New method could reveal hidden supermassive black hole binaries

Artistic impression of gravitationally lensed starlight (orange) by a supermassive black hole binary. The Einstein ring is shown in blue.Credit: Physics simulation enhanced using AI

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To the point:

• New method: Researchers at Oxford University and the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in the Potsdam Science Park propose a new way to detect supermassive black hole binaries using gravitational lensing.


• Gravitational lensing: Black holes act as natural telescopes, bending light with their gravity. This magnification creates bright images of stars from the same galaxy that lie behind the supermassive black hole binary.


• Detectable signals: As the binary orbits, it produces repeating flashes of lensed starlight. Current and upcoming wide-field surveys may detect these bursts in the future. These bursts can provide information about the black holes’ properties and enabling entirely new studies.

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Bright flashes of lensed starlight guide the way

New method

Tightly bound supermassive black hole binaries form naturally when galaxies merge, but only widely separated systems have confidently been observed to date. In a paper published today in Physical Review Letters, the researchers suggest hunting down the hidden systems by searching for repeating flashes of light from individual stars lying behind the black holes as they are temporarily magnified by gravitational lensing as the binary orbits.

Supermassive black holes reside at the centers of most galaxies. When two galaxies collide and merge, their central black holes eventually form a bound pair, known as a supermassive black hole binary. These systems play a crucial role in galaxy evolution and are among the most powerful sources of gravitational waves in the Universe. While future space-based gravitational-wave observatories like LISA will be able to probe such binaries directly, researchers are now showing that they may already be detectable using existing and upcoming electromagnetic surveys.

Gravitational lensing

“Supermassive black holes act as natural telescopes,” says Miguel Zumalacárregui from the Max Planck Institute for Gravitational Physics. “Because of their enormous mass and compact size, they strongly bend passing light. Starlight from the same host galaxy can be focused into extraordinarily bright images, a phenomenon known as gravitational lensing.”

For a single supermassive black hole, extremely strong lensing occurs only when a star lies almost exactly along the line of sight. In contrast, a supermassive black hole binary acts as a pair of lenses. This produces a diamond-shaped structure, known as a caustic curve, along which stars can experience dramatic magnification.

“The chances of starlight being hugely amplified increase enormously for a binary compared to a single black hole,” explains Bence Kocsis from the University of Oxford’s Department of Physics and a co-author of the study.

A further key difference is that black hole binaries are not static. While the pair orbits under gravity the system slowly loses energy by emitting gravitational waves. As a result, the binary separation shrinks over time and the orbit gradually speeds up.

“As the binary moves, the caustic curve rotates and changes shape, sweeping across a large volume of stars behind it. If a bright star lies within this region, it can produce an extraordinarily bright flash each time the caustic passes over it,” says Hanxi Wang, a PhD student in Kocsis’ group who led the study “This leads to repeating bursts of starlight, which provide a clear and distinctive signature of a supermassive black hole binary.”

Valuable information from detectable signals

The researchers show that the timing and brightness of these bursts encode valuable information about the black hole binary. As the binary inspirals, gravitational-wave emission subtly alters the caustic structure, imprinting a characteristic modulation in both the frequency and peak brightness of the flashes. By measuring these patterns, astronomers could infer key properties of the underlying black hole binary, including its masses and orbital evolution.

With powerful wide-field surveys coming online such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, researchers are optimistic that such repeating lensing bursts could be observed in the coming years.

“The prospect of identifying inspiraling supermassive black hole binaries years before future space-based gravitational wave detectors come online is extremely exciting,” concludes Kocsis. “It opens the door to true multi-messenger studies of black holes, allowing us to test gravity and black hole physics in entirely new ways.”

Source: Max Planck Institute for Gravitational Physics/News

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Media contact:

Dr. Elke Müller
Press Officer AEI Potsdam, Scientific Coordinator
Tel: +49 331 567-7303
Email: elke.mueller@aei.mpg.de

Science contact:

Dr. Miguel Zumalacarregui
Group; Leader
Tel: +49 331 567-7322
Fax: +49 331 567-7298
Email: miguel.zumalacarregui@aei.mpg.de

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Publication

Wang, H.; Zumalacarregui, M.; Kocsis, B.
Black holes as telescopes: Discovering supermassive binaries through quasi-periodic lensed starlight. Physical Review Letters 136, 061403 (2026)

MPG.PuRe - pre-print - publisher-version

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Are astronomers seeing a signal from giant black holes?

Artist’s impression of the IPTA experiment — an array of pulsars around the Earth embedded in a gravitational wave background from supermassive black hole binaries. The signals from the pulsars measured with a network of global radio telescopes are affected by the gravitational waves and allow for the study of the origin of the background. Image by Carl Knox (OxGrav).

