Kilonova Neutron Star Collision Probably Left Behind a Black Hole

Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

In February of 2016, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the first-ever detection of gravitational waves. A little over a century after they were first predicted by Einstein’s Theory of General Relativity, we finally had proof that this phenomenon existed. In August of 2017, another major breakthrough occurred when 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 the neutron star merger occurred. While many studies have focused on the by-products of this merger, a new study by researchers from Trinity University, the University of Texas at Austin and Eureka Scientific, has chosen to focus on the remnant, which they claim is likely a black hole.

For the sake of their study, which recently appeared online under the title “GW170817 Most Likely Made a Black Hole“, the team consulted data from the Chandra X-ray Observatory to examine what resulted of the supernova merger. This data was obtained during Director’s Discretionary Time observations that were made on December 3rd and 6th, 2017, some 108 days after the merger.

This data showed a light-curve increase in the X-ray band which was compatible to the radio flux increase that was reported by a previous study conducted by the same team. These combined results suggest that radio and X-ray emissions were being produced at the same source, and that the rising light-curve that followed the merger was likely due to an increase in accelerated charged particles in the external shock – the region where an outflow of gas interacts with the interstellar medium.

As they indicate in their study, this could either be explained as the result of a more massive neutron star being formed from the merger, or a black hole:

“The merger of two neutron stars with mass 1.48 ± 0.12 M and 1.26 ± 0.1 M — where the merged object has a mass of 2.74 +0.04-0.01 M… could result in either a neutron star or a black hole. There might also be a debris disk that gets accreted onto the central object over a period of time, and which could be source of keV X-rays.”

The team also ruled out various possibilities of what could account for this rise in X-ray luminosity. Basically, they concluded that the X-ray photons were not coming from a debris disk, which would have been left over from the merger of the two neutron stars. They also deduced that they would not be produced by a relativistic jet spewing from the remnant, since the flux would be much lower after 102 days.

 

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

All of this indicated that the remnant was more likely to be a black hole than a hyper-massive neutron star. As they explained:

“We show next that if the merged object were a hyper-massive neutron star endowed with a strong magnetic field, then the X-ray luminosity associated with the dipole radiation would be larger than the observed luminosity 10 days after the event, but much smaller than the observed flux at t ~ 100 days. This argues against the formation of a hyper-massive neutron star in this merger.”

Last, but not least, they considered the X-ray and radio emissions that were present roughly 100 days after the merger. These, they claim, are best explained by continued emissions coming from the merger-induced shock (and the not remnant itself) since these emissions would continue to propagate in the interstellar medium around the remnant. Combined with early X-ray data, this all points towards GW170817 now being a black hole.

The first-ever detection of gravitational waves signaled the dawn of a new era in astronomical research. Since that time, observatories like LIGO, Advanced Virgo, and GEO 600 have also benefited from information-sharing and new studies that have indicated that mergers are more common than previously thought, and that gravity waves could be used to probe the interior of supernovae.

With this latest study, scientists have learned that they are not only able to detect the waves caused by black hole mergers, but even the creation thereof. At the same time, it shows how the study of the Universe is growing. Not only is astronomy advancing to the point where we are able to study more and more of the visible Universe, but the invisible Universe as well.

Further Reading: LIGO, arXiv

GW170817 Update: Surprises From First Gravitational Wave Observed Independently

This is the first optical image ever to show an event initially detected as a Gravitational Wave (GW), designated GW170817, shown left. Afterglow, designated as SSS17a, is left over from the explosion of two neutron stars that collided in galaxy NGC 4993 (shown centre). Only 10.9 hours after triggering the largest astronomical search in history, GW170817’s afterglow was discovered by the Swope 1-m telescope at the Las Campanas Observatory, in Chile. Four days later, image on right shows afterglow dimming in brightness and changing from blue to red. CREDIT: Las Campanas Observatory, Carnegie Institution of Washington (Swope + Magellan)

“This is quite literally a physics gold mine!” said Masao Sako, with the University of Pennsylvania.

For over a week now, the astronomy and astrophysics communities have been buzzing with the news of the latest gravitational wave discovery. And this discovery has been big.

Four days before the Great American Solar Eclipse on August 21, a newly discovered gravitational wave caused more astronomers (8,223+), using more telescopes (70), to publish more papers (100 — see the list below) in less time than for any other astronomical event in history. The sixth gravitational wave (GW) to be discovered by the Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo GW observatories, which occurred on August 17, 2017 at 12:41:04 UTC, was surprising in two ways already reported.

GW event six, designated GW170817, did not result from the collision and subsequent explosion of two black holes. All previous GW events, including the first ever discovered in 2015, had involved the collision of black holes with typically 40 times the mass of the Sun between them. Here however, the GW was evidently triggered by the collision and explosion of two neutron stars, having only 3 times the Sun’s mass in total.

Afterglow of GW170817 is shown in close-ups captured by the NASA Hubble Space Telescope, showing it dimming in brightness over days and weeks. CREDIT: NASA and ESA: A. Levan (U. Warwick), N. Tanvir (U. Leicester), and A. Fruchter and O. Fox (STScI)

Crucially, GW170817 occurred ten times closer to Earth than all earlier GW events. Earlier GWs involved black hole collisions at more than 1.3 billion light-years (400 million parsecs or Mpc). GW170817, in comparison, was known within hours of its discovery to lie within only 130 million light-years (40 Mpc). That vastly improved astronomer’s odds of detecting the event independently, because in cosmological terms, it occurred within less than 1% of the universe’s Hubble length of 14 billion light-years (4,300 Mpc).

Not widely reported is that our current astronomical theory regarding GW170817 still depends significantly on observations yet to be made. In brief, astronomers currently believe that GW170817 was triggered by the merger of two neutron stars, which triggered the explosion of a Short Gamma-Ray Burst (SGRB), which emitted only a fraction of the gamma-ray energy in our direction normally associated with SGRBs, because it was the first SGRB observed at such a large angle away from the direction of its focused jets of gamma-rays. The SGRB associated with GW170817 emitted its jet at roughly 30 degrees away from our line-of-sight. All other SGRBs have been observed at only a few degrees from alignment with their jets. The exact angle of the newly discovered SGRB’s jet is important in understanding how its afterglow compares with other SGRB afterglows. Significant properties reported for the GW, including its distance, depend on the angle at which the two neutron stars collided relative to Earth.

