Researchers have found a new way to search for gravitational waves, waves caused by the explosion, rotation or aggregation of space objects in space. Physicists first detected waves in 2015 with laser-based detectors, and other scientists are pursuing them with Earth-based radio telescopes. Now, the prey has gone into space. A new study has found that data from the Fermi Gamma-Ray Space Telescope can theoretically sense a passing wave. Although the technique is still not accurate enough for actual identification, it is already helping other researchers sharpen their analyzes.
Matthew Kerr, leader of the Naval Research Laboratory’s gamma ray astronomy team, said the discovery that Fermi could do it was “a big surprise for us.” When the telescope was launched about 14 years ago, “it was far from radar.”
Gravitational waves, predicted by Albert Einstein’s general theory of relativity, occur when massive masses – such as black holes or neutrons, the dense core of a burned star – move violently, revolve around, and collide with each other. Since 2015, they have detected two large Earth-based detectors, the Laser Interferometer Gravitational-Wave Observatory (LIGO) Virgo in the United States and Europe, a combination of dozens of black holes and a pair of neutrons. Detectors fire lasers through a kilometer-long vacuum tube. When a wave travels, it changes the length of the tube to 1 / 10,000th of the width of a proton, which the lasers detect.
Radio astronomers are looking for larger prey than LIGO and Virgo: they are looking for mega-mergers, the union of supermassive black holes each weighing billions of suns. Such black holes are hidden in the center of the galaxy; When two galaxies merge, it is thought that the black holes orbit each other closely and merge slowly. Conventional telescopes are never going to pick up such a pair in a distant galaxy, says Chiara Mingarelli, a gravitational wave theorist at the University of Connecticut in Strauss. Gravitational waves “may be the only evidence we will ever see.”
This is because the waves produced by such spiral pairs are long – it takes years to cross a cycle – and they require a net across a galaxy to catch them. Instead of using lasers and vacuum tubes, radio astronomers revert to pulsar, neutron stars that emit radiation from their poles. When they rotate, that radiation spreads across the sky like a supercharged lighthouse beam. On Earth, astronomers see flashes hundreds of times a second from some pulsars, which come regularly like the tick of an atomic clock. A transient gravitational wave will slightly change the distance between a pulsar and the Earth, so for many years observing the arrival of pulses from a collection of pulsars across the Milky Way – known as a pulsar timing array (PTA) – astronomers hope to detect slight differences between gravitational waves. Gives the signal to pass.
Last year, using data collected over a dozen years, PTA teams in North America and Europe announced that they had picked up obscure statistical signals indicating something known as a gravitational wave background, an echo of the integration of all supermassive black holes across a vast expanse. The swath of the universe analyzing a few more years of data, which the parties are now doing, could reinforce those claims.
And this time Fermi is on the field. Pulsars emit gamma rays in addition to flooding their radio waves. But many astronomers doubted that their instruments would be enough to detect gravitational waves. Kerr and colleagues decided to find out. They searched the Fermi archives for 12.5 years for gamma ray photons from about 30 suitable pulsars. Unlike radio PTA, which targets specific pulsars for a fixed period of time, Fermi constantly sees a large portion of the sky, so several pulsars are almost always visible. But photons in the gamma range are so rare, “Fermi can see all week and see no photons,” Kerr said.
Nevertheless, the team reports today Science, Their troll gamma ray through the Fermi archive has generated enough photons to create PTA. Like their radio colleagues, Kerr and his team were not able to detect the background of gravitational waves with certainty. But they were able to set a higher limit on the quality of their signals. Kerr admits that the gamma-based threshold is about one-third tighter than that of radio PTA, but that it will improve as the firm collects more data. “So, if Fermi doesn’t fall from the sky, we will have comparative sensitivity in 5 to 10 years,” he said.
“It’s a really interesting piece of paper,” said Maura McLaughlin of the University of West Virginia, a leader in Nanograve, one of the Radio PTA teams. While the gamma ray effort is still playing catchup, it could already contribute. “One of the most useful things that gamma ray data can do is help us understand the effects of interstellar media,” said McLaughlin, a major source of noise in PTA research. This whisp of particles and radiation can bend the path of radio waves and slow down some frequencies compared to others, sending out signals. But gamma rays get a free pass, and by comparing pulsar signals from radio and gamma rays, researchers can better understand interstellar sound and detect fingerprints of potential gravitational waves. Gamma ray signals are “an independent measure,” said Mingarelli, also a member of the Nanograve team. “It adds to our gravitational wave detection toolkit.”
Once the background of PTAs-radio and gamma ray-gravitational waves has been identified, the next goal will be to find out the distinctive supermassive black hole binaries to see how these rotating behemoths affect the galaxies around them. “It’s a whole new way to observe the universe,” Mingarelli said. “Who knows what we’ll find?”
