Gravitationally Lensed Gravitational Waves from Black Holes Around Black Holes

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llustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole.
Credit: Caltech/R. Hurt (IPAC)

Diagram of a binary black hole system orbiting within the disk of a supermassive black hole
The observer is located at N in this diagram.
Credit:Leong et al. 2025

Gravitational-wave detectors have captured the chirps of dozens of merging black holes. Could any of these mergers have happened in the disk around a supermassive black hole?

Black Holes Around Black Holes

At the centers of galaxies across the universe, the disks surrounding accreting supermassive black holes — known as active galactic nuclei — provide an extreme ecosystem for stars and stellar-mass black holes. When a pair of black holes within an active galactic nucleus disk merges, the collision produces gravitational waves that can be picked up by detectors on Earth. If, from our perspective, that merger takes place behind the supermassive black hole, the gravitational-wave signal will be gravitationally lensed: split into two “images” of the same wave with slightly different properties.

Detecting a gravitationally lensed gravitational-wave signal from merging black holes would provide valuable information about the population of black holes that reside in active galactic nucleus disks, as well as the properties of the disks themselves.

Constraints able to be placed on the fraction of binary black hole mergers happening in active galactic nucleus disks as a function of the number of observations, Nobs, and the distance between the binary system and the central supermassive black hole, indicated by the fill pattern. The filled area shows the values that are ruled out. This plot assumes that no gravitationally lensed gravitational waves are observed. Adapted from Leong et al. 2025

Lensing Likelihood

So far, no gravitationally lensed gravitational waves have been detected — but luckily, even this non-detection contains valuable information. To explore the implications of this non-detection, Samson Leong (The Chinese University of Hong Kong) and collaborators developed an analytical model that describes a binary black hole pair orbiting and merging within the disk of an active galactic nucleus. The team calculated the probability that gravitational waves from the merger of these black holes would be gravitationally lensed from the perspective of a distant observer. This probability is dependent upon the orientation of the disk relative to the viewer, as well as the distance from the binary system to the central supermassive black hole.

Then, given the fact that none of the dozens of mergers detected so far have had gravitationally lensed signals, Leong’s team constrained the fraction of observed mergers happening in active galactic nucleus disks. With only about 100 binary black hole merger observed to date, the constraining power of the non-detection is limited. For now, all that can be said is that no more than 47% of the observed mergers took place in the disks around active galactic nuclei. As the number of detected black hole mergers grows, the constraint will grow more stringent; if no lensed events have been observed after roughly 1,000 mergers have been detected, that would mean that no more than 5% of the mergers took place within an active galactic nucleus disk.

Similar to the previous figure, but this time emphasizing the impact of the orbital distance of the merging black holes. The vertical dotted lines indicate the locations of potential migration traps. Adapted from Leong et al. 2025

To Be Constrained

This estimate is based on the assumption that all black holes in active galactic nucleus disks merge within the migration trap nearest the central supermassive black hole. Several migration traps — particular orbital radii within the disk where black holes are expected to collect — are predicted to exist. If the black holes instead merge within a migration trap at a much larger radius, many more observations will be needed to narrowly constrain the number of mergers happening within accretion disks.

Future observations may yield new information about active galactic nucleus accretion disks. In particular, it may be possible to discern the minimum size of an accretion disk, as well as where within the disk binary black holes are most likely to merge.

By Kerry Hensley

Citation

“Constraining Binary Mergers in Active Galactic Nuclei Disks Using the Nonobservation of Lensed Gravitational Waves,” Samson H. W. Leong et al 2025 ApJL 979 L27. doi:10.3847/2041-8213/ad9ead

 

Source: American Astronomical Society/AAS-Nova

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Are Binary Black Hole Spins and Mass Ratios Correlated?