The collision angle determined roughly based on the GW itself is probably OK. Only radio maps of the SGRB region at 100 days however, will provide astronomers with the most precise measurements of the resulting explosion’s velocities and directions over time to date. Only then will astronomers learn more about the exact angle of the SGRB’s jet, providing potentially a more accurate estimate of the angle at which the neutron stars collided. More surprises could be in store as a result, including refinements of the properties reported.

ANIMATION (you may have to click image for animation in some browsers): This time-lapse image of the afterglow of GW170817 shows it continuing to increase in radio wavelength brightness over the first month, and was provided by the National Radio Astronomy Observatory Very Large Array radio telescope. CREDIT: NRAO/VLA

Unlike previous events, GW170817 was close enough that within 1.74 seconds of its initial detection by LIGO, it’s gamma radiation was detected by the Fermi Gamma-Ray space telescope. The INTEGRAL Gamma-Ray space observatory detected it too, and it was later designated SGRB 170817A. As an SGRB alone, the event would have triggered alerts to observatories worldwide and aloft, each aiming to detect the explosion’s faint optical afterglow. SGRB optical afterglows have been used to pinpoint the exact positions of SGRBs, not only on the sky, but also in terms of their distance from Earth.

Astronomers in this case had the first GW ever to coincide with, and be independently corroborated by, any observable counterpart, and alerts became a call to astronomical arms. Even though its exact position on the sky was uncertain by many degrees, GW170817 was so close that astronomers were able to quickly narrow down its exact location.

“With a previously-compiled list of nearby galaxies having positions and distances culled from the massive on-line archive of the NASA/IPAC Extragalactic Database (NED), our team rapidly zeroed in on the host galaxy of the event,” said Barry Madore, of Carnegie Observatories.

Precisely because GW170817 occurred at only 130 million light-years, the number of candidate galaxies to observe was only several dozen. In contrast, for previous GW discoveries occurring at billions of light-years, thousands of galaxies would have to be observed. Within 11 hours of the explosion, its afterglow was discovered in the lenticular galaxy NGC 4993, by the Swope 1-m telescope in Chile. They obtained the first-ever visual image of an event associated with a GW.

“Where observation is concerned, chance favors only the prepared mind,” added Madore, quoting Louis Pasteur from 150 years ago. Madore is also a researcher with the Swope team and a co-author on six papers reporting Swope’s discovery of the afterglow and some of its implications. “When alerts were sent out to the LIGO/VIRGO gravity wave detection consortium on the night of August 17, 2017, our team of astronomers was indeed prepared.”

New images of the afterglow of GW170817, aka SGRB 170817A, initially designated as Swope Supernova Survey SSS17a, revealed a bright blue astronomical transient, later designated as AT2017gfo by the International Astronomical Union (IAU).

“There will be more such events, no doubt; but this image taken at the Henrietta Swope 1m telescope at the Las Campanas Observatory in Chile was the first in history, and it truly ushered in the Era of Multi-Messenger Astronomy,” said Madore.

Radio observatories joined the hunt, including the Karl G. Jansky Very Large Array (VLA), the Australia Telescope Compact Array (ATCA) and the Giant Metrewave Radio Telescope (GMRT). So did the Swift ultraviolet and Chandra X-ray space observatory satellites. By day one after the explosion, all frequencies of the electromagnetic spectrum were being observed in the direction of NGC 4993. On multiple wavelengths, multiple “messengers” of GW170817’s existence began to reveal more than the sum of their parts.

Change in brightness of GW170817’s afterglow over time since explosion (merger), is shown in these light-curves. Brightness in 14 different optical wavelengths is shown, including invisible ultraviolet, and visible blue, green, and yellow, and invisible infrared wavelengths in orange and red. Afterglow fades quickly in all wavelengths, except infrared. In infrared, afterglow continues to brighten until ~3 days after explosion, before beginning to fade. CREDIT: Las Campanas Observatory, Carnegie Institution of Washington (Swope + Magellan)

AT2017gfo brightened over the next few days after explosion, in near infrared observations continued by Swope. Their light-curves show the changes in the afterglow’s brightness over time. At three days post explosion, the near-infrared afterglow stops brightening and begins to fade. As with other SGRB afterglows, AT2017gfo faded completely from visual observation over the course of days to weeks, but observations in X-rays and radio continue. Radio observations at 100 days post explosion, which will not occur until November 25, are crucial as said. Although a month away, planned radio observations will determine more than just the long-term evolution of the afterglow over 3 months. Indeed, our astronomical theory accounting for the event’s first three weeks, as already observed, analyzed, and reported, still depends to a surprising degree on an exact number of degrees. The number of degrees relative to Earth for this SGRB based on radio data however, will not be known for at least a month.

“With GW170817 we have for the first time truly independent verification of a gravitational wave source,” said Robert Quimby, of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo, and coauthor of a paper regarding the event’s implications. “The electromagnetic signature of this event can be uniquely matched to the predictions of binary neutron star mergers, and it is actually quite amazing how well the theory matches the data considering how few observational constraints were available to guide the model.”

“With GW170817, we can learn about nuclear physics, relativity, stellar evolution, and cosmology all in one shot,” added Sako, who is also a co-author on ten papers regarding the event. “Plus we now know how all of the heaviest elements in the Universe are created.”

Afterglow faded from optical observations over days to weeks. Here, however, as observed at radio frequencies by the Very Large Array radio telescope, the electromagnetic counterpart to GW170817 is seen brightening over the first month since explosion. CREDIT: Courtesy of Gregg Hallinan, California Institute of Technology, and the National Radio Astronomy Observatory Very Large Array radio telescope

EVENT CHRONOLOGY

T = 0 sec.: GW170817 detected by LIGO/VIRGO [1, 82]
T = 1.74 sec.: SGRB 170817A detected by Fermi Gamma-Ray Burst Monitor satellite immediately after GW170817 [52]
T = 28 min.: Gamma-ray Coordinates Network (GCN) Notice [53]
T = 40 min.: GCN Circular [53]
T = 5.63 hr.: First sky map locating GW170817 to within several degrees [53]
T = 10.9 hr.: Swope 1-m observatory discovers explosion’s afterglow, AT 2017gfo, in galaxy NGC 4993 [18, 24, 64, 75, 77]
T = 11.09 hr.: PROMPT 0.4m observatory detects AT 2017gfo [88]
T = 11.3 hr.: Hubble Space Telescope images AT 2017gfo [20]
T = 12-24 hr.: Magellan; Las Campanas Observatory; W. M. Keck Observatory; Blanco 4-m Cerro Tololo Inter-American Observatory; Gemini South; European Southern Observatory VISTA; Subaru among 6 Japanese telescopes; Pan-STARRS1; Very Large Telescope; 14 Australian telescopes; and Antarctic Survey Telescope optical observatories, and VLA, VLITE, ATCA, GMRT, and ALMA radio observatories, as well as Swift and NuSTAR ultraviolet satellite observatories

