New Gravitational Waves Detected From Four More Black Hole Mergers. Total Detections up to 11 Now

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves (GWs). Since that time, multiple detections have taken place and scientific collaborations between observatories  – like Advanced LIGO and Advanced Virgo – are allowing for unprecedented levels of sensitivity and data sharing.

Previously, seven such events had been confirmed, six of which were caused by the mergers of binary black holes (BBH) and one by the merger of a binary neutron star. But on Saturday, Dec. 1st, a team of scientists the LIGO Scientific Collaboration (LSC) and Virgo Collaboration presented new results that indicated the discovery of four more gravitational wave events. This brings the total number of GW events detected in the last three years to eleven.

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Gravitational waves were only recently observed, and now astronomers are already thinking of ways to use them: like accurately measuring the expansion rate of the Universe

Neutron stars scream in waves of spacetime when they die, and astronomers have outlined a plan to use their gravitational  agony to trace the history of the universe. Join us as we explore how to turn their pain into our cosmological profit.

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It Could be Possible to Transfer Data Through Gravitational Waves

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) made history when they announced the first detection of gravitational waves. Originally predicted made by Einstein’s Theory of General Relativity a century prior, these waves are essentially ripples in space-time that are formed by major astronomical events – such as the merger of a binary black hole pair.

This discovery not only opened up an exciting new field of research, but has opened the door to many intriguing possibilities. One such possibility, according to a new study by a team of Russian scientists, is that gravitational waves could be used to transmit information. In much the same way as electromagnetic waves are used to communicate via antennas and satellites, the future of communications could be gravitationally-based.

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Last Year’s Gravitational Wave Detections Failed to Provide a Hint of Any Extra Spatial Dimensions

In August of 2017, astronomers made another major breakthrough when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves that were believed to be caused by the merger of two neutron stars. Since that time, scientists at multiple facilities around the world have conducted follow-up observations to determine the aftermath this merger, as even to test various cosmological theories.

For instance, in the past, some scientists have suggested that the inconsistencies between Einstein’s Theory of General Relativity and the nature of the Universe over large-scales could be explained by the presence of extra dimensions. However, according to a new study by a team of American astrophysicists, last year’s kilonova event effectively rules out this hypothesis.

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Superfast Jet of Material Blasted Out From Last Year’s Neutron Star Merger

In August of 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected waves that were believed to be caused by a neutron star merger. This “kilonova” event, known as GW170817, was the first astronomical event to be detected in both gravitational and electromagnetic waves – including visible light, gamma rays, X-rays, and radio waves.

In the months that followed the merger, orbiting and ground-based telescopes around the world have observed GW170817 to see what has resulted from it. According to a new study by an international team of astronomers, the merger produced a narrow jet of material that made its way into interstellar space at velocities approaching the speed of light.

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Okay, Last Year’s Kilonova Did Probably Create a Black Hole

In August of 2017, another major breakthrough occurred when the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected waves that were believed to be caused by a neutron star merger. Shortly thereafter, scientists at LIGO, Advanced Virgo, and the Fermi Gamma-ray Space Telescope were able to determine where in the sky this event (known as a kilonova) occurred.

This source, known as GW170817/GRB, has been the target of many follow-up surveys since it was believed that the merge could have led to the formation of a black hole. According to a new study by a team that analyzed data from NASA’s Chandra X-ray Observatory since the event, scientists can now say with greater confidence that the merger created a new black hole in our galaxy.

The study, titled “GW170817 Most Likely Made a Black Hole“, recently appeared in The Astrophysical Journal Letters. The study was led by David Pooley, an assistant professor in physics and astronomy at Trinity University, San Antonio, and included members from the University of Texas at Austin, the University of California, Berkeley, and Nazarbayev University’s Energetic Cosmos Laboratory in Kazakhstan.

