(ORDO NEWS) — Coalescing supermassive black holes at the centers of merging galaxies flood the universe with low-frequency gravitational waves.
Astronomers have been looking for these waves with large radio telescopes to detect the subtle influence of these space-time pulsations on the radio waves emitted by pulsars in our galaxy.
Now, an international team of scientists has shown that high-energy light collected by NASA‘s Fermi Gamma-ray space-based gamma-ray telescope can also be used for searches.
Using gamma rays instead of radio waves allows for a clearer view of pulsars and provides an independent and complementary way to detect gravitational waves.
Research results from an international team of scientists, including Aditya Parthasarathy and Michael Cramer of the Max Planck Institute for Radio Astronomy in Bonn, Germany, are published in the journal Science this week.
A sea of gravitational waves
At the center of most galaxies – clusters of hundreds of billions of stars like our Milky Way – is a supermassive black hole. Galaxies are attracted to each other by their huge gravity, and when they merge, their black holes sink into a new center.
As black holes swirl inward and merge, they create long gravitational waves that extend hundreds of trillions of kilometers between wave crests. The universe is full of such merging supermassive black holes, and they fill it with a sea of low-frequency space-time ripples.
Astronomers have been searching for these waves for decades by monitoring the pulses of pulsars, the dense remnants of massive stars. Pulsars rotate with extreme regularity, and astronomers know exactly when to expect each pulse.
However, a sea of gravitational waves subtly alters the timing of the arrival of pulses on Earth, and precise monitoring of multiple pulsars across the sky can reveal its presence. Previously, exceptionally large radio telescopes, which collect and analyze radio waves, were used to search for these waves.
But now an international team of scientists is looking for these tiny changes in more than a decade of data collected by NASA’s Fermi Gamma-ray Space Telescope, and their analysis shows that detection of these waves is possible with just a few years of additional observations.
“Fermi is studying the universe in gamma rays, the most energetic form of light. We were surprised at how well he finds the pulsars we need to look for gravitational waves – there are already more than 100 of them,” said Matthew Kerr, a research physicist at the US Army US Marine Research Laboratory in Washington DC. “Gamma rays have some special characteristics that together make them a very powerful tool in this study.”
The results of the study, led by Kerr and Aditya Parthasarathy, a researcher at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, were published in the April 07 issue of Science.
Light takes many forms. Low-frequency radio waves can pass through some objects, and high-frequency gamma rays, when they collide with matter, turn into showers of energetic particles. Gravitational waves also cover a wide spectrum, and more massive objects tend to generate longer waves.
It is impossible to build a detector large enough to detect the trillion-kilometre waves generated by merging supermassive black holes, so astronomers use naturally occurring detectors called pulsar timing arrays.
These are collections of millisecond pulsars that shine with both radio waves and gamma rays and make about a hundred revolutions per second.
Like lighthouses, these beams of radiation pulse regularly as they sweep over the Earth, and as they pass through a sea of gravitational waves, they are imprinted with the faint hum of distant massive black holes.
Initially, pulsars were detected using radio telescopes, and experiments to synchronize pulsars with radio telescopes have been going on for almost two decades. These large dishes provide the greatest sensitivity to gravitational wave effects, but interstellar effects complicate the analysis of radio data.
Space is mostly empty, but the vast distance between the pulsar and the Earth, the radio waves still collide with a lot of electrons.
Just as a prism bends visible light, interstellar electrons bend radio waves and change the timing of their arrival. Energetic gamma rays are not affected in this way, so they are an additional and independent method for determining the age of a pulsar.
“Fermi’s results are already 30% superior to those of radio pulsars when it comes to potential detection of the gravitational wave background,” Parthasarathy said. “Another five years of collecting and analyzing pulsar data and he’ll be just as capable, and another bonus is that he won’t have to worry about all those stray electrons.”
The Gamma Pulsar Synchronization Array, which was not anticipated until the launch of Fermi, represents a powerful new possibility in gravitational wave astrophysics.
“Detection of the gravitational wave background with pulsars is within reach, but remains challenging. The independent method unexpectedly demonstrated here by Fermi is great news, both for confirming future findings and for demonstrating its synergy with radio experiments,” – concludes Michael Cramer, director of the MPIfR and head of its research department “Fundamental Physics in Radio Astronomy”.
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