PROPERTIES

Position: Right Ascension 13h09m48.085s ± 0.018s; Declination -23d22m53.343s ± 0.218s (J2000 equinox); 10.6s or 7,000 light-years (2.0 kiloparsecs or kpc) from the nucleus of lenticular galaxy NGC 4993 [18]
Distance: 140 ± 40 million light-years (41 ± 13 Mpc), with 30% scatter based on 3 GW-based estimates [1, 25, 82], and 131 ± 9 million light-years (39.3 ± 2.7 Mpc), with 7% scatter based on 3 distance indicators, including GW-based as well as new Fundamental Plane relation-based distances for NGC 4993 [41, 43], and Tully-Fisher relation-based distances for galaxies in the group of galaxies including NGC 4993 from the NASA/IPAC Extragalactic Database (NED)
Mass: Neutron stars total 2.82 +0.47 -0.09 Sun’s mass [82]; mass ejected in elements heavier than iron is 0.03 ± 0.01 Sun’s mass or 10,000 Earth masses, based on 4 estimates [24, 59, 82, 93], including gold amounting to 150 ± 50 Earth masses [60]
Luminosity: Peaks at 0.5 days after explosion, at ~1042 erg/s, equivalent to 260 million Suns [24]
SGRB jet angle: 31 ± 3 degrees away from line-of-sight to Earth, based on 9 estimates [2, 25, 34, 35, 36, 44, 58, 62, 82]
SGRB jet speed: 30% speed of light, based on 4 estimates [20, 42, 59, 75]
Names: GW170817, SGRB 170817A, AT 2017gfo = IAU designation for SGRB afterglow, aka SSS17a, DLT17ck, J-GEM17btc, and MASTER OTJ130948.10-232253.3

IMPLICATIONS

Astronomy (1): Confirms binary neutron star collisions as a source for GW and SGRB events [1, 82]
Astronomy (2): GWs provide a new way of measuring neutron star diameters [8]
Astronomy (3): Gives universal expansion rate, or Hubble constant, as H0 = 71 ± 10 km -1 Mpc-1, with 14% accuracy, based on 6 GW-based estimates for GW170817 ranging from 69 to 74 km -1 Mpc-1, bridging current estimates [1, 22, 36, 60, 74, 82]; accuracy will improve to 4% with future similar events [74]
General Relativity (1): Confirms GW velocity equals speed of light to within 1 part per 1,000,000,000,000,000 or 1/1015 [7, 21, 70, 91]
General Relativity (2): Confirms equivalence of gravitational energy and inertial energy, or Weak Equivalence Principle, to within 1 part per 1,000,000,000 or 1/109 [7, 11, 91, 92]
Physics: Confirms binary neutron star collisions are significant production sites for elements heavier than iron, including gold, platinum, and uranium [17, 69]
Life on Earth: Indicates a higher deadly rate of gamma-rays for extraterrestrial life [15]
GW170817 (1): Predicted one binary neutron star collision per year similar to GW170817 within a distance from Earth of 130 million light-years [40 Mpc] [24]
GW170817 (2): Predicted to produce a 10 Giga-Hertz afterglow that peaks at ~100 days with a radio magnitude of ~10 milli-Janskys [24]
GW170817 (3): Predicted to remain visible in radio for 5-10 years, and for decades with next-generation radio observatories [2]

BIBLIOGRAPHY

96 papers on GW170817 released on arXiv during week of October 16-20

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Weekly Space Hangout – Oct 18, 2017: Weekly News Roundup

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First Cosmic Event Observed in Both Gravitational Waves and Light

Artist's illustration of two merging neutron stars. The narrow beams represent the gamma-ray burst while the rippling spacetime grid indicates the isotropic gravitational waves that characterize the merger. Swirling clouds of material ejected from the merging stars are a possible source of the light that was seen at lower energies. Credit: National Science Foundation/LIGO/Sonoma State University/A. Simonnet

About 130 million years ago, in a galaxy far away, two neutron stars collided. The cataclysmic crash produced gravitational waves, ripples in the fabric of space and time. This event is now the 5th observation of gravitational waves by the Laser Interferometer Gravitational wave Observatory (LIGO) and Virgo collaboration, and the first detected that was not caused by the collision of two black holes.

But this event — called a kilonova — produced something else too: light, across multiple wavelengths.

Continue reading “First Cosmic Event Observed in Both Gravitational Waves and Light”

Scientist Find Treasure Trove of Giant Black Hole Pairs

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.

For decades, astronomers have known that Supermassive Black Holes (SMBHs) reside at the center of most massive galaxies. These black holes, which range from being hundreds of thousands to billions of Solar masses, exert a powerful influence on surrounding matter and are believed to be the cause of Active Galactic Nuclei (AGN). For as long as astronomers have known about them, they have sought to understand how SMBHs form and evolve.

In two recently published studies, two international teams of researchers report on the discovery of five newly-discovered black hole pairs at the centers of distant galaxies. This discovery could help astronomers shed new light on how SMBHs form and grow over time, not to mention how black hole mergers produce the strongest gravitational waves in the Universe.

The first four dual black hole candidates were reported in a study titled “Buried AGNs in Advanced Mergers: Mid-Infrared Color Selection as a Dual AGN Finder“, which was led by Shobita Satyapal, a professor of astrophysics at George Mason University. This study was accepted for publication in The Astrophysical Journal and recently appeared online.

Optical and x-ray data on two of the new black hole pairs discovered. Credit: NASA/CXC/Univ. of Victoria/S.Ellison et al./George Mason Univ./S.Satyapal et al./SDSS

The second study, which reported the fifth dual black hole candidate, was led by Sarah Ellison – an astrophysics professor at the University of Victoria. It was recently published in the Monthly Notices of the Royal Astronomical Society under the title “Discovery of a Dual Active Galactic Nucleus with ~8 kpc Separation. The discovery of these five black hole pairs was very fortuitous, given that pairs are a very rare find.

