Cosmic “beacons” emitting gamma rays could reveal gravitational waves around us

An illustration of the Fermi Gamma Ray Space Telescope detecting gamma rays from distant pulsars.

A team of astronomers has compiled 12.5 years of data Fermi gamma ray space telescope to form a gamma-ray pulsar timing array, a system of cosmic beacons that could help reveal ripples in spacetime.

Since the first observation of gravitational waves in 2016, astronomers and astrophysicists have tried to determine the background of the gravitational waves, practically the entire ocean of it waves in spacetime. Rapid rotations and collisions of the most massive objects in the universe, things like black holes and neutron stars, create gravitational waves that can be detected on Earth.

That LIGO and Virgo Interferometers have picked up gravitational waves from multiple black hole mergers times the size of our Sun, known as stellar-mass black holes. But scientists would also like to see much larger waves, like those that would emerge from two supermassive black holes smashing into each other. That is a challenge.

Gravitational waves from stellar-mass black hole mergers are “tens to hundreds of kilometers long, and so we need detectors that are only a few kilometers long,” wrote Aditya Parthasarathy, an astronomer at the Max Planck Institute for Radio Astronomy in Bonn, in an E- Mail , Germany and co-author of the new paper. “To detect the billions of kilometers long gravitational waves from supermassive black hole mergers, we need a detector that spans the entire galaxy!”

We cannot construct a galaxy-wide detector. But we can exploit naturally occurring pulsars, which is what the researchers behind the new work envisioned. They built on an existing idea, called a pulsar timing array, which relies on radio waves emitted by the rapidly spinning remnants of dead stars. These pulsars rotate in a predictable pathway that allows researchers to document subtle changes in the time it takes for the pulses to reach Earth. These changes are due to distortions in spacetime – gravitational waves – causing the pulse to arrive a little earlier or later than usual.

By stringing together the signals from the pulsars into networks, astronomers can build galactic-scale observatories. The latest team’s novel approach looks for the gamma rays produced by some of the pulsars being discovered by the Fermi Gamma Ray Space Telescope. your research is released in the journal Science.

Last yearThe North American Nanohertz Gravitational-Wave Observatory released a 12.5-year dataset describing a pattern in the light from 45 Milky Way pulsars, a low-frequency signal that “was what we expect the first evidence of the gravitational-wave background to look like,” , according to the lead author of the study. This data comes from two radio telescopes: the Green Bank Telescope in West Virginia and the Arecibo Telescope in Puerto Rico collapsed in 2020.

But timing pulsar radio waves is not a foolproof way to find the gravitational-wave background. Parthasarathy noted that over the long distances it takes radio waves to reach Earth, pulsars encounter scatter choices that can disrupt the waves’ journey. “However, gamma-ray photons do not see the scattered electrons, and thus gamma-ray observations are devoid of this main source of noise,” Parthasarathy said. “Hence, the gamma-ray pulsar timing array is a more direct probe to study the gravitational-wave background signal.”

Directness aside, timing pulsars by their gamma rays would provide astronomers with a radio-independent probe of the gravitational-wave background—a proposition a more complete picture of what is actually going on.

The gravitational-wave background is somewhat similar to the cosmic microwave background, the earliest light we can see in the universe and present everywhere you look in the sky. But “in a way, it’s more dynamic than that [cosmic microwave background]because it tracks the last few billion years of the universe’s evolution and the loudest (closest) sources may have just been strong [gravitational wave] Sources for hundreds of thousands of years, which is basically nothing on this scale,” said Matthew Kerr, an astronomer at the US Naval Research Laboratory and co-author of the paper, in an email to Gizmodo.

Kerr added that the waves are “a great probe of the dynamics of the inner parts of galaxies and merger history. But they only start when supermassive black holes exist, which takes a long time as galaxies have to condense, star form and grow.”

The Fermi approach is not yet as sensitive as that of radio telescopes – the most recent results are about 30% as good like the radiopulsar timing arrays — but astronomers think Fermi will be just as good at detecting the gravitational-wave background in about five years.

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