An artist's rendition of two black holes approaching a collision.
Credit: LIGO/Caltech/MIT/R. Hurt (IPAC)

When researchers scour the detections of merging black holes made by gravitational wave observatories, they use models and statistics to make careful inferences about the population of black holes in our universe. In a recent article, researchers explored whether an emerging trend in gravitational wave data is real or an artifact of previous analysis methods.

llustration of the first black hole merger detected by LIGO.
Credit: Aurore Simmonet (Sonoma State University)

A New Window on the Universe

The detection of gravitational waves from merging black holes in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave scientists a new way to investigate black holes. By analyzing the spacetime ripples from colliding black holes, researchers hope to understand how the black holes formed (through the collapse of massive stars or the successive mergers of existing black holes?) and how they came to exist in binary systems (by first belonging to a stellar binary system or by forming solo and linking up with another black hole later?).

One potential result that has emerged from several analyses of gravitational wave signals is that the effective spins and the ratios of the masses of merging black hole binaries appear to be anticorrelated. But as with all results that are extracted delicately, statistically from complex data sets, it’s important to ask if this is a real feature of the data, with real implications for how black hole binary systems are assembled, or if it’s a result of our models or statistical analyses.

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). Credit: Kerry Hensley

Statistical Investigation

Christian Adamcewicz (Monash University and OzGrav) and collaborators approached this question by applying a new statistical treatment to detections of black hole mergers. This new treatment features a new model for effective spin and allows for a subpopulation of black hole binaries with zero effective spin, which hasn’t yet been ruled out and might have an impact that hasn’t been accounted for.

The team applied their population model to the third catalog of gravitational wave signals from the LIGO and Virgo detectors and used Bayesian statistical methods to extract the properties of the overarching population of black holes. They found that the previously reported anticorrelation between effective spin and mass ratio is likely real, ruling out the possibility of there being no correlation at 99.7% probability.

More Work, a Paradox, and Astrophysical Possibilities

Adamcewicz and collaborators acknowledge that this work doesn’t provide a final verdict on this question (as they put it, “a modeler’s job is never done”), and that other statistical effects need to be rooted out. One lingering possibility is that this result is due to the amalgamation paradox, which arises when trends present in different factors disappear or flip when the factors are considered together.

If the observed anticorrelation holds up to further statistical scrutiny, a number of astrophysical phenomena could be responsible for this effect. Extensive mass transfer between black hole progenitor stars, stars evolving within a common envelope and accreting matter at a high rate, or even black hole binary systems assembled within the accretion disks of active galactic nuclei should all be investigated with future black hole population models.

By Kerry Hensley

Citation

“Evidence for a Correlation Between Binary Black Hole Mass Ratio and Black Hole Spins,” Christian Adamcewicz et al 2023 ApJ 958 13. doi:10.3847/1538-4357/acf763

Source: American Astronomical Society/AAS-Nova

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Could We Detect the Merger of Stellar-Mass and Supermassive Black Holes?

An illustration of the warping of spacetime around a stellar-mass black hole that is orbiting a supermassive black hole
Credit: NASA

We’ve detected gravitational waves from mergers of compact objects like stellar-mass black holes, and we’ve found promising evidence for the spacetime disturbances from binary supermassive black holes. But what about when these two mass scales meet — could we detect the merger of a stellar-mass black hole with a supermassive black hole?

Infographic showing the typical frequency ranges of the gravitational waves produced by different sources
Credit: ESA

Stellar-Mass and Supermassive

Many galaxies host a central supermassive black hole, which may have the opportunity to consume stellar-mass black holes from its surroundings. Based on theoretical calculations, it’s likely fairly rare for a stellar-mass black hole to merge with a supermassive black hole, with each galaxy experiencing just a few dozen of these events every billion years.

Surprisingly, adding another supermassive black hole into the mix may greatly increase the odds of such an interaction. When stellar-mass black holes encounter a supermassive black hole binary, the likelihood of a merger is boosted up to hundreds of thousands of events per galaxy per billion years. The gravitational waves from this type of merger are too low frequency to be detected with our current observatories, but a recent research article has explored the possibility of detecting gravitational waves from these encounters in the not-too-distant future.