As Shobita Satyapal explained in a Chandra press statement:

“Astronomers find single supermassive black holes all over the universe. But even though we’ve predicted they grow rapidly when they are interacting, growing dual supermassive black holes have been difficult to find.

The black hole pairs were discovered by combining data from a number of different ground-based and space-based instruments. This included optical data from the Sloan Digital Sky Survey (SDSS) and the ground-based Large Binocular Telescope (LBT) in Arizona with near-infrared data from the Wide-Field Infrared Survey Explorer (WISE) and x-ray data from NASA’s Chandra X-ray Observatory.

For the sake of their studies, Satyapal, Ellison, and their respective teams sought to detect dual AGNs, which are believed to be a consequence of galactic mergers. They began by consulting optical data from the SDSS to identify galaxies that appeared to be in the process of merging. Data from the all-sky WISE survey was then used to identify those galaxies that displayed the most powerful AGNs.

Illustration of a pair of black holes. Credit: NASA/CXC/A.Hobart

They then consulted data from the Chandra’s Advanced CCD Imaging Spectrometer (ACIS) and the LBT to identify seven galaxies that appeared to be in an advanced stage of merger. The study led by Ellison also relied on optical data provided by the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to pinpoint one of the new black hole pairs.

From the combined data, they found that five out of the seven merging galaxies hosted possible dual AGNs, which were separated by less than 10 kiloparsecs (over 30,000 light years). This was evidenced by the infrared data provided by WISE, which was consistent with what is predicated of rapidly growing supermassive black holes.

In addition, the Chandra data showed closely-separated pairs of x-ray sources, which is also consistent with black holes that have matter slowly being accreted onto them. This infrared and x-ray data also suggested that the supermassive black holes are buried in large amounts of dust and gas. As Ellison indicated, these findings were the result of painstaking work that consisted of sorting through multiple wavelengths of data:

“Our work shows that combining the infrared selection with X-ray follow-up is a very effective way to find these black hole pairs. X-rays and infrared radiation are able to penetrate the obscuring clouds of gas and dust surrounding these black hole pairs, and Chandra’s sharp vision is needed to separate them”.

Artist’s impression of binary black hole system in the process of merging. Credit: Bohn et al.

Before this study, less than ten pairs of growing black holes had been confirmed based on X-ray studies, and these were mostly by chance. This latest work, which detected five black hole pairs using combined data, was therefore both fortunate and significant. Aside from bolstering the hypothesis that supermassive black holes form from the merger of smaller black holes, these studies also have serious implications for gravitational wave research.

“It is important to understand how common supermassive black hole pairs are, to help in predicting the signals for gravitational wave observatories,” said Satyapa. “With experiments already in place and future ones coming online, this is an exciting time to be researching merging black holes. We are in the early stages of a new era in exploring the universe.”

Since 2016, a total of four instances of gravitational waves have been detected by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the VIRGO Observatory. However, these detections were the result of black hole mergers where the black holes were all smaller and less massive  – between eight and 36 Solar masses.

Supermassive Black Holes, on the other hand, are much more massive and will likely produce a much larger gravitational wave signature as they continue to draw closer together. And in a few hundred million years, when these pairs eventually do merge, the resulting energy produced by mass being converted into gravitational waves will be incredible.

Artist’s conception of two merging black holes, similar to those detected by LIGO on January 4th, 2017. Credit: LIGO/Caltech

At present, detectors like LIGO and Virgo are not able to detect the gravitational waves created by Supermassive Black Hole pairs. This work is being done by arrays like the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), which relies on high-precision millisecond pulsars to measure the influence of gravitational waves on space-time.

The proposed Laser Interferometer Space Antenna (LISA), which will be the first dedicated space-based gravitational wave detector, is also expected to help in the search. In the meantime, gravitational wave research has already benefited immensely from collaborative efforts like the one that exists between Advanced LIGO and Advanced Virgo.

In the future, scientists also anticipate that they will be able to study the interiors of supernovae through gravitational wave research. This is likely to reveal a great deal about the mechanisms behind black hole formation. Between all of these ongoing efforts and future developments, we can expect to “hear” a great deal more of the Universe and the most powerful forces at work within it.

Be sure to check out this animation that shows what the eventual merger of two of these black hole pairs will look like, courtesy of the Chandra X-ray Observatory:

Further Reading: Chandra HarvardarXiv, MNRAS

LIGO Scientists who Detected Gravitational Waves Awarded Nobel Prize in Physics

Barry C. Barish and Kip S. Thorne, two of the recipients for the 2017 Nobel Prize in physics for their work with gravitational wave research. Credit: Caltech

In February of 2016, scientists working for the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. 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.

Not only was the first-time detection of gravity waves an historic accomplishment, it ushered in a new era of astrophysics. It is little wonder then why the three researchers who were central to the first detection have been awarded the 2017 Nobel Prize in Physics. The prize was awarded jointly to Caltech professors emeritus Kip S. Thorne and Barry C. Barish, along with MIT professor emeritus Rainer Weiss.

To put it simply, gravitational waves are ripples in space-time that are formed by major astronomical events – such as the merger of a binary black hole pair. They were first predicted over a century ago by Einstein’s Theory of General Relativity, which indicated that massive perturbations would alter the structure of space-time. However, it was not until recent years that evidence of these waves was observed for the first time.

The first signal was detected by LIGO’s twin observatories – in Hanford, Washington, and Livingston, Louisiana, respectively – and traced to a black mole merger 1.3 billion light-years away. To date, four detections have been, all of which were due to the mergers of black-hole pairs. These took place on December 26, 2015, January 4, 2017, and August 14, 2017, the last being detected by LIGO and the European Virgo gravitational-wave detector.

For the role they played in this accomplishment, one half of the prize was awarded jointly to Caltech’s Barry C. Barish – the Ronald and Maxine Linde Professor of Physics, Emeritus – and Kip S. Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus. The other half was awarded to Rainer Weiss, Professor of Physics, Emeritus, at the Massachusetts Institute of Technology (MIT).

As Caltech president Thomas F. Rosenbaum – the Sonja and William Davidow Presidential Chair and Professor of Physics – said in a recent Caltech press statement:

“I am delighted and honored to congratulate Kip and Barry, as well as Rai Weiss of MIT, on the award this morning of the 2017 Nobel Prize in Physics. The first direct observation of gravitational waves by LIGO is an extraordinary demonstration of scientific vision and persistence. Through four decades of development of exquisitely sensitive instrumentation—pushing the capacity of our imaginations—we are now able to glimpse cosmic processes that were previously undetectable. It is truly the start of a new era in astrophysics.”

