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Nachrichten, Termine, Experten
Nachrichten, Termine, Experten
The most sensitive map of the gravitational wave sky to date was produced by an international collaboration of researchers including numerous scientists from the Max Planck Institute for Radio Astronomy in Bonn, Germany. To achieve this goal, the scientists analysed 4.5 years of pulsar data taken with the MeerKAT radio telescope in South Africa, one of the most powerful radio telescopes in the world.
A Pulsar Timing Array uses the clock-like predictability of so-called pulsars (pulsating radio stars) to detect faint deviations from their regular behaviour caused by gravitational waves – minuscule ripples in the fabric of spacetime. As an international collaboration of radio astronomers from Australia, Germany, the UK, South Africa, the Netherlands, Italy and France, the MeerKAT Pulsar Timing Array (MPTA) has created a Galaxy-sized gravitational wave detector by monitoring the regular pulses of pulsars with the MeerKAT radio telescope in South Africa to nanosecond precision.
The gravitational waves observed with pulsar timing arrays are caused by some of the universe’s most powerful sources, from supermassive black hole binaries to events just moments after the Big Bang. Matt Miles, a researcher at Swinburne University of Technology in Melbourne, Australia, and one of the leaders of the MeerKAT Pulsar Timing Array data release explains: “Ringing throughout cosmic history, the accumulation of all these waves forms a gravitational wave background, a cosmic hum that provides valuable clues about the hidden processes shaping our universe.”
Creating a map of the gravitational waves across the sky allows searching for areas with an anomalous excess of gravitational radiation, so-called ‘hot spots’, caused by a single, stand-out supermassive black hole binary, promising insights about the gravitational wave background origin. Equipped with the largest number of pulsars jointly used in any such analysis, and profiting from the high-quality data of the MeerKAT radio telescope, a group of researchers led by Kathrin Grunthal, a PhD student at the Max Planck Institute for Radio Astronomy (MPIfR), published the most informative map of the gravitational wave sky. She reflects: “By looking for variations in the gravitational waves across the sky, we hunt for the fingerprint of the astrophysical processes behind the gravitational wave signal.”
While most parts of the sky show no sign of anisotropy, the research team revealed a small number of intriguing features which will be followed up upon in future work. Rowina Nathan, co-author of the Anisotropy paper comments: “Mapping the gravitational-wave background provides us with a more complete picture of the Universe.”
This gravitational wave sky map builds upon the notable evidence of the gravitational wave background signal within the MeerKAT Pulsar Timing Array dataset. “Previous maps have assumed there was no signal present. Now that we have evidence for gravitational waves it changes the mathematics. Ours is the first timing array map to take this into account,” comments MPIfR astronomer David Champion. Compared to similar global PTA efforts such as the European Pulsar Timing Array (who, along with international collaborators, published the first evidence for these low frequency gravitational waves in 2023), the MeerKAT Pulsar Timing array needed just one-third of the observational time span to match their sensitivity.
“This is testament to the outstanding capabilities of the MeerKAT radio telescope, in which our institute has been a leading collaborator, providing hardware, software and scientific work across a whole range of projects,” says MPIfR director Michael Kramer.
The work by the MeerKAT Pulsar Timing Array collaboration represents a significant leap towards the future of gravitational wave research based on radio astronomy. MeerKAT is a precursor for the Square Kilometre Array (SKA) and will eventually be integrated into the SKA Observatory’s SKA-Mid telescope, currently under construction in South Africa. The results from the MeerKAT Pulsar Timing Array collaboration demonstrate the crucial role these next-generation radio telescopes will play in global efforts to explore the low-frequency gravitational wave universe.
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Further Information
Pulsars: Pulsars are the remains of massive star explosions, where the core survived as a neutron star, a very compact object of 1.4 solar mass within a 13 km radius. The fastest pulsars rotate at a speed of 700 turns per second, and emit a beam of radiation from their magnetic poles. From an observer's point of view, they behave like cosmic lighthouses. With a radio telescope they are seen as a series of pulses or « ticks » arriving at very regular intervals providing a natural and precise clock-like signal. Such a clock signal is expected to be perturbed by low frequency gravitational waves.