Predicted gravitational wave amplitude as a function of frequency for resolved (purple lines) and unresolved (grey lines) systems, compared to LISA’s estimated sensitivity. Click to enlarge. Credit: Naoz & Haiman 2023

Simulating Gravitational Waves

Smadar Naoz (University of California, Los Angeles) and Zoltán Haiman (Columbia University) simulated the gravitational waves that would result from a stellar-mass black hole spiraling in to merge with one member of a supermassive black hole binary. This type of merger is called an extreme-mass-ratio inspiral. First, Naoz and Haiman estimated the number of extreme-mass-ratio inspirals as a function of the mass of the black holes in the binary system. Perhaps counterintuitively, stellar-mass black holes are much more likely to merge with the less massive black hole in a binary system, thanks to gravitational perturbations.

The team then calculated the amplitude of the gravitational waves produced in each merger and found that future observatories should be able to detect these events. They focused on the Laser Interferometer Space Antenna (LISA) — a proposed space-based gravitational wave observatory that would consist of three spacecraft trailing Earth in its orbit — which should detect individual extreme-mass-ratio inspirals as well as a background signal composed of thousands of events too faint to be detected individually. During the proposed 4-year LISA mission, the observatory could detect hundreds of individual sources.

Observing gravitational waves from a stellar-mass black hole as it spirals toward a supermassive black hole can help us understand many aspects of how supermassive black holes grow and merge. In particular, these observations may help us put a number on how many companions a supermassive black hole is likely to have; do these behemoths mostly fly solo, or are pairs, triples, or quartets more likely? Hopefully, it’s just a matter of time before LISA is in place in its berth in space — the planned launch date is 2037 — and ready to open a new window onto gravitational waves.

Illustration of the three LISA spacecraft trailing behind Earth
Credit: NASA/JPL-Caltech/NASAEA/ESA/CXC/STScl/GSFCSVS/S.Barke; CC BY 4.0

Citation

“The Enhanced Population of Extreme Mass-Ratio Inspirals in the LISA Band from Supermassive Black Hole Binaries,” Smadar Naoz and Zoltán Haiman 2023 ApJL 955 L27. doi:10.3847/2041-8213/acf8c9

Source:  American Astronomical Society/AAS-Nova

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Juggling Black Holes in Star Clusters

The reddish stars of the globular cluster Liller 1 glow behind bright blue stars in the foreground.
Credit: ESA/Hubble & NASA, F. Ferraro; CC BY 4.0

Title: Demographics of Hierarchical Black Hole Mergers in Dense Star Clusters
Authors: Giacomo Fragione and Frederic A. Rasio
First Author’s Institution: Northwestern University
Status: Published in ApJ

Observations of gravitational waves from binary black hole mergers made by LIGO, Virgo, and KAGRA have revolutionized our understanding of the demographics of compact objects like neutron stars and black holes. At the range detectable by these instruments, for example, they have shed light on the distribution of black hole masses, which gives us a glimpse into the late-stage evolution of massive stars. In particular, stellar evolution models predict a dearth of black hole remnants with masses between fifty and a few hundred times the mass of our Sun, because of the so-called pair-instability process. However, LIGO–Virgo–KAGRA observations have indicated the existence of black hole binaries where one member of the binary is in this pair-instability range, meaning that the simple stellar evolution picture doesn’t fully explain the observed population!

Binary black holes are believed to form by two main channels: in isolation, from a binary star system, and dynamically, as depicted in Figure 1. In the former (left), the binary system evolves through a common-envelope phase, which, if the conditions are right, will result in a binary black hole system. In the dynamical pathway (right), interactions between stars in the dense centers of clusters are frequent and can lead to the formation of binary black hole systems. Subsequent interactions can tighten the orbit, leading to mergers.

Focusing on the latter channel, today’s authors explore how such a mechanism could produce intermediate-mass black holes (those with masses between 100 and 1,000 solar masses), specifically through hierarchical black hole mergers. Hierarchical black hole mergers are those in which one (or both) of the component black holes is a so-called “later-generation” black hole: the remnant of a previous binary black hole merger in the center of a dense star cluster. (A first-generation black hole is one produced at the end of a star’s life, a second-generation one is formed from two first-generation black holes, and so on.) These sorts of repeated mergers of stellar-mass black holes would yield black holes in the intermediate-mass black hole range, providing a natural answer to the provenance of these black holes. Today’s authors use a new modeling framework to predict the properties of star clusters that can produce a detectable distribution of hierarchical black hole mergers.