This accomplishment was all the more impressive considering that Albert Einstein, who first predicted their existence, believed gravitational waves would be too weak to study. However, by the 1960s, advances in laser technology and new insights into possible astrophysical sources led scientists to conclude that these waves might actually be detectable.

The first gravity wave detectors were built by Joseph Weber, an astrophysics from the University of Maryland. His detectors, which were built in the 1960s, consisted of large aluminum cylinders  that would be driven to vibrate by passing gravitational waves. Other attempts followed, but all proved unsuccessful; prompting a shift towards a new type of detector involving interferometry.

One such instrument was developed by Weiss at MIT, which relied on the technique known as laser interferometry. In this kind of instrument, gravitational waves are measured using widely spaced and separated mirrors that reflect lasers over long distances. When gravitational waves cause space to stretch and squeeze by infinitesimal amounts, it causes the reflected light inside the detector to shift minutely.

At the same time, Thorne – along with his students and postdocs at Caltech – began working to improve the theory of gravitational waves. This included new estimates on the strength and frequency of waves produced by objects like black holes, neutron stars and supernovae. This culminated in a 1972 paper which Throne co-published with his student, Bill Press, which summarized their vision of how gravitational waves could be studied.

That same year, Weiss also published a detailed analysis of interferometers and their potential for astrophysical research. In this paper, he stated that larger-scale operations – measuring several km or more in size – might have a shot at detecting gravitational waves. He also identified the major challenges to detection (such as vibrations from the Earth) and proposed possible solutions for countering them.

Barry C. Barish and Kip S. Thorne, two of three recipients of the 2017 Nobel Prize in Physics. Credit: Caltech

In 1975, Weiss invited Thorne to speak at a NASA committee meeting in Washington, D.C., and the two spent an entire night talking about gravitational experiments. As a result of their conversation, Thorne went back to Calteh and proposed creating a experimental gravity group, which would work on interferometers in parallel with researchers at MIT, the University of Glasgow and the University of Garching (where similar experiments were being conducted).

Development on the first interferometer began shortly thereafter at Caltech, which led to the creation of a 40-meter (130-foot) prototype to test Weiss’ theories about gravitational waves. In 1984, all of the work being conducted by these respective institutions came together. Caltech and MIT, with the support of the National Science Foundation (NSF) formed the LIGO collaboration and began work on its two interferometers in Hanford and Livingston.

The construction of LIGO was a major challenge, both logistically and technically. However, things were helped immensely when Barry Barish (then a Caltech particle physicist) became the Principal Investigator (PI) of LIGO in 1994. After a decade of stalled attempts, he was also made the director of LIGO and put its construction back on track. He also expanded the research team and developed a detailed work plan for the NSF.

As Barish indicated, the work he did with LIGO was something of a dream come true:

“I always wanted to be an experimental physicist and was attracted to the idea of using continuing advances in technology to carry out fundamental science experiments that could not be done otherwise. LIGO is a prime example of what couldn’t be done before. Although it was a very large-scale project, the challenges were very different from the way we build a bridge or carry out other large engineering projects. For LIGO, the challenge was and is how to develop and design advanced instrumentation on a large scale, even as the project evolves.”

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

By 1999, construction had wrapped up on the LIGO observatories and by 2002, LIGO began to obtain data. In 2008, work began on improving its original detectors, known as the Advanced LIGO Project. The process of converting the 40-m prototype to LIGO’s current 4-km (2.5 mi) interferometers was a massive undertaking, and therefore needed to be broken down into steps.

The first step took place between 2002 and 2010, when the team built and tested the initial interferometers. While this did not result in any detections, it did demonstrate the observatory’s basic concepts and solved many of the technical obstacles. The next phase – called Advanced LIGO, which took placed between 2010 and 2015 – allowed the detectors to achieve new levels of sensitivity.

These upgrades, which also happened under Barish’s leadership, allowed for the development of several key technologies which ultimately made the first detection possible. As Barish explained:

“In the initial phase of LIGO, in order to isolate the detectors from the earth’s motion, we used a suspension system that consisted of test-mass mirrors hung by piano wire and used a multiple-stage set of passive shock absorbers, similar to those in your car. We knew this probably would not be good enough to detect gravitational waves, so we, in the LIGO Laboratory, developed an ambitious program for Advanced LIGO that incorporated a new suspension system to stabilize the mirrors and an active seismic isolation system to sense and correct for ground motions.”

Rainer Weiss, famed MIT physicist and partial winner of the 2017 Nobel Prize in Physics. Credit: MIT/Bryce Vickmark

Given how central Thorne, Weiss and Barish were to the study of gravitational waves, all three were rightly-recognized as this year’s recipients of the Nobel Prize in Physics. Both Thorne and Barish were notified that they had won in the early morning hours on October 3rd, 2017. In response to the news, both scientists were sure to acknowledge the ongoing efforts of LIGO, the science teams that have contributed to it, and the efforts of Caltech and MIT in creating and maintaining the observatories.

“The prize rightfully belongs to the hundreds of LIGO scientists and engineers who built and perfected our complex gravitational-wave interferometers, and the hundreds of LIGO and Virgo scientists who found the gravitational-wave signals in LIGO’s noisy data and extracted the waves’ information,” said Thorne. “It is unfortunate that, due to the statutes of the Nobel Foundation, the prize has to go to no more than three people, when our marvelous discovery is the work of more than a thousand.”

“I am humbled and honored to receive this award,” said Barish. “The detection of gravitational waves is truly a triumph of modern large-scale experimental physics. Over several decades, our teams at Caltech and MIT developed LIGO into the incredibly sensitive device that made the discovery. When the signal reached LIGO from a collision of two stellar black holes that occurred 1.3 billion years ago, the 1,000-scientist-strong LIGO Scientific Collaboration was able to both identify the candidate event within minutes and perform the detailed analysis that convincingly demonstrated that gravitational waves exist.”

Looking ahead, it is also pretty clear that Advanved LIGO, Advanced Virgo and other gravitational wave observatories around the world are just getting started. In addition to having detected four separate events, recent studies have indicated that gravitational wave detection could also open up new frontiers for astronomical and cosmological research.