Pulsar Timing Array (PTA): A Pulsar Timing Array is a network of pulsars that are observed with one, or ideally more, radio telescopes to search for and detect gravitational waves in the nanohertz range (i.e., with wavelengths on the order of several light-years). The PTA is made up by an ensemble of millisecond pulsars observed in different directions from Earth. Due to the precision of their pulse period and, due to their distribution on the sky, they represent a gravitational wave detector spanning large distances across the Galaxy. The analysis of their pulse arrival times allows, after correction of a whole series of effects, the inference of gravitational waves in the nanohertz range.
The use of these pulsars as a Galactic gravitational wave detector was suggested by M. Sazhin (1978, Sternberg Astronomical Institute, Moscow and S. Detweiler (1979, Yale University). Sazhin proposed that ultralong gravitational waves could be detected by their perturbation on electromagnetic pulses propagation. Detweiler showed that given published pulsar data, one can set an upper limit of the dimensionless amplitude of 10-11 to the energy density of a stochastic gravitational wave background with periods 1 year. A few years later, Hellings and Downs (1983, Jet Propulsion Laboratory) introduced for the first time the concept of a Pulsar Timing Array (PTA). They showed that if one is able to time a network of stably rotating pulsars with high precision, one can measure the background emission of a population of distant compact binary sources. In particular, it was demonstrated that one can infer the quadrupolar nature of the gravitational wave signal from the angular correlation between pairs of pulsars, i.e. the way the pulsars are affected according to their relative position on the sky. This defines the principle of detecting ultra-low-frequency gravitational waves with what we call today a Pulsar Timing Array (PTA).
When technology started to allow such precise measurement, reaching typically a dating of pulsation arrival time (the “ticks”) better than the microsec level, several groups in the world started to monitor the fastest and most stable known millisecond pulsars.
MeerKAT: Built and operated by the South African Radio Astronomy Observatory (SARAO), the 64 dish MeerKAT is the largest radio telescope in the Southern hemisphere and one of two SKA precursor instruments based in South Africa. Located in the Karoo semi- desert, the radio telescope will soon be expanded with an additional number of dishes, in the context of the “MeerKAT+” project, jointly funded in 2019 by SARAO and the Max-Planck- Gesellschaft (MPG) in Germany and since 2020 with the Istituto Nazionale di Astrofisica (INAF). The German Center for Astrophysics (DZA) will also participate and create additional technical and scientific opportunities. The telescope will later be gradually integrated into SKAO's Mid telescope in South Africa.
SKAO: The SKAO, formally known as the SKA Observatory, is an intergovernmental organisation composed of Member States from five continents and headquartered in the UK. Its mission is to build and operate cutting-edge radio telescopes to transform our understanding of the Universe, and deliver benefits to society through global collaboration and innovation.Its two telescopes, each composed of hundreds of dishes and thousands of antennas, will be constructed in South Africa and Australia and be the two most advanced radio telescopes on Earth. A later expansion is envisioned in both countries and other African partner countries.
Together with other state-of-the-art research facilities, the SKAO’s telescopes will explore the unknown frontiers of science and deepen our understanding of key processes, including the formation and evolution of galaxies, fundamental physics in extreme environments and the origins of life. Through the development of innovative technologies and its contribution to addressing societal challenges, the SKAO will play its part to address the United Nations’ Sustainable Development Goals and deliver significant benefits across its membership and beyond.
In addition to the countries of the locations (South Africa and Australia) and the headquarters (UK), China, Germany, India, Italy, Canada, the Netherlands, Portugal, Switzerland and Spain are members of the SKAO. France, Japan, Sweden and South Korea are observers.
The SKAO recognises and acknowledges the Indigenous peoples and cultures that have traditionally lived on the lands on which the SKAO facilities are located.