Figure 1: Cartoon depiction of the two primary channels for binary black hole formation (left and right columns), with time increasing from top to bottom in each column. At left, a binary star system enters a common-envelope phase that shrinks the orbit of the binary. If this results in ejection of the envelope, the system will eventually form a binary black hole system that will ultimately merge. At right, a binary system of a black hole and a star will become a system of binary black holes through a three-body interaction in a dense star cluster. Later interactions will shrink the orbit until the two black holes merge. Credit: Mapelli 2020; CC BY 4.0

Demographics of Cluster Hosts

The primary challenge associated with this channel of intermediate-mass black hole formation is the “recoil kick” that results from asymmetry in the merger process. In some cases, the velocity with which the resulting black hole forms can exceed the escape velocity of the star cluster in which it formed, leading to ejection (and thus limiting the possibility of a subsequent merger). Using the modeling framework developed in a previous work, today’s authors are able to predict the properties of clusters that will be able to retain a binary black hole remnant after the merger, as displayed in Figure 2. From these panels it is clear that hierarchical black holes are likely only produced and retained in the most massive and densest clusters, as these will be the ones with the deepest potential wells and highest rates of interaction.

In these massive and dense clusters, repeated black hole mergers can eventually result in the formation of a single massive black hole >1,000 times the mass of our Sun that dominates the interactions and binary merger process. It turns out that these massive black holes generally grow as a result of mergers with first-generation black holes, as mergers of two hierarchically produced black holes (i.e., two second- or third-generation black holes) tend to impart a strong kick on the remnant, driving it to escape from the cluster. Therefore, robustly modeling the effect of kicks is crucial to understanding the rates of intermediate-mass black hole formation by this hierarchical merger process.

Figure 2: The probability that a merger remnant will survive in the cluster (color scale) as a function of cluster mass and density, going from the merger of two first-generation black holes (left), to the merger of a first- and second-generation black hole (middle), and the merger of two second-generation black holes (right). The points correspond to observations of different types of star clusters from the literature. Notice how the remnant of two first-generation black holes merging is relatively easy to retain, but it becomes progressively more difficult as we introduce later generations. Adapted from Fragione and Rasio 2023

Figure 3: The merger rates as a function of cosmological redshift (i.e., over time in the universe’s history) for combinations of merging black holes of different generations (e.g., 1G refers to a first-generation black hole). The left panel is the prediction including star clusters with masses up to 10 million solar masses, while the right panel only includes clusters up to 1 million solar masses. There are no mergers beyond 3G in the right panel, demonstrating that only the most massive clusters can host late-generation black-hole mergers. Adapted from Fragione and Rasio 2023

Merger Rates and Assorted Sundries

With their framework in hand and the properties of the cluster hosts understood, today’s authors then average over the distribution of star clusters of different masses as a function of time to predict merger rates of various generations of hierarchical black holes. In Figure 3, the authors demonstrate that massive clusters (with masses up to 107 solar masses) are necessary to produce later generations of black hole mergers. That is, they find that the rates of hierarchical black hole mergers fall as the maximum cluster mass is lowered from 10 million to 1 million solar masses, as the lower-mass clusters are less likely to hold onto the resulting black holes. In kind, the merger rates for component black holes later than third-generation disappear.

Relatedly, they compare the distribution of merging black hole masses detected with the LIGO–Virgo–KAGRA observatories to those predicted by their model. From such analysis, they demonstrate that within their framework, several of the observed events can only be produced by accounting for hierarchical black hole mergers, with a few appearing to come from mergers of second- and third-generation black holes!

These results, while preliminary, can be extended (by incorporating other metrics, such as the black hole spins) to statistically measure the likelihood of individual gravitational wave events being associated with hierarchical mergers. However, they represent an exciting step towards understanding where these intermediate-mass black holes come from and provide a compelling, natural explanation for how a large population of massive black holes can form and continue to evolve over cosmic time!

Original astrobite edited by Mark Popinchalk.