For instance, a recent study by a team of researchers from the Monash Center for Astrophysics proposed a theoretical concept known as ‘orphan memory’. According to their research, gravitational waves not only cause waves in space-time, but leave permanent ripples in its structure. By studying the “orphans” of past events, gravitational waves can be studied both as they reach Earth and long after they pass.

In addition, a study was released in August by a team of astronomers from the Center of Cosmology at the University of California Irvine that indicated that black hole mergers are far more common than we thought. After conducting a survey of the cosmos intended to calculate and categorize black holes, the UCI team determined that there could be as many as 100 million black holes in the galaxy.

Another recent study indicated that the Advanced LIGO, GEO 600, and Virgo gravitational-wave detector network could also be used to detect the gravitational waves created by supernovae. By detecting the waves created by star that explode near the end of their lifespans, astronomers could be able to see inside the hearts of collapsing stars for the first time and probe the mechanics of black hole formation.

The Nobel Prize in Physics is one of the highest honors that can be bestowed upon a scientist. But even greater than that is the knowledge that great things resulted from one’s own work. Decades after Thorne, Weiss and Barish began proposing gravitational wave studies and working towards the creation of detectors, scientists from all over the world are making profound discoveries that are revolutionizing the way we think of the Universe.

And as these scientists will surely attest, what we’ve seen so far is just the tip of the iceberg. One can imagine that somewhere, Einstein is also beaming with pride. As with other research pertaining to his theory of General Relativity, the study of gravitational waves is demonstrating that even after a century, his predictions were still bang on!

And be sure to check out this video of the Caltech Press Conference where Barish and Thorn were honored for their accomplishments:

Further Reading: NASA, Caltech

LIGO and Virgo Observatories Detect Black Holes Colliding

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.

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century ago, has opened up new avenues of research for cosmologists and astrophysicists. Since that time, more detections have been made, all of which were said to be the result of black holes merging.

The latest detection took place on August 14th, 2017, when three observatories – the Advanced LIGO and the Advanced Virgo detectors – simultaneously detected the gravitational waves created by merging black holes. This was the first time that gravitational waves were detected by three different facilities from around the world, thus ushering in a new era of globally-networked research into this cosmic phenomena.

The study which detailed these observations was recently published online by the LIGO Scientific Collaboration and the Virgo Collaboration. Titled “GW170814 : A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence“, this study has also been accepted for publication in the scientific journal Physical Review Letters.

Aerial view of the Virgo Observatory. Credit: The Virgo collaboration/CCO 1.0

The event, designated as GW170814, was observed at 10:30:43 UTC (06:30:43 EDT; 03:30:43 PDT) on August 14th, 2017. The event was detected by the National Science Foundation‘s two LIGO detectors (located in Livingston, Louisiana, and Hanford, Washington) and the Virgo detector located near Pisa, Italy – which is maintained by the National Center for Scientific Research (CNRS) and the National Institute for Nuclear Physics (INFN).

Though not the first instance of gravitational waves being detected, this was the first time that an event was detected by three observatories simultaneously. As France Córdova, the director of the NSF, said in a recent LIGO press release:

“Little more than a year and a half ago, NSF announced that its Laser Interferometer Gravitational Wave Observatory had made the first-ever detection of gravitational waves, which resulted from the collision of two black holes in a galaxy a billion light-years away. Today, we are delighted to announce the first discovery made in partnership between the Virgo gravitational-wave observatory and the LIGO Scientific Collaboration, the first time a gravitational wave detection was observed by these observatories, located thousands of miles apart. This is an exciting milestone in the growing international scientific effort to unlock the extraordinary mysteries of our universe.”

Based on the waves detected, the LIGO Scientific Collaboration (LSC) and Virgo collaboration were able to determine the type of event, as well as the mass of the objects involved. According to their study, the event was triggered by the merger of two black holes – which were 31 and 25 Solar Masses, respectively. The event took place about 1.8 billion light years from Earth, and resulted in the formation of a spinning black hole with about 53 Solar Masses.

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

What this means is that about three Solar Masses were converted into gravitational-wave energy during the merger, which was then detected by LIGO and Virgo. While impressive on its own, this latest detection is merely a taste of what gravitational wave detectors like the LIGO and Virgo collaborations can do now that they have entered their advanced stages, and into cooperation with each other.

Both Advanced LIGO and Advanced Virgo are second-generation gravitational-wave detectors that have taken over from previous ones. The LIGO facilities, which were conceived, built, and are operated by Caltech and MIT, collected data unsuccessfully between 2002 and 2010. However, as of September of 2015, Advanced LIGO went online and began conducting two observing runs – O1 and O2.

Meanwhile, the original Virgo detector conducted observations between 2003 and October of 2011, once again without success. By February of 2017, the integration of the Advanced Virgo detector began, and the instruments went online by the following April. In 2007, Virgo and LIGO also partnered to share and jointly analyze the data recorded by their respective detectors.

In August of 2017, the Virgo detector joined the O2 run, and the first-ever simultaneous detection took place on August 14th, with data being gathered by all three LIGO and Virgo instruments. As LSC spokesperson David Shoemaker – a researcher with the Massachusetts Institute of Technology (MIT) – indicated, this detection is just the first of many anticipated events.

Artist’s impression of two merging black holes, which has been theorized to be a source of gravitational waves. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS

“This is just the beginning of observations with the network enabled by Virgo and LIGO working together,” he said. “With the next observing run planned for fall 2018, we can expect such detections weekly or even more often.”

Not only will this mean that scientists have a better shot of detecting future events, but they will also be able to pinpoint them with far greater accuracy. In fact, the transition from a two- to a three-detector network is expected to increase the likelihood of pinpointing the source of GW170814 by a factory of 20. The sky region for GW170814 is just 60 square degrees – more than 10 times smaller than with data from LIGO’s interferometers alone.

In addition, the accuracy with which the distance to the source is measured has also benefited from this partnership. As Laura Cadonati, a Georgia Tech professor and the deputy spokesperson of the LSC, explained:

“This increased precision will allow the entire astrophysical community to eventually make even more exciting discoveries, including multi-messenger observations. A smaller search area enables follow-up observations with telescopes and satellites for cosmic events that produce gravitational waves and emissions of light, such as the collision of neutron stars.”

Artist’s impression of gravitational waves. Credit: NASA

In the end, bringing more detectors into the gravitational-wave network will also allow for more detailed test’s of Einstein’s theory of General Relativity. Caltech’s David H. Reitze, the executive director of the LIGO Laboratory, also praised the new partnership and what it will allow for.