The European Pulsar Timing Array (EPTA): Europe has been a pioneer in this research program. Heir to the already existing “European Pulsar Network” (EPN) and “PULSE: European Pulsar Research” collaboration that won the Descartes Prize of the European Commission in 2005, the European Pulsar Timing Array (EPTA) was officially born in 2006, gathering the « pulsar » teams attached to the largest radio telescope facilities across the continent: the 100-m radio telescope in Effelsberg (Germany), the Westerbork Synthesis Radio Telescope (Netherlands), the Lovell Telescope at Jodrell Bank Observatory (United Kingdom), the Sardinia Radio Telescope (Italy) and the Nançay Radio Telescope (France) observatories. In each of these places, the local groups had developed state-of-the-art instrumentation and the data pipeline able to properly measure and accurately time pulsars. In the following years, they were joined by other groups who also brought their theoretical expertise and their skills in gravitational wave data analysis: the Universities at Birmingham, Cambridge, Milano, as well as the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and the Observatory in Paris.
The EPTA significantly expanded its capability in 2008 thanks to the ERC-funded “Large European Array for Pulsars” (LEAP). With monthly observations, the LEAP uses the coherently added sensitivity of the EPTA telescopes to synthesize a dish with an effective diameter of up to 200m. With observations spanning 25 years, those instruments have accumulated about 60,000 measurements for the 25 most stable millisecond pulsars, allowing an effective cadence of a few days and reaching a timing precision better than a microsec for most of them.
These numbers define the sensitivity and frequency domain of the array: a few 10-16 in gravitational energy density on average over the sky, between 1.3 nHz and 5.8 μHz in frequency, with the local sensitivity in a region of the celestial sphere depending on the actual distribution and stability of the pulsars in the array.
Kathrin Grunthal
Research Department Fundamental Physics in Radio Astronomy
Max-Planck-Institut für Radioastronomie, Bonn
Fon: +49(0)228-525-189
E-mail: kgrunthal@mpifr-bonn.mpg.de
Prof. Dr. Michael Kramer
Head of Research Dept. “Fundamental Physics in Radio Astronomy”
Max-Planck-Institut für Radioastronomie, Bonn
Fon: +49 228 525-299 (Sekretariat)
E-mail: mkramer@mpifr-bonn.mpg.de
Dr. David Champion
Research Department Fundamental Physics in Radio Astronomy
Max-Planck-Institut für Radioastronomie, Bonn
Fon: +49 228 525-315
E-mail: champion@mpifr-bonn.mpg.de
Grunthal, K. et al., Mapping the gravitational-wave sky: Results from the 4.5 year MeerKAT Pulsar Timing Array Data Release, 2024, MNRAS, DOI: 10.1093/mnras/stae2573
https://doi.org/10.1093/mnras/stae2573
Miles, M.T. et al.: The MeerKAT Pulsar Timing Array: The first search for gravitational waves with the MeerKAT radio telescope, 2024, MNRAS, DOI: 10.1093/mnras/stae2571
https://doi.org/10.1093/mnras/stae2571
Miles, M.T. et al.: The MeerKAT Pulsar Timing Array: The 4.5-year data release and the noise and stochastic signals of the millisecond pulsar population, 2024, MNRAS, DOI: 10.1093/mnras/stae2572
https://doi.org/10.1093/mnras/stae2572
https://www.mpifr-bonn.mpg.de/pressreleases/2024/11-n1h5 (until the embargo expires on Tuesday, December 03, 01:01 CET)
https://www.mpifr-bonn.mpg.de/pressreleases/2024/11 (after the embargo has expired)
A MeerKAT antenna in front of an artistic representation of supermassive black holes and the space-t ...
Carl Knox, OzGrav, Swinburne University of Technology and South African Radio Astronomy Observatory (SARAO)
Gravitational Wave Sky Map from the full 4.5yr-long MeerKAT pulsar timing array dataset. The white s ...
K. Grunthal et al., 2024
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A MeerKAT antenna in front of an artistic representation of supermassive black holes and the space-t ...
Carl Knox, OzGrav, Swinburne University of Technology and South African Radio Astronomy Observatory (SARAO)
Gravitational Wave Sky Map from the full 4.5yr-long MeerKAT pulsar timing array dataset. The white s ...
K. Grunthal et al., 2024
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