World-wide radio telescope network strengthens evidence for signal that may hint at ultra-low frequency gravitational waves

An international team of astronomers has discovered what could be the early sign of a background signal arising from supermassive black holes, observed through low-frequency gravitational waves. These scientists are comparing data collected from several instruments, including the National Science Foundation’s Green Bank Telescope (GBT.)

Gravitational Waves ripple through spacetime at a light-year-scale, and could originate from mergers of the most massive black holes in the Universe—or from events occurring soon after the formation of the Universe in the Big Bang.

The International Pulsar Timing Array (IPTA) joins the work of several astrophysics collaborations from around the world, including independent data sets of the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array in Australia (PPTA). The IPTA has shared a new data release, known as Data Release 2 (DR2) consisting of precision timing data from 65 millisecond pulsars—stellar remnants which spin hundreds of times per second, sweeping narrow beams of radio waves that appear as pulses due to the spinning. 20 of these pulsars were observed by the Green Bank Telescope.

“The GBT contributes to the IPTA as one of the most important telescopes used by NANOGrav. The combination of the GBT’s excellent sensitivity, instruments, and ability to see so much of the sky make it a critical part of the IPTA’s efforts,” shares Dr. Ryan Lynch, a Green Bank Observatory scientist and NANOGrav member.

Research of the combined IPTA DR2, and other independent data sets from the three constituent collaborations, has revealed strong evidence for this new low-frequency gravitational wave background signal, correlated many of the pulsars. The characteristics of this common-among-pulsars signal are in broad agreement with those expected from a GW “background” (GWB). This background is formed by many different overlapping GW signals emitted from the cosmic population of supermassive binary black holes (i.e., two supermassive black holes orbiting each other and eventually merging), analogous to background noise from the many overlapping voices in a crowded hall. This result further strengthens the gradual emergence of similar signals that have been found in the individual data sets of the participating collaborations over the past few years.

But scientists caution they do not yet have definitive evidence for the GWB, and are still looking into what else this signal could be, and gathering more information to strengthen their findings. The “smoking gun” for a gravitational wave detection is a unique relationship in the strength of the signal between pulsars in different parts of the sky. While these “spatial correlations” have not yet been detected, the existing signal is consistent with what scientists expect to see at first. The IPTA is working diligently to analyze more recent data, which could confirm the nature of the new signal. In addition, contributions from new telescopes such as MeerKAT and from other collaborations, such as the India Pulsar Timing Array, will be important in the future. Dr. Maura McLaughlin of West Virginia University, who uses the GBT for data collection for NANOGrav, says that, “If the signal we are currently seeing is the first hint of a GWB, then based on our simulations, it is possible we will have more definite measurements of the spatial correlations necessary to conclusively identify the origin of the common signal in the near future.”

“The IPTA is a great example of scientists and instruments from around the world coming together to advance our understanding of the cosmos,” shares Lynch. The Green Bank Observatory is developing new technology to enhance the GBT’s capabilities for this research, “New instruments, like our upcoming ultrawideband receiver [funded by the Moore Foundation], will ensure that the GBT continues to make essential contributions to NANOGrav and the IPTA. If what we are seeing here is indeed the signature of gravitational waves, then the next few years are going to be really exciting.”

Written by Jill Malusky

Source: Green Bank Telescope (GBT)

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International Pulsar Timing Array Steering Committee, Megan DeCesar - megandecesar@gmail.com

Green Bank Observatory, Jill Malusky - jmalusky@nrao.edu

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Gravitational Wave Search Provides Insights into Galaxy Evolution and Mergers

NANOGrav Animation of Gravitational Waves

from NRAO Outreach on Vimeo.

In this artist animation, the Earth is constantly jostled by low-frequency gravitational waves from supermassive black hole binaries in distant galaxies. Astrophysicists are using pulsars as a galaxy-sized detector to measure the Earth’s motion from these waves. Credit: B. Saxton (NRAO/AUI/NSF)

The Earth is constantly jostled by low-frequency gravitational waves from supermassive black hole binaries in distant galaxies. Astrophysicists are using pulsars as a galaxy-sized detector to measure the Earth’s motion from these waves. Credit: B. Saxton (NRAO/AUI/NSF)


The recent LIGO detection of gravitational waves from merging black holes with tens of solar masses has confirmed that distortions in the fabric of space-time can be observed and measured [1]. Researchers from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have spent the past decade searching for low-frequency gravitational waves emitted by black hole binaries with masses many millions of times larger than those seen by LIGO.

Analysis of NANOGrav's nine-year dataset provides very constraining limits on the prevalence of such supermassive black hole binaries throughout the Universe. Given scientists’ current understanding of how often galaxies merge, these limits point to fewer detectable supermassive black hole binaries than were previously expected. This result has significant impacts on our understanding of how galaxies and their central black holes co-evolve.

Low-frequency gravitational waves are very difficult to detect, with wavelengths spanning light-years and originating from black hole binaries in galaxies spread across the sky. The combination of all these giant binary black holes leads to a constant "hum" of gravitational waves that models predict should be detectable at Earth. Astrophysicists call this effect the "stochastic gravitational wave background," and detecting it requires special analysis techniques.

