Pulsar clocks open new window on gravitational waves

29 June 2023

A team of European astronomers, together with Indian and Japanese colleagues, has for the first time found strong evidence of ultra-low-frequency gravitational waves, which probably come from pairs of supermassive black holes at the centre of merging galaxies.

It is the result of more than 25 years of observations with the most sensitive radio telescopes in Europe and India, including the Westerbork Synthesis Radio Telescope (WSRT). In doing so, they have opened a new window on gravitational wave research. These gravitational waves contain information about the universe’s best-kept secrets. The research has been published in a series of articles in the professional journal Astronomy & Astrophysics.

Also Nikhef researcher Anuradha Samajdar was involved in the research from her previous position as a Humboldt Fellow at the University of Potsdam, Germany. Since May this year, Samajdar is assistant professor at the GRASP Institute at Utrecht University.

The scientists work together within the European Pulsar Timing Array (EPTA) and the Indian Pulsar Timing Array (InPTA). The EPTA is a collaboration of scientists from more than 10 institutions across Europe in which astronomers and theoretical physicists use observations of very regular pulses from pulsars – extinct stars – as a gravitational wave detector the size of our Milky Way galaxy. From the Netherlands, astronomers from ASTRON and Radboud University are involved. Other teams from around the world have independently obtained the same results, also announced today.

The astronomically large gravitational wave detector, which spans twenty-five specifically chosen pulsars, scattered across the Milky Way, gives the possibility to investigate gravitational waves at lower frequencies (wavelengths of a few light years) than is possible with other detectors such as LIGO and Virgo (wavelengths of several kilometres). At these nanohertz frequencies, unique sources and phenomena can be observed.

The gravitational waves now seen are most likely a sum of signals from a very large number of supermassive black holes orbiting each other very slowly. The researchers see the results as the beginning of a new exploration of the universe.

“It is exciting to see the hints of supermassive black holes, with a million up to tens of billions of solar masses,” says Samajdar. “Less massive black holes (of the order of tens of times the mass of the sun) have already been detected and studied by LIGO and Virgo. Being able to investigate now also supermassive black holes will give us valuable information in a new mass regime where we do not know much yet.”

Samajdar is member of EPTA and of the Virgo and the Einstein Telescope collaborations. She continues: “The future Einstein Telescope will be able to also detect neutron star collisions for a much longer duration than is possible with current detectors. Neutron stars are compact objects with masses much lower than that of black holes. With the bandwidth of Einstein Telescope we will observe these signals for hours, giving valuable information due to the length of the signal and the improved sensitivity of the instrument. So in the future we will be able to study the population of compact objects in the universe in a whole range of masses with many different observation methods.”

Source: ASTRON and NOVA

Image credit: Danielle Futselaar/MPIfR

Read here the complete press release of ASTRON and NOVA