By Astrobites

Source: American Astronomical Society/AAS Nova

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About the author, Sahil Hegde:

I am an astrophysics PhD student at UCLA working on using semi-analytic models to study the formation of the first stars and galaxies in the universe. I completed my undergraduate at Columbia University, and am originally from the San Francisco Bay Area. Outside of astronomy you’ll find me playing tennis, surfing (read: wiping out), and playing board games/TTRPGs!

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Redefining a Heavy Collision

Artist’s impression of the collision of two black holes that produced the gravitational-wave signal GW190521.
[LIGO/Caltech/MIT/R. Hurt (IPAC)]

Could the biggest — literally — gravitational-wave discovery yet be something other than what it initially seemed? A new study suggests that the most massive merger of black holes detected by LIGO/Virgo may have included a surprising lightweight.

The rapidly expanding “stellar graveyard”, a plot of the masses of the different components of observed compact binary mergers. GW190521, top center, is more massive than any other binary merger we’ve observed. [LIGO-Virgo/Northwestern U./Frank Elavsky & Aaron Geller]

Echoes of a Surprising Merger

In May 2019, a collision of two black holes shook spacetime, registering in the LIGO and Virgo gravitational-wave detectors as the heaviest black-hole merger discovered yet. Initial analysis of GW190521 suggested that the participants in this cosmic collision were ~85 and ~66 times the mass of the Sun, and that they formed a final black hole of ~142 solar masses — an unexpectedly heavy outcome that lands in the elusive category of intermediate-mass black holes.

But GW190521 raised eyebrows for another reason as well: the estimated masses of the two merging black holes fell between 65 and 120 solar masses, a region known as the pair-instability mass gap. This range of masses should be inherently off-limits for black holes born from collapsed stars, based on our current understanding of stellar evolution processes.

While there are many hypotheses about how mass-gap black holes could potentially form, two scientists have focused on an alternative angle: what if we were simply wrong in our estimate of GW190521’s component masses?

Checking Our Assumptions

How do we measure component masses from a gravitational-wave signal? Decades of theoretical research have produced a vast array of model signals for mergers with different parameters. By comparing the observed gravitational-wave signal to the various models, we can calculate which ones fit best. But this comparison relies on what are called priors — a set of assumptions that go into the analysis and affect the outcome.

In a recent publication, scientists Alexander Nitz and Collin Capano (Max Planck Institute for Gravitational Physics and Leibniz University Hannover, Germany) reanalyze the gravitational-wave signal for GW190521 using a different set of priors and constraints than the original analysis completed by the LIGO collaboration.

Nitz and Capano find that their analysis admits two possible solutions for GW190521: one similar to that found by the LIGO collaboration — and another, in which the component black holes are ~16 and ~170 solar masses. This second option becomes even more heavily favored when the authors analyze the gravitational-wave signal simultaneously with an electromagnetic flare that may have been associated with the merger.

The observed gravitational-wave signal of GW190521 in each of the three detectors (black), plotted with two best-fit models: one for when the component mass ratio is between 1 and 2 (blue) and one for a mass ratio between 2 and 25 (orange). [Nitz & Capano 2021]

An Uneven Pair? 

 

What does this outcome tell us? The masses in Nitz and Capano’s second solution both lie outside of the pair-instability mass gap, neatly resolving the paradox previously created by this merger.

If the authors’ interpretation is correct, then GW190521 would represent the first detected intermediate-mass-ratio inspiral — a type of merger in which one component is substantially larger than the other. This signal then provides an exciting milestone and an opportunity to learn more about the different types of dramatic collisions that occur in our galaxy.

Citation

“GW190521 May Be an Intermediate-mass Ratio Inspiral,” Alexander H. Nitz and Collin D. Capano 2021 ApJL 907 L9. doi:10.3847/2041-8213/abccc5

Source:American Astronomical  Society (AAS NOVA)

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Black Hole Collision May Have Exploded with Light

Artist's concept of a supermassive black hole and its surrounding disk of gas. Embedded within this disk are two smaller black holes orbiting one another. Using data from the Zwicky Transient Facility (ZTF) at Palomar Observatory, researchers have identified a flare of light suspected to have come from one such binary pair soon after they merged into a larger black hole. The merger of the black holes would have caused them to move in one direction within the disk, plowing through the gas in such a way to create a light flare. The finding, while not confirmed, could amount to the first time that light has been seen from a coalescing pair of black holes. These merging black holes were first spotted on May 21, 2019, by the National Science Foundation's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector, which picked up gravitational waves generated by the merger. Credit: Caltech/R. Hurt (IPAC)