Illustration of the kilonova merger (top), and the resulting object (left and right) over time. Credit: NASA/CXC/Trinity University/D. Pooley et al. Illustration: NASA/CXC/M.Weiss

For the sake of their study, the team analyzed X-ray data from Chandra taken in the days, weeks, and months after the detection of gravitational waves by LIGO and gamma rays by NASA’s Fermi mission. While nearly every telescope in the world had observed the source, X-ray data was critical to understanding what happened after the two neutron stars collided.

While a Chandra observation two to three days after the event failed to detect an X-ray source, subsequent observations taken 9, 15, and 16 days after the event resulted in detections. The source disappeared for a time as GW170817 passed behind the Sun, but additional observations were made about 110 and 160 days after the event, both of which showed significant brightening.

While the LIGO data provided astronomers with a good estimate of the resulting object’s mass after the neutron stars merged (2.7 Solar Masses), this was not enough to determine what it had become. Essentially, this amount of mass meant that it was either the most massive neutron star ever found or the lowest-mass black hole ever found (the previous record holders being four or five Solar Masses). As Dave Pooley explained in a NASA/Chandra press release:

“While neutron stars and black holes are mysterious, we have studied many of them throughout the Universe using telescopes like Chandra. That means we have both data and theories on how we expect such objects to behave in X-rays.”

Illustration of the resulting black hole caused by GW170817. Credit: NASA/CXC/M.Weiss

If the neutron stars merged to form a heavier neutron star, then astronomers would expect it to spin rapidly and generate and very strong magnetic field. This would have also created an expanded bubble of high-energy particles that would result in bright X-ray emissions. However, the Chandra data revealed X-ray emissions that were several hundred times lower than expected from a massive, rapidly-spinning neutron star.

By comparing the Chandra observations with those by the NSF’s Karl G. Jansky Very Large Array (VLA), Pooley and his team were also able to deduce that the X-ray emission were due entirely to the shock wave caused by the merger smashing into surrounding gas. In short, there was no sign of X-rays resulting from a neutron star.

This strongly implies that the resulting object was in fact a black hole. If confirmed, these results would indicate that the formation process of a blackhole can sometimes be complicated. Essentially, GW170817 would have been the result of two stars undergoing a supernova explosion that left behind two neutron stars in a sufficiently tight orbit that they eventually came together. As Pawan Kumar explained:

“We may have answered one of the most basic questions about this dazzling event: what did it make? Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now.”

Simulated view of a black hole. Credit: Bronzwaer/Davelaar/Moscibrodzka/Falcke, Radboud University

Looking ahead, the claims put forward by Pooley and his colleagues could be tested by future X-ray and radio observations. Next-generation instruments – like the Square Kilometer Array (SKA) currently under construction in South Africa and Australia, and the ESA’s Advanced Telescope for High-ENergy Astrophysics (Athena+) – would be especially helpful in this regard.

If the remnant turns out to be a massive neutron star with a strong magnetic field after all, then the source should get much brighter in the X-ray and radio wavelengths in the coming years as the high-energy bubble catches up with the decelerating shock wave. As the shock wave weakens, astronomers expect that it will continue to become fainter than it was when recently observed.

Regardless, future observations of GW170817 are bound to provide a wealth of information, according to J. Craig Wheeler, a co-author on the study also from the University of Texas. “GW170817 is the astronomical event that keeps on giving,” he said. “We are learning so much about the astrophysics of the densest known objects from this one event.”

If these follow-up observations find that a heavy neutron star is what resulted from the merger, this discovery would challenge theories about the structure of neutron stars and how massive they can get. On the other hand, if they find that it formed a tiny black hole, then it will challenge astronomers notions about the lower mass limits of black holes. For astrophysicists, it’s basically a win-win scenario.

As co-author Bruce Grossan of the University of California at Berkeley added:

“At the beginning of my career, astronomers could only observe neutron stars and black holes in our own galaxy, and now we are observing these exotic stars across the cosmos. What an exciting time to be alive, to see instruments like LIGO and Chandra showing us so many thrilling things nature has to offer.”