“With this first joint detection by the Advanced LIGO and Virgo detectors, we have taken one step further into the gravitational-wave cosmos,” he said. “Virgo brings a powerful new capability to detect and better locate gravitational-wave sources, one that will undoubtedly lead to exciting and unanticipated results in the future.”

The study of gravitational waves is a testament to the growing capability of the world’s science teams and the science of interferometry. For decades, the existence of gravitational waves was merely a theory; and by the turn of the century, all attempts to detect them had yielded nothing. But in just the past eighteen months, multiple detections have been made, and dozens more are expected in the coming years.

What’s more, thanks to the new global network and the improved instruments and methods, these events are sure to tell us volumes about our Universe and the physics that govern it.

Further Reading: NSF, LIGO-Caltech, LIGO DD

Gravitational Waves will let us see Inside Stars as Supernovae Happen

Artistic representation of the material around the supernova 1987A. Credit: ESO/L. Calçada

On February 11th, 2016, scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) announced the first detection of gravitational waves. This development, which confirmed a prediction made by Einstein’s Theory of General Relativity a century ago, has opened up new avenues of research for cosmologists and astrophysicists. Since that time, more detections have been made, all of which were said to be the result of black holes merging.

However, according to a team of astronomers from Glasgow and Arizona, astronomers need not limit themselves to detecting waves caused by massive gravitational mergers. According to a study they recently produced, the Advanced LIGO, GEO 600, and Virgo gravitational-wave detector network could also detect the gravitational waves created by supernova. In so doing, astronomers will able to see inside the hearts of collapsing stars for the first time.

The study, titled “Inferring the Core-Collapse Supernova Explosion Mechanism with Three-Dimensional Gravitational-Wave Simulations“, recently appeared online. Led by Jade Powell, who recently finished her PhD at the Institute for Gravitational Research at the University of Glasgow, the team argue that current gravitational wave experiments should be able to detect the waves created by Core Collapse Supernovae (CSNe).

Otherwise known as Type II supernovae, CCSNe are what happens when a massive star reaches the end of its lifespan and experiences rapid collapse. This triggers a massive explosion that blows off the outer layers of the star, leaving behind a remnant neutron star that may eventually become a black hole. In order for a star to undergo such collapse, it must be at least 8 times (but no more than 40 to 50 times) the mass of the Sun.

When these types of supernovae take place, it is believed that neutrinos produced in the core transfer gravitational energy released by core collapse to the cooler outer regions of the star. Dr. Powell and her colleagues believe that this gravitational energy could be detected using current and future instruments. As they explain in their study:

“Although no CCSNe have currently been detected by gravitational-wave detectors, previous studies indicate that an advanced detector network may be sensitive to these sources out to the Large Magellanic Cloud (LMC). A CCSN would be an ideal multi-messenger source for aLIGO and AdV, as neutrino and electromagnetic counterparts to the signal would be expected. The gravitational waves are emitted from deep inside the core of CCSNe, which may allow astrophysical parameters, such as the equation of state (EOS), to be measured from the reconstruction of the gravitational-wave signal.”

Dr. Powell and her also outline a procedure in their study that could be implemented using the Supernova model Evidence Extractor (SMEE). The team then conducted simulations using the latest three-dimensional models of gravitational-wave core collapse supernovae to determine if background noise could be eliminated and proper detection of CCSNe signals made.

As Dr. Powell explained to Universe Today via email:

“The Supernova Model Evidence Extractor (SMEE) is an algorithm that we use to determine how supernovae get the huge amount of energy they need to explode. It uses Bayesian statistics to distinguish between different possible explosion models. The first model we consider in the paper is that the explosion energy comes from the neutrinos emitted by the star. In the second model the explosion energy comes from rapid rotation and extremely strong magnetic fields.”

From this, the team concluded that in a three-detector network researchers could correctly determine the explosion mechanics for rapidly-rotating supernovae, depending on their distance. At a distance of 10 kiloparsecs (32,615 light-years) they would be able to detect signals of CCSNe with 100% accuracy, and signals at 2 kiloparsecs (6,523 light-years) with 95% accuracy.

In other words, if and when a supernova takes place in the local galaxy, the global network formed by the Advanced LIGO, Virgo and GEO 600 gravitational wave detectors would have an excellent chance of picking up on it. The detection of these signals would also allow for some groundbreaking science, enabling scientists to “see” inside of exploding stars for the first time. As Dr. Powell explained:

“The gravitational waves are emitted from deep inside the core of the star where no electromagnetic radiation can escape. This allows a gravitational wave detection to tell us information about the explosion mechanism that can not be determined with other methods. We may also be able to determine other parameters such as how rapidly the star is rotating.”

Illustration showing the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other. Credit: LIGO/T. Pyle

Dr. Powell, having recently completed work on her PhD will also be taking up a postdoc position with the RC Centre of Excellence for Gravitational Wave Discovery (OzGrav), the gravitational wave program hosted by the University of Swinburne in Australia. In the meantime, she and her colleagues will be conducting targeted searchers for supernovae that occurred during the first and seconds advanced detector observing runs.

While there are no guarantees at this point that they will find the sought-after signals that would demonstrate that supernovae are detectable, the team has high hopes. And given the possibilities that this research holds for astrophysics and astronomy, they are hardly alone!

Further Reading: arXiv

Cosmic Census Says There Could be 100 Million Black Holes in our Galaxy Alone

Artist's conception shows two merging black holes similar to those detected by LIGO on January 4th, 2017. Credit: LIGO/Caltech

In January of 2016, researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made history when they announced the first-ever detection of gravitational waves. Supported by the National Science Foundation (NSF) and operated by Caltech and MIT, LIGO is dedicated to studying the waves predicted by Einstein’s Theory of General Relativity and caused by black hole mergers.

According to a new study by a team of astronomers from the Center of Cosmology at the University of California Irvine, such mergers are far more common than we thought. After conducting a survey of the cosmos intended to calculate and categorize black holes, the UCI team determined that there could be as many as 100 million black holes in the galaxy, a finding which has significant implications for the study of gravitational waves.

The study which details their findings, titled “Counting Black Holes: The Cosmic Stellar Remnant Population and Implications for LIGO“, recently appeared in the Monthly Notices of the Royal Astronomical Society. Led by Oliver D. Elbert, a postdoc student with the department of Physics and Astronomy at UC Irvine, the team conducted an analysis of gravitational wave signals that have been detected by LIGO.