Possible light flare observed from small black holes within the disk of a massive black hole

When two black holes spiral around each other and ultimately collide, they send out ripples in space and time called gravitational waves. Because black holes do not give off light, these events are not expected to shine with any light waves, or electromagnetic radiation. But some theorists have come up with ways in which a black hole merger might explode with light. Now, for the first time, astronomers have seen evidence for one of these light-producing scenarios.

With the help of Caltech's Zwicky Transient Facility (ZTF), funded by the National Science Foundation (NSF) and located at Palomar Observatory near San Diego, the scientists have spotted what might be a flare of light from a pair of coalescing black holes. The black hole merger was first witnessed by the NSF's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector on May 21, 2019, in an event called S190521g. As the black holes merged, jiggling space and time, they sent out gravitational waves.

While this was happening, ZTF was performing its robotic survey of the sky that captured all kinds of objects that flare, erupt, or otherwise vary in the night sky. One flare the survey caught, generated by a distant active supermassive black hole, or quasar, called J1249+3449, was pinpointed to the region of the gravitational-wave event S190521g.

"This supermassive black hole was burbling along for years before this more abrupt flare," says Matthew Graham, a research professor of astronomy at Caltech and the project scientist for ZTF. "The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event. In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities." Graham is lead author of the new study, published today, June 25, in the journal Physical Review Letters.

"ZTF was specifically designed to identify new, rare, and variable types of astronomical activity like this," says NSF Division of Astronomical Science Director Ralph Gaume. "NSF support of new technology continues to expand how we can track such events."

How do two merging black holes erupt with light? In the scenario outlined by Graham and his colleagues, two partner black holes were nestled within a disk surrounding a much larger black hole.

"At the center of most galaxies lurks a supermassive black hole. It's surrounded by a swarm of stars and dead stars, including black holes," says co-author K. E. Saavik Ford of the City University of New York (CUNY) Graduate Center, the Borough of Manhattan Community College (BMCC), and the American Museum of Natural History (AMNH). "These objects swarm like angry bees around the monstrous queen bee at the center. They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance. But in a supermassive black hole's disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up," she says.

Once the black holes merge, the new, now-larger black hole experiences a kick that sends it off in a random direction, and it plows through the gas in the disk. "It is the reaction of the gas to this speeding bullet that creates a bright flare, visible with telescopes," says co-author Barry McKernan, also of the CUNY Graduate Center, BMCC, and AMNH.

Such a flare is predicted to begin days to weeks after the initial splash of gravitational waves produced during the merger. In this case, ZTF did not catch the event right away, but when the scientists went back and looked through archival ZTF images months later, they found a signal that started days after the May 2019 gravitational-wave event. ZTF observed the flare slowly fade over the period of a month.

The scientists attempted to get a more detailed look at the light of the supermassive black hole, called a spectrum, but by the time they looked, the flare had already faded. A spectrum would have offered more support for the idea that the flare came from merging black holes within the disk of the supermassive black hole. However, the researchers say they were able to largely rule out other possible causes for the observed flare, including a supernova or a tidal disruption event, which occurs when a black hole essentially eats a star.

What is more, the team says it is not likely that the flare came from the usual rumblings of the supermassive black hole, which regularly feeds off its surrounding disk. Using the Catalina Real-Time Transient Survey, led by Caltech, they were able to assess the behavior of the black hole over the past 15 years, and found that its activity was relatively normal until May of 2019, when it suddenly intensified.

"Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular," says co-author Mansi Kasliwal (MS '07, PhD '11), an assistant professor of astronomy at Caltech. "The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins."

The newly formed black hole should cause another flare in the next few years. The process of merging gave the object a kick that should cause it to enter the supermassive black hole's disk again, producing another flash of light that ZTF should be able to see.