Indeed, looking farther out into the cosmos and deeper back in time has revealed much about the Universe that was previously unknown. And with improved instruments being developed for the sole purpose of studying astronomical phenomena in greater detail and at even greater distances, there seems to be no limit to what we might learn. And be sure to check out this video of the GW170817 merger, courtesy of the Chandra X-ray Observatory:

Further Reading: Chandra, The Astrophysical Journal Letters

How to Listen to the Background Hum of Gravitational Waves From all the Black Holes Colliding into Each Other

The first-ever detection of gravitational waves (which took place in September of 2015) triggered a revolution in astronomy. Not only did this event confirm a theory predicted by Einstein’s Theory of General Relativity a century before, it also ushered in a new era where the mergers of distant black holes, supernovae, and neutron stars could be studied by examining their resulting waves.

In addition, scientists have theorized that black hole mergers could actually be a lot more common than previously thought. According to a new study conducted by pair of researchers from Monash University, these mergers happen once every few minutes. By listening to the background noise of the Universe, they claim, we could find evidence of thousands of previously undetected events.

Their study, titled “Optimal Search for an Astrophysical Gravitational-Wave Background“, recently appeared in the journal Physical Review X. The study was conducted by Rory Smith and Eric Thrane, a senior lecturer and a research fellow at Monash University, respectively. Both researchers are also members of the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav).

Drs. Eric Thrane and Rory Smith. Credit: Monash University

As they state in their study, every 2 to 10 minutes, a pair of stellar-mass black holes merge somewhere in the Universe. A small fraction of these are large enough that the resulting gravitational wave event can be detected by advanced instruments like the Laser Interferometer Gravitational-Wave Observatory and Virgo observatory. The rest, however, contribute to a sort of stochastic background noise.

By measuring this noise, scientists may be able to study much more in the way of events and learn a great deal more about gravitational waves. As Dr Thrane explained in a Monash University press statement:

“Measuring the gravitational-wave background will allow us to study populations of black holes at vast distances. Someday, the technique may enable us to see gravitational waves from the Big Bang, hidden behind gravitational waves from black holes and neutron stars.”

Drs Smith and Thrane are no amateurs when it comes to the study of gravitational waves. Last year, they were both involved in a major breakthrough, where researchers from LIGO Scientific Collaboration (LSC) and the Virgo Collaboration measured gravitational waves from a pair of merging neutron stars. This was the first time that a neutron star merger (aka. a kilonova) was observed in both gravitational waves and visible light.

The pair were also part of the Advanced LIGO team that made the first detection of gravitational waves in September 2015. To date, six confirmed gravitational wave events have been confirmed by the LIGO and Virgo Collaborations. But according to Drs Thrane and Smith, there could be as many as 100,000 events happening every year that these detectors simply aren’t equipped to handle.

In February 2016, LIGO detected gravity waves for the first time. As this artist's illustration depicts, the gravitational waves were created by merging black holes. The third detection just announced was also created when two black holes merged. Credit: LIGO/A. Simonnet.
Artist’s impression of merging binary black holes. Credit: LIGO/A. Simonnet.

These waves are what come together to create a gravitational wave background; and while the individual events are too subtle to be detected, researchers have been attempting to develop a method for detecting the general noise for years. Relying on a combination of computer simulations of faint black hole signals and masses of data from known events, Drs. Thrane and Smith claim to have done just that.

From this, the pair were able to produce a signal within the simulated data that they believe is evidence of faint black hole mergers. Looking ahead, Drs Thrane and Smith hope to apply their new method to real data, and are optimistic it will yield results. The researchers will also have access to the new OzSTAR supercomputer, which was installed last month at the Swinburne University of Technology to help scientists to look for gravitational waves in LIGO data.

