Although astrophysicists have never sensed supermassive black hole binary systems, a galaxy-sized detector made up of dead stars is hot on its trail.
In a new study led by Northwestern University, astrophysicists crunched 12.5 years of data from 45 dead stars (called pulsars) to put the best limits yet on the signals of gravitational waves emitted by pairs of monster black holes. Knowing these limits will help astrophysicists limit the number of binaries in the nearby universe, confirm or reject current binary candidates and, one day, detect gravitational waves from these complex pairs.
In another breakthrough, the study also found that when searching for pairs of supermassive black holes, researchers need to account for a constant hum of background noise emitted by the symphony of gravitational waves from all the supermassive black hole binaries in the universe.
The study, titled “12.5-year NANOGrav dataset: Bayesian limits on gravitational waves from binaries of supermassive black holes,” was accepted by Astrophysical Journal Letters It will be published this summer. It is currently posted on arXiv Prepress server.
“We really think that the detection of a binary of supermassive black holes through gravitational waves is just around the corner,” said Caitlin Witt of Northwestern, who led the study.
“This will be an important discovery for many scientific fields. It will enable us to do more experiments such as the gravity test to explore whether binaries of supermassive black holes evolve the way we think they do, and it will teach us how to look for them in future surveys. We will also be able to look back.” through cosmic time and trace the history of the universe in which we live.”
Witt is the first CIERA-Adler Postdoctoral Fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the Adler Planetarium.
too big to discover
Supermassive black holes lie at the center of most galaxies, and they can be several billion times the mass of our Sun. Compared to typical stellar-mass black holes, which are 10 to 100 times more massive than our Sun, supermassive black holes are unfathomably gigantic.
When two galaxies – each with a supermassive black hole – merge together, they can create a binary system of these monstrous black holes.
“One day, our galaxy will collide with the Andromeda galaxy,” Witt said. “After millions of years, the black holes finally found each other to form a small friendly system. Detecting gravitational waves from systems like these will help us understand how galaxies interact and how the universe evolves.”
In 2016, an international team led by Northwestern University professor Vicky Kalogera used the Laser Gravitational-Wave Observatory (LIGO) to first detect gravitational waves from the merger of two stellar-mass black holes, which resulted in clear, short-lived ripples. in spacetime. But binaries of supermassive black holes are too big and too far apart for ground-based instruments like LIGO to detect them.
These pairs of monsters create very long waves that can take years or even decades for their gravitational waves to wash completely over the Earth. Even when NASA and the European Space Agency launched LISA (the space gravitational-wave detector for which Northwestern professor Shane Larson is co-principal investigator) in the early 2000s, it still could not detect such massive waves.
“Ligo can only detect which wavelengths fit into its arms,” Witt said. “We have to look for much lower wave frequencies. We’re sensitive to pairs of supermassive black holes that can take a month or even 15 years to orbit each other. So, we’re looking for a steady signal that can blend into a background.”
Pulsars tick like a clock
To get around this hurdle, an international collaboration of researchers created the North American Nanohertz Gravitational-Wave Observatory (NANOGrav), which searches for gravitational waves using pulsars, a type of rapidly spinning neutron star that was born in the supernova explosion of an eventual massive star. of her life. Just like a lighthouse, a pulsar emits a flashing beam of light as it rotates.
“Because pulsars rotate so steadily, we see little flashes of light ticking like a clock,” Witt said. “We watch this light using ground-based radio telescopes. If the clock’s tick arrives a little early or late, that’s a sign that it may have been affected by a gravitational wave.”
NANOGrav tracks 75 pulsars — 45 of which were used in this study — located across the night sky. And it takes milliseconds for their light beams to flash away from Earth. So, in this case, “a little early or a little late” might mean a fraction of a nanosecond. Therefore, NANOGrav techniques must be highly sensitive to capture these almost imperceptible changes.
By looking across the entire sky, Witt and the NANOGrav team are looking for specific patterns from all of the pulsars together. According to the theory, binaries of supermassive black holes should emit gravitational waves that stretch and compress (or compress) space-time on their way to Earth. Distorted space-time will affect beams of pulsar-light in a way that points to an elusive pair of monster black holes.
Red noise can fool us
But, of course, pulsars generate their own noise, which can muddy the signals.
“Pulsars have some internal noise called ‘red noise,'” Witt said. The red noise looks similar to the broad gravitational wave noise we’re looking for. We have to tease that away.”
Last year, the NANOGrav team published a study that found a red noise process in all pulsars that share the same common characteristics. Without further evidence, NANOGrav cannot attribute this to gravitational waves. In the new study, Witt and her team find that this red noise still needs to be carefully accounted for in order to finally detect gravitational waves from individual supermassive black hole binaries.
“When a gravitational wave becomes detectable, it looks very similar to red noise at first glance,” Witt said. “Red noise can deceive us. Our new study tells us that we must look closely to avoid confusion. It will be important to monitor when we finally discover gravitational waves.”
Although NANOGrav has yet to detect supermassive black hole binaries with gravitational waves, Witt’s new paper brings the field closer than ever before. By taking advantage of the 12.5-year dataset, the researchers created new models to accurately account for uncertainties in the pulsar data and implemented new techniques for calculating red noise.
Confirmation of candidates
These new models provide the minimalistic data yet on the strength of gravitational waves emitted by pairs of supermassive black holes. Previously, other researchers had detected supermassive black hole binaries with light-based telescopes. NANOGrav can finally confirm that these potential candidates are indeed supermassive black hole binaries.
“With our new methods, we may be able to confirm this sooner,” Witt said. “Or, if we continue to collect and analyze the data, we might be able to rule it out as a candidate. It could just be another strange thing going on in the galaxy.”
Zaven Arzoumanian et al, The 12.5-year NANOGrav Data Set: Bayesian Limits on Gravitational Waves from Single Supermassive Black Hole Binaries, arXiv (2023). doi: 10.48550/arxiv.2301.03608
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