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Gravitational wave wallpaper search –

Searching for long-wavelength gravitational-wave signals by NASA’s Fermi satellite Merging supermassive black holes at the centers of interacting galaxies is filling the universe with extremely low-frequency gravitational waves. Astronomers use large radio telescopes to search for these waves to observe the subtle effects of these spacetime ripples on radio waves emitted by pulsars in our galaxy. Press release from the Max Planck Institute for Radio Astronomy (MPIfR), Bonn.

source: MPIfR.

The Fermi Large Area Telescope (LAT) is located on the Fermi satellite in orbit 500 km above Earth and collects gamma rays from millisecond pulsars. These high-energy photons travel through the Milky Way, encountering a sea of ​​low-frequency gravitational waves produced by pairs of supermassive black holes that coalesce at the centers of merging galaxies. The ripples of spacetime, at wavelengths of a few light-years, cause each photon to arrive a little earlier or later than expected. Observing gamma rays from many of these millisecond pulsars – an experiment known as the Pulsar Timing Array (PTA) – can reveal this telltale signature. So far, sensitive radio telescopes have only been used in pulsar timing arrays. Now, Fermi data enables a gamma-ray-based pulsar timing array that provides new and clear insight into these types of gravitational waves. © Daniëlle Futselaar / MPIfR (

April 7, 2022. An international team of scientists has now shown that the high-frequency gamma radiation recorded by the Fermi telescope NASA, can be used for this search. Using gamma rays instead of radio waves allows a clearer view of pulsars and provides an independent and complementary method for detecting gravitational waves.

The findings of an international team of scientists, including Aditya Parthasarathy and Michael Kramer of the Max Planck Institute for Radio Astronomy in Bonn, were published in Science this week.

sea ​​of ​​gravitational waves

At the core of most galaxies – clusters of hundreds of billions of stars such as our Milky Way – is a supermassive black hole. Galaxies are attracted to each other by their intense gravity, and when they merge, central black holes sink into the new center. As these now roll inward and merge with each other, they create extremely long-wavelength gravitational waves hundreds of trillions of kilometers or several light years between wave crests. The universe is full of such supermassive merging black holes, and they fill it with a sea of ​​low-frequency space-time waves.

Astronomers have been searching for these waves for decades by systematically observing the pulsations of pulsars, the dense remnants of massive stars. Pulsars rotate very regularly, and astronomers know exactly when to expect each pulsation. However, the sea of ​​gravitational waves subtly alters the timing of the pulsations that reach Earth, and careful observation of the many pulsars across the sky can reveal their presence.

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So far, the search for these waves has only used large radio telescopes that collect and analyze radio waves. An international team of scientists is now looking for these tiny fluctuations in space-time in gamma-ray data collected over a period of more than ten years using the Fermi satellite. NASA was registered. Their analysis shows that detecting these waves could be possible with just a few years of additional observations.

“Fermi studies the universe with gamma rays, the most energetic form of light. We were surprised by how much it detected the kind of pulsars we need to look for these gravitational waves — we’ve found more than 100 so far,” said Matthew Kerr, a scientist at the US Naval Research Laboratory in Washington. “The Fermi telescope and the gamma ray telescope have some special properties that together make them a very powerful tool in this study.”

The results of the study, which was led by Kerr and Aditya Parthasarathy, collaborators at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, were published in Science (April 7 issue).

cosmic clocks

Light takes many forms. Low-frequency radio waves can penetrate some objects, while high-frequency gamma rays explode into energetic showers of particles when they hit matter. Gravitational waves also cover a wide spectrum, with more massive objects tending to produce longer waves.

Since it is impossible to build a detector large enough to detect waves of trillions of kilometers in wavelength caused by merging supermassive black holes, astronomers use detectors already in nature in the form of so-called star timing arrays. These are clusters of millisecond pulsars that glow in both radio waves and gamma rays and rotate on their axis hundreds of times every second. Like beacons, these beams appear to pulse regularly as they sweep across the Earth, and when they break through a sea of ​​gravitational waves, they are imprinted with the faint noise of distant massive black holes.

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Unique measuring technology

Originally, pulsars were discovered using radio telescopes, and pulsar timing array experiments using radio telescopes have been in the works for nearly two decades. Large parabolic mirrors are more sensitive to the effects of gravitational waves, but interstellar effects complicate the analysis of radio data. Space is often empty, but when crossing the vast distance between a pulsar and Earth, radio waves still encounter many electrons. Just as a prism bends visible light, interstellar electrons bend radio waves, changing their arrival time. High-energy gamma rays are not affected in this way, so they are a complementary and independent method for “pulsar timing”.

“Fermi’s results are 30% better than radio pulsar timing arrays when it comes to gravitational wave background detection,” says Aditya Parthasarathy. “If we collect and analyze pulsars data for another five years, the system will be just as good, with the added advantage of not having to worry about all those stray electrons.”

The timing matrix of gamma-wavelength pulsars that had not been visualized prior to the Fermi launch represents a powerful new addition to gravitational wave astrophysics.

“Detecting the gravitational-wave background of pulsars is within reach, but remains challenging. The independent method, as demonstrated here unexpectedly by Fermi, is great news, to confirm future results and to demonstrate synergies with radio experiments,” concludes Michael Kramer, Director of MPIfR and chair of the “Radioastronomical Fundamental Physics” research division.

additional information

The Fermi Gamma Ray Space Telescope is a collaborative effort in astrophysics and particle physics developed by the United Nations’ Goddard Space Flight Center NASA in Greenbelt, Maryland. Fermi was developed in cooperation with the US Department of Energy, with significant contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

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The FERMI-LAT collaboration includes an international team of scientists including Aditya Parthasarathy and Michael Kramer, both from the Max Planck Institute for Radio Astronomy.

original version

The gamma-ray pulsar timing matrix constrains the nanogravitational wave background
Fermi-LAT Collaboration, Science, 7 April 2022.
DOI: 10.1126/science.abm3231.

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