This computer is different from those used by the LIGO community, which includes the supercomputers at CalTech and MIT. Rather than relying on more traditional central processing units (CPUs), OzGrav uses graphical processor units – which can be hundreds of times faster for some applications. According to Professor Matthew Bailes, the Director of the OzGRav supercomputer:

“It is 125,000 times more powerful than the first supercomputer I built at the institution in 1998… By harnessing the power of GPUs, OzStar has the potential to make big discoveries in gravitational-wave astronomy.”

What has been especially impressive about the study of gravitational waves is how it has progressed so quickly. From the initial detection in 2015, scientists from Advanced LIGO and Virgo have now confirmed six different events and anticipate detecting many more. On top of that, astrophysicists are even coming up with ways to use gravitational waves to learn more about the astronomical phenomena that cause them.

All of this was made possible thanks to improvements in instrumentation and growing collaboration between observatories. And with more sophisticated methods designed to sift through archival data for additional signals and background noise, we stand to learn a great deal more about this mysterious cosmic force.

Further Reading: Monash, Physical Review X

Here’s Something Strange, the Afterglow From Last Year’s Kilonova is Continuing to Brighten

In August of 2017, a major breakthrough occurred when scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves that were believed to be caused by the collision of two neutron stars. This source, known as GW170817/GRB, was the first gravitational wave (GW) event that was not caused by the merger of two black holes, and was even believed to have led to the formation of one.

As such, scientists from all over the world have been studying this event ever since to learn what they can from it. For example, according to a new study led by the McGill Space Institute and Department of Physics, GW170817/GRB has shown some rather strange behavior since the two neutron stars colliding last August. Instead of dimming, as was expected, it has been gradually growing brighter.

The study that describes the team’s findings, titled “Brightening X-Ray Emission from GW170817/GRB 170817A: Further Evidence for an Outflow“, recently appeared in The Astrophysical Journal Letters. The study was led by John Ruan of McGill University’s Space Institute and included members from the Canadian Institute for Advanced Research (CIFAR), Northwestern University, and the Leicester Institute for Space and Earth Observation.

Chandra images showing the X-ray afterglow of the GW170817/GRB event. Credit: NASA/CXC/McGill University/J. Ruan et al.

For the sake of their study, the team relied on data obtained by NASA’s Chandra X-ray Observatory, which showed that the remnant has been brightening in the X-ray and radio wavelengths in the months since the collision took place. As Daryl Haggard, an astrophysicist with McGill University whose research group led the new study, said in a recent Chandra press release:

“Usually when we see a short gamma-ray burst, the jet emission generated gets bright for a short time as it smashes into the surrounding medium – then fades as the system stops injecting energy into the outflow. This one is different; it’s definitely not a simple, plain-Jane narrow jet.”

What’s more, these X-ray observations are consistent with radiowave data reported last month by another team of scientists, who also indicated that it was continuing to brighten during the three months since the collision. During this same period, X-ray and optical observatories were unable to monitor GW170817/GRB because it was too close to the Sun at the time.

However, once this period ended, Chandra was able to gather data again, which was consistent with these other observations. As John Ruan explained:

“When the source emerged from that blind spot in the sky in early December, our Chandra team jumped at the chance to see what was going on. Sure enough, the afterglow turned out to be brighter in the X-ray wavelengths, just as it was in the radio.”

Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)

This unexpected behavior has led to a serious buzz in the scientific community, with astronomers trying to come up with explanations as to what type of physics could be driving these emissions. One theory is a complex model for neutron star mergers known as “cocoon theory”. In accordance with this theory, the merger of two neutron stars could trigger the release of a jet that shock-heats the surrounding gaseous debris.

This hot “cocoon” around the jet would glow brightly, which would explain the increase in X-ray and radiowave emissions. In the coming months, additional observations are sure to be made for the sake of confirming or denying this explanation. Regardless of whether or not the “cocoon theory” holds up, any and all future studies are sure to reveal a great deal more about this mysterious remnant and its strange behavior.