Pulsars are the cores of massive stars left behind after stars go supernova. The fastest pulsars rotate hundreds of times each second and emit a pulse of radio waves every few milliseconds. These millisecond pulsars (MSPs) are considered nature's most precise clocks and are ideal for detecting the small signal from gravitational waves. "This measurement is possible because the gravitational wave background imprints a unique signature onto the radio waves seen from a collection of MSPs," said Justin Ellis, Einstein Fellow at NASA’s Jet Propulsion Laboratory, California Institute of Technology in Pasadena, California, and a co-author on the report published in Astrophysical Journal.

Astrophysicists use computer models to predict how often galaxies merge and form supermassive black hole binaries. Those models use several simplifying assumptions about how black hole binaries evolve when they predict the strength of the stochastic gravitational wave background. By using information about galaxy mergers and constraints on the background, the scientists are able to improve their assumptions about black hole binary evolution.

Ellis continues: "After nine years of observing a collection of MSPs, we haven't detected the stochastic background but we are beginning to rule out many predictions based on current models of galaxy evolution. We are now at a point where the non-detection of gravitational waves is actually improving our understanding of black hole binary evolution." "Pulsar timing arrays like NANOGrav are making novel observations of the evolution and nature of our Universe," says Sarah Burke Spolaor, Jansky Fellow at the National Radio Astronomy Observatory (NRAO) in Soccoro, New Mexico, and a co-author on the paper.

According to Spolaor, there are two possible interpretations of this non-detection. “Some supermassive black hole binaries may not be in circular orbits or are significantly interacting with gas or stars. This would drive them to merge faster than simple models have assumed in the past,” she said. An alternate explanation is that many of these binaries inspiral too slowly to ever emit detectable gravitational waves.

NANOGrav is currently monitoring 54 pulsars, using the National Science Foundation's Green Bank Telescope in West Virginia and Arecibo Radio Observatory in Puerto Rico, the two most sensitive radio telescopes at these frequencies [2]. Their array of pulsars is continually growing as new MSPs are discovered. In addition, the group collaborates with radio astronomers in Europe and Australia as part of the International Pulsar Timing Array, giving them access to many more pulsar observations. Ellis estimates that this increase in sensitivity could lead to a detection in as little as five years.

In addition, this measurement helps constrain the properties of cosmic strings, very dense and thin cosmological objects, which many theorists believe evolved when the Universe was just a fraction of a second old. These strings can form loops, which then decay through gravitational wave emission. The most conservative NANOGrav limit on cosmic string tension is the most stringent limit to date, and will continue to improve as NANOGrav continues operating.

"These new results from NANOGrav have the most important astrophysical implications yet," said Scott Ransom, an astronomer with the NRAO in Charlottesville, Virginia. "As we improve our detection capabilities, we get closer and closer to that important threshold where the cosmic murmur begins to be heard. At that point, we’ll be able to perform entirely new types of physics experiments on cosmic scales and open up a new window on the Universe, just like LIGO just did for high-frequency gravitational waves." NANOGrav is a collaboration of over 60 scientists at over a dozen institutions in the United States and Canada whose goal is detecting low-frequency gravitational waves to open a new window on the Universe. The group uses radio pulsar timing observations to search for the ripples in the fabric of spacetime. In 2015, NANOGrav was awarded $14.5 million by the National Science Foundation (NSF) to create and operate a Physics Frontiers Center.

TThe Physics Frontier Centers bring people together to address frontier science, and NANOGrav’s work in low-frequency gravitational wave physics is a great example," said Jean Cottam Allen, the NSF program director who oversees the Physics Frontiers Center program. "We’re delighted with their progress thus far, and we’re excited to see where it will lead."

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Notes

[1] LIGO is the Laser Interferometer Gravitational-Wave Observatory (https://www.ligo.caltech.edu)

Press Release: Gravitational waves detected 100 years after Einstein's prediction http://www.nsf.gov/news/news_summ.jsp?cntn_id=137628&org=NSF&from=news

[2] National Science Foundation (http://www.nsf.gov)

Press Release: Advancing physics frontiers: Newest collaborative centers set to blaze trails in basic research http://www.nsf.gov/news/news_summ.jsp?cntn_id=134586 

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Reference:

"The NANOGrave Nine-year Data Set: Limits on the Isotropic Stochastic Gravitational Wave Background," Z. Arzoumanian et al., 2016, appears in the Astrophysical Journal [http://apj.aas.org].

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Contacts:

Elizabeth Ferrara
NANOGrav press officer
elizabeth.ferrara@nanograv.org
301-286-7057

Charles Blue
NRAO Public Information Officer
cblue@nrao.edu
(434) 296-0314

Source: National Radio Astronomy Observatory (NRAO)