices of the Royal Astronomical Society.

Collecting data from ‘lighthouse’ stars

CHIME is a radio telescope located at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia. Unlike traditional radio telescopes, CHIME has a stationary cylindrical design that captures a wide-field view of the sky. Cherry Ng, a lead author of the paper and a postdoctoral research fellow at the Dunlop Institute, described CHIME as a “transit telescope” in a written interview with The Varsity.

“What it means is we don’t point and track a specific point on the sky, instead, the sky drifts overhead during the course of an observation,” Ng added. CHIME performs by capturing radio waves from distant pulsars. Pulsars are rapidly rotating neutron stars, which are compact remnants of massive stars that are undergoing supernova explosions. These stars emit electromagnetic radiation from their magnetic poles.

Pulsars are often compared to lighthouses — when facing the earth, their radiation can be collected by radio telescopes, and when facing away, the emission signal is out of view. As a result, pulsars are observed as periodic pulses of radio emissions.

So how are radio emission signals from pulsars used to calculate the magnetic field of the Milky Way Galaxy? The answer lies in the term ‘faraday rotation.’

As the polarized radio emission of pulsars travels toward the Earth, it is encountered by free electrons. The electrons generate a parallel magnetic field along the path of the radio wave and this generated field rotates the plane of polarization of the incoming radio wave along the line of sight. By quantifying this rotation you can derive the galactic magnetic field.

“Using rotation measure [sic], you are able to discern information about the magnetic field structure along the line of sight of your observation. So, when we observe pulsar signals that are very far away in our galaxy, we are able to turn that data into measurements of the strength and direction of magnetic fields in the interstellar medium between Earth and the pulsar,” explained Ayush Pandhi, a former undergraduate research student and another lead author of the paper, in an email to The Varsity.

The advantages of the CHIME telescope

What was different about the data collected from CHIME compared to previous telescopes? Due to its unique design and ability to scan a large portion of the sky at a time, CHIME was able to collect data from 10 different pulsars simultaneously. Besides a large collection area, CHIME operates at a much lower frequency with a wide range of 400–800 megahertz compared to most radio facilities.

One of the biggest selling-points of CHIME is its location. While most of the world’s radio telescopes are located in the Southern Hemisphere, CHIME is situated in the Northern Hemisphere. “This means that it is looking at a lot of new pulsar rotation measures that haven’t previously been observed and the lower frequency leads to more precise results,” wrote Pandhi.

In fact, the current study reported a total of 80 new measurements of galactic magnetism — 55 completely new and 25 improved data points. This was a marked 20 per cent increase in the known pulsar sample size in the Northern Hemisphere, helping provide a more complete picture of the under-explored magnetic field of the Milky Way.

“A big advantage of CHIME is that we get a lot of observation time, we were able to combine multiple observing sessions to maximize signal-to-noise, and were able to detect pulsars that were further away than previous studies,” added Ng.

Looking forward

When asked about any difficulties they faced during the course of the study, Ng recalled the struggle of working with a new telescope without a set manual to follow. She wrote, “We needed to make sure what we are seeing are not artifacts of the instrument and accurately quantify the uncertainty in our measurements.”

Pandhi similarly noted that, “Learning how to work with that [back-end software] system and create the pipelines required for this project were fairly challenging aspects of this study.”

With everything set in place now, Ng hopes to collect more measures and “be able to revisit the large-scale Galactic magnetic field structure.” Understanding this will uncover the properties of the galaxy we are in. “The magnetic field also traces the shape… of the galaxy. [It] plays a key role in the formation and evolution of stars and galaxies,” reminded Ng.

Ng had some advice for undergraduate students who are interested in research. “I am particularly impressed by how much my student Ayush was able to achieve during the few months of the summer undergrad research program,” she wrote. “There is a lot of value and opportunity to make forefront discoveries by participating in these kinds of programs, I’d encourage [U of T] students to get in touch if they are interested!”