As Melania Nynka, another McGill postdoctoral researcher and a co-author on the paper indicated, GW170817/GRB presents some truly unique opportunities for astrophysical research. “This neutron-star merger is unlike anything we’ve seen before,” she said. “For astrophysicists, it’s a gift that seems to keep on giving.”

It is no exaggeration to say that the first-ever detection of gravitational waves, which took place in February of 2016, has led to a new era in astronomy. But the detection of two neutron stars colliding was also a revolutionary accomplishment. For the first time, astronomers were able to observe such an event in both light waves and gravitational waves.

In the end, the combination of improved technology, improved methodology, and closer cooperation between institutions and observatories is allowing scientists to study cosmic phenomena that was once merely theoretical. Looking ahead, the possibilities seem almost limitless!

Further Reading: Chandra X-Ray Observatory, The Astrophysical Journal Letters

Astronomers Set the Limit for Just How Massive Neutron Stars Can Be

In February of 2016, scientists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Since that time, the study of gravitational waves has advanced considerably and opened new possibilities into the study of the Universe and the laws which govern it.

For example, a team from the University of Frankurt am Main recently showed how gravitational waves could be used to determine how massive neutron stars can get before collapsing into black holes. This has remained a mystery since neutron stars were first discovered in the 1960s. And with an upper mass limit now established, scientists will be able to develop a better understanding of how matter behaves under extreme conditions.

The study which describes their findings recently appeared in the scientific journal The Astrophysical Journal Letters under the title “Using Gravitational-wave Observations and Quasi-universal Relations to Constrain the Maximum Mass of Neutron Stars“. The study was led by Luciano Rezzolla, the Chair of Theoretical Astrophysics and the Director of the Institute for Theoretical Physics at the University of Frankfurt, with assistance provided by his students, Elias Most and Lukas Wei.

Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold. Credit: Dana Berry, SkyWorks Digital, Inc.

For the sake of their study, the team considered recent observations made of the gravitational wave event known as  GW170817. This event, which took place on August 17th, 2017, was the sixth gravitational wave to be discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo Observatory. Unlike previous events, this one was unique in that it appeared to be caused by the collision and explosion of two neutron stars.

And whereas other events occurred at distances of about a billion light years, GW170817 took place only 130 million light years from Earth, which allowed for rapid detection and research. In addition, based on modeling that was conducted months after the event (and using data obtained by the Chandra X-ray Observatory) the collision appeared to have left behind a black hole as a remnant.

The team also adopted a “universal relations” approach for their study, which was developed by researchers at Frankfurt University a few years ago. This approach implies that all neutron stars have similar properties which can be expressed in terms of dimensionless quantities. Combined with the GW data, they concluded that the maximum mass of non-rotating neutron stars cannot exceed 2.16 solar masses.

 

Artist’s impression of gravitational-wave emissions from a collapsing star. Credit: aktuelles.uni-frankfurt.de

As Professor Rezzolla explained in a University of Frankfurt press release:

“The beauty of theoretical research is that it can make predictions. Theory, however, desperately needs experiments to narrow down some of its uncertainties. It’s therefore quite remarkable that the observation of a single binary neutron star merger that occurred millions of light years away combined with the universal relations discovered through our theoretical work have allowed us to solve a riddle that has seen so much speculation in the past.”

This study is a good example of how theoretical and experimental research can coincide to produce better models ad predictions. A few days after the publication of their study, research groups from the USA and Japan independently confirmed the findings. Just as significantly, these research teams confirmed the studies findings using different approaches and techniques.

In the future, gravitational-wave astronomy is expected to observe many more events. And with improved methods and more accurate models at their disposal, astronomers are likely to learn even more about the most mysterious and powerful forces at work in our Universe.

Further Reading: Goethe University Frankfurt am Main, The Astrophysical Journal Letters