The Physical Review Letters paper, titled, "A Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event GW190521g," was funded by the NSF, NASA, the Heising-Simons Foundation, and the GROWTH (Global Relay of Observatories Watching Transients Happen) program. Other co-authors include: K. Burdge, S.G. Djorgovski, A.J. Drake, D. Duev, A.A. Mahabal, J. Belecki, R. Burruss, G. Helou, S.R. Kulkarni, F.J. Masci, T. Prince, D. Reiley, H. Rodriguez, B. Rusholme, R.M. Smith, all from Caltech; N.P. Ross of the University of Edinburgh; Daniel Stern of the Jet Propulsion Laboratory, managed by Caltech for NASA; M. Coughlin of the University of Minnesota; S. van Velzen of University of Maryland, College Park and New York University; E.C. Bellm of the University of Washington; S.B. Cenko of NASA Goddard Space Flight Center; V. Cunningham of University of Maryland, College Park; and M.T. Soumagnac of the Lawrence Berkeley National Laboratory and the Weizmann Institute of Science.

In addition to the NSF, ZTF is funded by an international collaboration of partners, with additional support from NASA, the Heising-Simons Foundation, members of the Space Innovation Council at Caltech, and Caltech itself.

Written by Whitney Clavin

Contact:

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

Source: Caltech/News

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Could gravitational waves reveal how fast our universe is expanding?

Artist’s depiction of the last instances of a neutron star and black hole merger, as the neutron star is destroyed by the tidal pull of the black hole (at the center of the disk). Image: A. Tonita, L. Rezzolla, F. Pannarale

Signals from rare black hole-neutron star pairs could pinpoint rate at which universe is growing, researchers say.

Since it first exploded into existence 13.8 billion years ago, the universe has been expanding, dragging along with it hundreds of billions of galaxies and stars, much like raisins in a rapidly rising dough.

Astronomers have pointed telescopes to certain stars and other cosmic sources to measure their distance from Earth and how fast they are moving away from us — two parameters that are essential to estimating the Hubble constant, a unit of measurement that describes the rate at which the universe is expanding.

But to date, the most precise efforts have landed on very different values of the Hubble constant, offering no definitive resolution to exactly how fast the universe is growing. This information, scientists believe, could shed light on the universe’s origins, as well as its fate, and whether the cosmos will expand indefinitely or ultimately collapse.

Now scientists from MIT and Harvard University have proposed a more accurate and independent way to measure the Hubble constant, using gravitational waves emitted by a relatively rare system: a black hole-neutron star binary, a hugely energetic pairing of a spiraling black hole and a neutron star. As these objects circle in toward each other, they should produce space-shaking gravitational waves and a flash of light when they ultimately collide.

In a paper published today in Physical Review Letters, the researchers report that the flash of light would give scientists an estimate of the system’s velocity, or how fast it is moving away from the Earth. The emitted gravitational waves, if detected on Earth, should provide an independent and precise measurement of the system’s distance. Even though black hole-neutron star binaries are incredibly rare, the researchers calculate that detecting even a few should yield the most accurate value yet for the Hubble constant and the rate of the expanding universe.

“Black hole-neutron star binaries are very complicated systems, which we know very little about,” says Salvatore Vitale, assistant professor of physics at MIT and lead author of the paper. “If we detect one, the prize is that they can potentially give a dramatic contribution to our understanding of the universe.”

Vitale’s co-author is Hsin-Yu Chen of Harvard.

Competing constants

Two independent measurements  of the Hubble constant were made recently, one using NASA's Hubble Space Telescope and another using the European Space Agency's Planck satellite. The Hubble Space Telescope’s measurement is based on observations of a type of star known as a Cepheid variable, as well as on observations of supernovae. Both of these objects are considered “standard candles,” for their predictable pattern of brightness, which scientists can use to estimate the star’s distance and velocity.

The other type of estimate is based on observations of the fluctuations in the cosmic microwave background — the electromagnetic radiation that was left over in the immediate aftermath of the Big Bang, when the universe was still in its infancy. While the observations by both probes are extremely precise, their estimates of the Hubble constant disagree significantly.

“That’s where LIGO comes into the game,” Vitale says.

