by Niccolò Di Lalla

Our Universe is believed to be filled with a chaotic sea of low-frequency gravitational waves, perturbations in space-time caused by orbiting pairs of supermassive black holes at the centers of merging galaxies. These waves can be light-years long and astronomers have been chasing them for decades using large radio telescopes around the globe. Now a powerful new tool has been developed and the hunt has moved to space using gamma rays, the highest-energy form of light.

Launched 14 years ago, the Fermi Gamma-ray Space Telescope (Fermi) maps the sources of these high-energy photons across the entire sky. Fermi’s main detector, the Large Area Telescope (LAT), was assembled at SLAC, and SLAC and KIPAC have had a long association with the instrument. A new study from the Fermi-LAT collaboration, published in Science on April 7, 2022, reveals that data collected by Fermi offer some valuable advantages over radio waves and can be used as an independent and complementary way to detect long gravitational waves.

Gravitational waves

Gravitational waves are generated when massive objects, such as black holes or neutron stars, are accelerated due to orbital motion in a binary system. Despite having been predicted more than 100 years ago, their existence wasn’t confirmed until 2015, when a signal generated by the merger of two stellar-mass black holes was detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO, previously discussed in this blog in July, 2016 and September, 2016).

Gravitational waves created by a pair of stellar-mass objects that spiral and eventually collide into each other are hundreds of miles long and can be detected by interferometers on Earth such as LIGO. In contrast, supermassive black hole binaries—each weighing billions of Suns—produce waves a fraction of the size of the Milky Way and form a chaotic background of low-frequency space-time ripples. These gravitational waves are too long to be detected by conventional interferometers as they can take years to pass the Earth. To find them, astronomers need detectors the size of our galaxy, called pulsar timing arrays (PTAs).

Pulsars and previous radio searches

To understand what a PTA is, let’s go back to what pulsars are: rapidly spinning neutron stars which, like cosmic lighthouses, appear to rhythmically pulse as the beam of light crosses the line of sight to Earth. Some pulsars, called millisecond pulsars, can rotate hundreds of times per second and emit broadband radiation with extreme regularity to function as celestial clocks, scattered across the sky and throughout the Milky Way. A long gravitational wave, passing between us and one of these pulsars, will stretch or squeeze the very fabric of space, causing the light to arrive either early or late by just billionths of a second. By precisely monitoring many pulsars making up an array in the sky over several years, scientists hope to reveal the presence of gravitational waves moving past them.

Previous searches for these gravitational waves have been performed exclusively using radio telescopes. Last year, several radio PTA teams reported the detection of a weak signal using data collected over a dozen years, which could represent the long-awaited signal coming from the gravitational wave background (GWB)—or a residual noise with a different origin. The analysis of radio data is intrinsically affected by interstellar effects: space is almost empty but the electrons spreading over the enormous distance between a pulsar and the Earth can bend the trajectory of radio waves and alter the arrival times of pulses at different frequencies. Analysis of a few more years of radio data may reinforce these claims, but an independent measurement would be required to confirm the result.

This is where the Fermi telescope joins the hunt.

Fermi and the gamma-ray PTA

Gamma rays are the most energetic form of light in the electromagnetic spectrum. Unlike with radio waves, gamma-ray observations are unaffected by the interstellar medium and can provide a complementary and independent confirmation of the radio results. Until this new study by Matthew Kerr (a research physicist at the U.S. Naval Research Laboratory in Washington DC), and his colleagues from the Fermi-LAT collaboration, no one knew how powerful the Fermi telescope really was at tracking pulsars and potentially detecting gravitational waves.

Fermi’s large field of view—up to one-sixth of the sky at any given time—allows the study of a large number of pulsars at any moment. Using the 35 brightest and most stable gamma-ray millisecond pulsars from more than 12 years of Fermi-LAT data, Kerr and his team searched for a signal compatible with the long GWB with two different techniques.

The first technique uses the same analysis tools developed for radio PTAs and aims to detect a common variation in gamma-ray pulse arrival times. The major difference for gamma-ray observations is that the signal is much weaker compared to radio PTA observations and that rather than measuring a waveform, gamma-ray detectors collect individual photons. Thus, the gamma-ray pulsar signals need to be “stacked” over multiple periods in order to obtain the shape of the pulse. The second technique is a new self-consistent analysis that uses forward modeling to characterize pulsar emissions and signals from the GWB in gamma-ray wavelengths. By comparing gamma-ray PTA data with the resulting pulsar signals predicted by the models, it is possible to infer the most likely set of parameters that describe the potential contribution to the pulsar signal from the GWB.

The two methods give consistent results and, although no potential GW signal was detected, the authors were able to set an upper limit of 1 x 10^-14 (95% confidence level) on the GWB amplitude reported in the figure above. The analysis shows that the gamma ray-based limit is currently only about one-third as sensitive as the radio PTA, but it will improve rapidly as Fermi continues to collect pulsar data. So, while the technique is not yet sensitive enough to make an actual detection, researchers think gravitational signals from supermassive black hole binaries could be within Fermi’s reach over the next five years, with the main advantage of not having to worry about interstellar effects on gamma rays.

This would be an excellent cross-check on the PTA analysis, and a completely different way to detect the effects of the ghostly gravitational waves that criss-cross our Universe.

Related reading

A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background — The Fermi-LAT Collaboration

Astronomers find a new way to detect gravitational waves — Science