LIGO’s two facilities, located in Livingston, Louisiana, and Hanford, Washington. Credit: ligo.caltech.edu

Their study began roughly a year and a half ago, shortly after LIGO announced the first detection of gravitational waves. These waves were created by the merger of two distant black holes, each of which was equivalent in mass to about 30 Suns. As James Bullock, a professor of physics and astronomy at UC Irvine and a co-author on the paper, explained in a UCI press release:

“Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein’s general theory of relativity. But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, ‘How common are black holes of this size, and how often do they merge?’”

Traditionally, astronomers have been of the opinion that black holes would typically be about the same mass as our Sun. As such, they sought to interpret the multiple gravitational wave detections made by LIGO in terms of what is known about galaxy formation. Beyond this, they also sought to create a framework for predicting future black hole mergers.

From this, they concluded that the Milky Way Galaxy would be home to up to 100 million black holes, 10 millions of which would have an estimated mass of about 30 Solar masses – i.e. similar to those that merged and created the first gravitational waves detected by LIGO in 2016. Meanwhile, dwarf galaxies – like the Draco Dwarf, which orbits at a distance of about 250,000 ly from the center of our galaxy – would host about 100 black holes.

They further determined that today, most low-mass black holes (~10 Solar masses) reside within galaxies of 1 trillion Solar masses (massive galaxies) while massive black holes (~50 Solar masses) reside within galaxies that have about 10 billion Solar masses (i.e. dwarf galaxies). After considering the relationship between galaxy mass and stellar metallicity, they interpreted a galaxy’s black hole count as a function of its stellar mass.

In addition, they also sought to determine how often black holes occur in pairs, how often they merge and how long this would take. Their analysis indicated that only a tiny fraction of black holes would need to be involved in mergers to accommodate what LIGO observed. It also offered predictions that showed how even larger black holes could be merging within the next decade.

As Manoj Kaplinghat, also a UCI professor of physics and astronomy and the second co-author on the study, explained:

“We show that only 0.1 to 1 percent of the black holes formed have to merge to explain what LIGO saw. Of course, the black holes have to get close enough to merge in a reasonable time, which is an open problem… If the current ideas about stellar evolution are right, then our calculations indicate that mergers of even 50-solar-mass black holes will be detected in a few years.”

In other words, our galaxy could be teeming with black holes, and mergers could be happening in a regular basis (relative to cosmological timescales). As such, we can expect that many more gravity wave detections will be possible in the coming years. This should come as no surprise, seeing as how LIGO has made two additional detections since the winter of 2016.

With many more expected to come, astronomers will have many opportunities to study black holes mergers, not to mention the physics that drive them!

Further Reading: UCI, MNRAS

LISA is On! Gravitational Wave Detection is Going to Space

Artist's impression of two merging black holes. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS
Artist's impression of two merging black holes. Credit: Bohn, Throwe, Hébert, Henriksson, Bunandar, Taylor, Scheel/SXS

The discovery of gravitational waves by the LIGO experiment in 2015 sent ripples through the scientific community. Originally predicted by Einstein’s Theory of General Relativity, the confirmation of these waves (and two subsequent detections) solved a long-standing cosmological mystery. In addition to bending the fabric of space-time, it is now known that gravity can also create perturbations that can be detected billions of light-years away.

Seeking to capitalize on these discoveries and conduct new and exciting research into gravitational waves, the European Space Agency (ESA) recently green-lighted the Laser Interferometer Space Antenna (LISA) mission. Consisting of three satellites that will measure gravitational waves directly through laser interferometry, this mission will be the first space-based gravitational wave detector.

This decision was announced yesterday (Tuesday, June 20th) during a meeting of ESA’s Science Program Committee (SPC). It’s implementation is part of the ESA’s Cosmic Vision plan – the current cycle of the agency’s long-term planning for space science missions – which began in 2015 and will be running until 2025. It is also in keeping with the ESA’s desire to study the “invisible universe“, a policy that was adopted in 2013. 

To accomplish this, the three satellites that make up the LISA constellation will be deployed into orbit around Earth. Once there, they will assume a triangular formation – spaced 2.5 million km (1.55 million mi) apart – and follow Earth’s orbit around the Sun. Here, isolated from all external influences but Earth’s gravity, they will then connect to each other by laser and begin looking for minute perturbations in the fabric of space-time.

Much like how the LIGO experiment and other gravitational wave detectors work, the LISA mission will rely on laser interferometry. This process consists of a beam of electromagnetic energy (in this case, a laser) being split in two and then recombined to look for patterns of interference. In LISA’s case, two satellites play the role of reflectors while the remaining one is the both source of the lasers and the observer of the laser beam.

When a gravitational wave passes through the triangle established by the three satellites, the lengths of the two laser beams will vary due to the space-time distortions caused by the wave. By comparing the laser beam frequency in the return beam to the frequency of the sent beam, LISA will be able to measure the level of distortion.

These measurements will have to be extremely precise, since the distortions they are looking for affect the fabric of space-time on the most minuscule of levels – a few millionths of a millionth of a meter over a distance of a million kilometers. Luckily, the technology to detect these waves has already been tested by the LISA Pathfinder mission, which deployed in 2015 and will conclude its mission at the end of the month.

Artist’s concept of the LISA mission. Credit: AEI/Milde Marketing/Exozet

In the coming weeks and months, the ESA will be looking over the design of the LISA mission and completing a cost assessment. If all goes as planned, the mission will be proposed for “adoption” before construction begins and it is expected to be launched by 2034. In the same meeting, the ESA also adopted another important mission that will be searching for exoplanets in the coming years.

This mission is known as the PLAnetary Transits and Oscillations of stars, or PLATO, mission. Like Kepler, this mission will monitor stars within a large sections of the sky to look for small dips in their brightness, which are caused by planets passing between the star and the observer (i.e. the transit method). Originally selected in February of 2014, this mission is now moving from the blueprint phase into construction and will launch in 2026.

It’s an exciting time for the European Space Agency. In recent years, it has committed itself to multiple endeavors in the hope of maintaining Europe’s commitment to and continued presence in space. These include studying the “invisible universe”, mounting missions to the Moon and Mars, maintaining a commitment to the International Space Station, and even building a successor to the ISS on the Moon!

Further Reading: ESA