Funded by a grant from the National Science Foundation, West Virginia University associate professor of physics and astronomy Maura McLaughlin is using the National Radio Astronomy Observatory Green Bank Telescope to observe pulsars for the International Pulsar Timing Array project.
McLaughlin is a member of NANOGrav – North American Nanohertz Observatory for Gravitational Radio Waves, which joined with colleagues in Europe and Australia for the IPTA.
“We [NANOGrav] are a group of scientists in America and Canada who are using radio timing observations of a type of star called a pulsar to detect gravitational waves,” McLaughlin said. “This is a really exciting project because gravitational waves were predicted to exist by [Albert] Einstein about a hundred years ago now in 1915 and there’s a lot of very good evidence for their existence, so far. But, we want to directly detect the effect of these gravitational waves on space time.”
Gravitational waves are created by objects as they move, whether it be a person walking down the street or stars a thousand lightyears away. The waves cause the space between two objects to ripple.
“If you’ve got two black holes that are orbiting each other out there in space – as they orbit each other, they’re causing space time to ripple and these ripples in space time travel through the universe and affect everything in the universe,” McLaughlin explained. “What they do is they cause the light travel times between objects to change.”
The project is observing pulsars, which are small stars that emit a beam of light, or pulse, like a lighthouse. Pulsars rotate rapidly – approximately 1,000 times a second. Using the GBT, McLaughlin is able to track the pulsars through these light beams or blips.
“The experiment we’re doing is we’re using the Green Bank Telescope and also the Arecibo Observatory in Puerto Rico to monitor the arrival times of the pulses,” McLaughlin said. “We’re measuring when the pulses arrive, very, very accurately and we’re searching for tiny little perturbations in the arrival times of the pulses due to these gravitational waves. If a gravitational wave passes between the Earth and a pulsar, it will stress or squeeze space time and that pulse will arrive a little bit sooner or later than it would otherwise. That’s like the basic picture of the experiment.”
Because pulsars are similar to a heartbeat – a steady pulse – if the blip arrives to the Earth faster or slower than it should, the observers chalk that up to the gravitational waves.
While it is great to prove that gravitational waves affect one pulsar, it’s better to show that they affect lots of pulsars. In her two weeks at the NRAO, McLaughlin and her students will be monitoring 42 pulsars.
While McLaughlin is working with the GBT, IPTA colleagues are observing pulsars in Australia and Europe, one of whom is her student and Marlinton native Natasha McMann, who spent the summer in Bielefeld, Germany.
“A big part of building these collaborations up is student exchanges,” McLaughlin said. “There’s nothing more valuable to getting a project going than getting a student of ours to actually go to Germany or go to Australia or vice versa for those students to come here and work with another faculty member and bring knowledge back, and get collaborations going.”
This particular project will last for a few years while the observers collect data to prove that there is a signature behind the gravitational waves.
“This is a very incremental project where a year or two from now, we’ll look at our data and say, ‘oh, there’s a hint of a signature,’ and then a year after that, we believe the signature and then a year after that, we’ll publish the signature,” McLaughlin said. “It’s not a thing that tomorrow we’re going to look at the data and say ‘Oh my God,’ and be done. It’s a very slow progress but that’s what we expect. The signature will slowly grow in significance in our data.”
During the observation, McLaughlin said may smaller scale milestones have been discovered, including interesting pulsar clusters.
“We’re finding lots of interesting pulsars,” she said. “We’re always looking for new pulsars. We’ve found some really interesting new pulsars recently. [One of my students] has been timing a pulsar that’s in orbit with another neutron star. These systems are very rare and they’re very relativistic – very interesting from a stellar revolution perspective. This is kind of a neat binary system of two stars in orbit around each other. There’s only about ten of these known so far, so that’s kind of cool that we found another one of those.”
The pulsars the group are observing are within the Milky Way Galaxy but outside of our solar system, hundreds of lightyears away. While it isn’t unheard of for a pulsar to have a partner star, it is somewhat rare.
“We know of about two thousand, three hundred pulsars and about two hundred of those have partners, so it’s not that rare,” McLaughlin said. “Ten percent of them have companion stars that they are in orbit with. Most of those companions are white dwarfs and about ten of them have other neutron star companions. A few have main sequence star companions, so stars like the sun. We recently found a really neat system. It’s a triple system with a pulsar and two other stars. We’re always finding kind of neat, unexpected combinations.”
A pulsar is created when a supernova explodes. It is a neutron star so it no longer burns fuel and it is considered inert.
“Their properties do change in time,” McLaughlin said of pulsars. “When they’re born, they’re spinning very quickly and then as they get older, they start spinning more and more slowly so they do evolve. The spin periods will change in time, but they will never explode or die. They’ll just spin more and more slowly, and eventually we’ll no longer be able to detect them with the GBT because they’ll just be too faint.”
McLaughlin is excited to be a part of the project because of what the research will prove.
“First, it’s exciting that we can test general relativity,” she said. “Einstein predicted these waves and we’ll be able to actually detect them and measure their properties, and say, ‘yes, Einstein’s theories are correct’ or maybe, ‘no, Einstein was not a hundred percent correct.’ The second exciting thing, and I think the more exciting thing is, that we’ll have a tool to study objects in the universe that we can’t see.
“We can’t see black holes, but once we can detect gravitational waves from them, even though they emit no light, we will actually be able to characterize these black hole binaries,” she continued. “We can measure their masses. We can measure their orbital periods. That will tell us a lot about how the universe has evolved and how galaxies have evolved. It’s just going to open a whole new window to the universe. The initial detection will be fun, but then we’ll actually be able to characterize the universe in ways that we just can’t now. We may even detect exotic things that we don’t even know about, who knows what we’ll see once we’re sensitive to these gravitational waves.”
Suzanne Stewart may be contacted at firstname.lastname@example.org