LIGO, or the Laser Interferometry Gravitational-Wave Observatory, detects gravitational waves — ripples in the Jell-O of space-time, produced by cataclysmic astrophysical phenomena.

“Gravitational waves provide a very direct and easy way of measuring the distances of their sources,” Vitale says. “What we detect with LIGO is a direct imprint of the distance to the source, without any extra analysis.”

In 2017, scientists got their first chance at estimating the Hubble constant from a gravitational-wave source, when LIGO and its Italian counterpart Virgo detected a pair of colliding neutron stars for the first time. The collision released a huge amount of gravitational waves, which researchers measured to determine the distance of the system from Earth. The merger also released a flash of light, which astronomers focused on with ground and space telescopes to determine the system’s velocity.

With both measurements, scientists calculated a new value for the Hubble constant. However, the estimate came with a relatively large uncertainty of 14 percent, much more uncertain than the values calculated using the Hubble Space Telescope and the Planck satellite.

Vitale says much of the uncertainty stems from the fact that it can be challenging to interpret a neutron star binary’s distance from Earth using the gravitational waves that this particular system gives off.   

“We measure distance by looking at how ‘loud’ the gravitational wave is, meaning how clear it is in our data,” Vitale says. “If it’s very clear, you can see how loud it is, and that gives the distance. But that’s only partially true for neutron star binaries.”

That’s because these systems, which create a whirling disc of energy as two neutron stars spiral in toward each other, emit gravitational waves in an uneven fashion. The majority of gravitational waves shoot straight out from the center of the disc, while a much smaller fraction escapes out the edges. If scientists detect a “loud” gravitational wave signal, it could indicate one of two scenarios: the detected waves stemmed from the edge of a system that is very close to Earth, or the waves emanated from the center of a much further system.

“With neutron star binaries, it’s very hard to distinguish between these two situations,” Vitale says.

A new wave

In 2014, before LIGO made the first detection of gravitational waves, Vitale and his colleagues observed that a binary system composed of a black hole and a neutron star could give a more accurate distance measurement, compared with neutron star binaries. The team was investigating how accurately one could measure a black hole’s spin, given that the objects are known to spin on their axes, similarly to Earth but much more quickly.

The researchers simulated a variety of systems with black holes, including black hole-neutron star binaries and neutron star binaries. As a byproduct of this effort, the team noticed that they were able to more accurately determine the distance of black hole-neutron star binaries, compared to neutron star binaries. Vitale says this is due to the spin of the black hole around the neutron star, which can help scientists better pinpoint from where in the system the gravitational waves are emanating.

“Because of this better distance measurement, I thought that black hole-neutron star binaries could be a competitive probe for measuring the Hubble constant,” Vitale says. “Since then, a lot has happened with LIGO and the discovery of gravitational waves, and all this was put on the back burner.”

Vitale recently circled back to his original observation, and in this new paper, he set out to answer a theoretical question:

“Is the fact that every black hole-neutron star binary will give me a better distance going to compensate for the fact that potentially, there are far fewer of them in the universe than neutron star binaries?” Vitale says.

To answer this question, the team ran simulations to predict the occurrence of both types of binary systems in the universe, as well as the accuracy of their distance measurements. From their calculations, they concluded that, even if neutron binary systems outnumbered black hole-neutron star systems by 50-1, the latter would yield a Hubble constant similar in accuracy to the former.

More optimistically, if black hole-neutron star binaries were slightly more common, but still rarer than neutron star binaries, the former would produce a Hubble constant that is four times as accurate.

“So far, people have focused on binary neutron stars as a way of measuring the Hubble constant with gravitational waves,” Vitale says. “We’ve shown there is another type of gravitational wave source which so far has not been exploited as much: black holes and neutron stars spiraling together,” Vitale says. “LIGO will start taking data again in January 2019, and it will be much more sensitive, meaning we’ll be able to see objects farther away. So LIGO should see at least one black hole-neutron star binary, and as many as 25, which will help resolve the existing tension in the measurement of the Hubble constant, hopefully in the next few years.”

This research was supported, in part, by the National Science Foundation and the LIGO Laboratory.

Jennifer Chu | MIT News Office

Source:  Massachusetts Institute of Technology - MIT