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NEWS | May 4, 2022

NRL Scientist Searches for Gravitational Waves From Monster Black Holes

By Paul Cage, U.S. Naval Research Laboratory Corporate Communications

A U.S. Naval Research Laboratory researcher is leading the way in helping understand gravitational waves generated by supermassive black holes in a new way.

These low-frequency waves stretch out so long that, even traveling at the speed of light, it takes years for each wave crest to pass the earth. The waves are produced when pairs of black holes, millions to billions of times more massive than the sun, spiral towards each other. Such pairs are formed when galaxies---each of which harbors its own supermassive black hole---collide and merge. Many black hole mergers are occurring through the universe, each producing gravitational waves, and they fill space with a gravitational wave background.

To detect these faint gravitational waves, scientists carefully monitor pulsars.  These neutron stars---the extremely dense remnants of exploded massive stars---emit regular trains of pulses. Gravitational waves bend spacetime, so as the waves pass through, they must take a slightly longer path than if the gravitational waves were absent, slowing the pulse down. By looking for tiny variations in the time it takes a pulse to reach earth, scientists can detect and characterize the waves. These experiments are called pulsar timing arrays, and to date they have used sensitive radio telescopes.

Now, astronomers are searching for these waves using gamma rays, the highest-energy form of light.  Matthew Kerr, Ph.D., who works in NRL’s High Energy Astrophysics and Applications Section, used 12.5 years of data from the Fermi Gamma-ray Space Telescope to form a gamma-ray pulsar timing array. Fermi is a space observatory used to perform gamma-ray astronomy observations from low Earth orbit and performs an all-sky survey studying astrophysical phenomena such as active galactic nuclei, pulsars, other high-energy sources.

Kerr’s, who is the paper’s co-lead, published the findings, Gamma-ray Pulsar Timing Array Constrains the Nanohertz Gravitational Wave Background, in Science recently.

“Pulsars have been one of the great successes for Fermi,” Kerr said.  “We’ve detected more than 100 gamma-ray millisecond pulsars. These are the kind that are used in pulsar timing arrays, so we decided to try to make a gamma-ray pulsar timing array.  It turned out to be surprisingly effective. Our results are almost as sensitive as those from radio telescopes, which are the size of football fields!”

Gamma rays offer a key benefit over radio waves. Space is mostly empty, but the pulsars in timing arrays are thousands of light years distant, and radio waves encounter electrons along the way.

“Just like light is split according to color when it passes through a prism, radio waves at different frequencies arrive at different times after passing through the interstellar medium,” Kerr said. “These delays mimic what we would see from gravitational waves, so radio astronomers have to try to remove them from the data, which can be challenging.”

Gamma rays are so much more energetic than radio waves that they aren’t affected in this way, eliminating any potential error. “And this lets us use the gamma-ray data to check for contamination in the radio data,” Kerr said.

Pulsars have a long history of providing celestial clocks and have been used in experiments that test the theory of general relativity. They also provide a time scale which can rival the precision of atomic clocks over long timescales. And arrays of pulsars can be used analogously to GPS, enabling navigation in environments where GPS is unavailable, like deep space.

“The gravitational wave background provides an ultimate limit on the observed stability of millisecond pulsars and so characterizing it is critical to these applications,” said Paul Ray, Ph.D., head of NRL’s High Energy Astrophysics and Applications Section. “The results are much more impressive than I had anticipated, given the small number of photons detected by Fermi from these pulsars, but even more exciting is how they are expected to improve over the next 5 years as the duration of the dataset is extended.”

About the U.S. Naval Research Laboratory

NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL is located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; Monterey, California, and employs approximately 3,000 civilian scientists, engineers and support personnel.
For more information, contact NRL Corporate Communications at (202) 480-3746 or nrlpao@nrl.navy.mil.

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