A long time ago in a galaxy far, far away…
Hundreds of millions of years before the first dinosaurs appeared on Earth, two sisters (cousins?) clashed with great fervour. The grand ordeal ended swiftly, with one sibling vanquishing the other. Their battle may have concluded, but their screams and roars wandered the Universe endlessly. After travelling the cosmos for eons, their cries finally fell on sympathetic ears, and the story of the two sisters was told at long last.
The Big News
A historic discovery was announced on the 29th of June, 2021 by the LIGO Scientific Collaboration (LSC), which confirmed the observation of neutron star-black hole (NSBH) binaries. This rare event was a big deal in the entire astrophysics community. The discovery pointed to an entirely new type of astronomical system formed by some of the most extreme and compact objects in the universe, a neutron star and a black hole (NSBH). These systems have been named GW200105 and GW200115.
Before you feel intimidated by these names, let me decode them for you. Since it’s a gravitational wave signal, it starts with GW. The rest of the numbers refer to the date the gravitational wave reached the detectors. Hence, GW200105 is a Gravitational Wave that was detected in the year 2020, on the 5th of January.
Anything flagged by the detection pipelines is considered an event. However, it is highly confusing to have multiple event entries in the database for the same stretch of data by multiple detectors. These could potentially form a part of a real signal! Hence they’re collectively put under a superevent. Gravitational wave astronomer Christopher Berry explains:
“Our candidates alerts don’t start out with the GW prefix, as we still need to do lots of work to check if they are real. Their names start with S for superevent (not for hope), then the date, and then a letter indicating the order it was uploaded to our database of candidates (we upload candidates with false alarm rates of around one per hour, so there are multiple database entries per day, and most are false alarms). S190425z was the 26th superevent uploaded on 2019 April 25th.”
Once the superevent is confirmed to be a real signal, the name switches to the convention followed for GWs.
(Courtesy: Carl Knox, OzGrav/Swinburne University)
This system was observed at the Advanced LIGO detector in Louisiana in the US. The Advanced Virgo detector in Italy detected the gravitational waves that were produced by the inward spiral of these objects. Another wave was detected later, this time by both the LIGO detectors in Louisiana and Hanford as well as the Virgo detector. Surprisingly this wave was of another NSBH binary barely detected within ten days of the first discovery!
About The Binaries
Initially, scientists were not sure what they were observing. After estimating the masses, the scientists guessed the nature of the stellar objects whose merger was observed. The signature of dense and compact objects are gravitational waves which were exactly what the LIGO and Virgo detectors found. However, no electromagnetic counterparts were detected in both these cases.
A binary is a system of two objects that could be two neutron stars, two black holes, etc. The binaries observed are a part of a neutron star-black hole system. The estimated masses of the two objects in the binary suggested that one is a black hole and other is a neutron star. The heavier objects involved were of \(8.9\ \mathrm{M}_\odot\) and \(5.7\ \mathrm{M}_\odot\) (\(\mathrm{M}_\odot\) denotes solar mass), so the only known objects they could be are black holes. The masses observed and predicted from the models of star formation and evolution also endorse this supposition.
The evidence for neutron stars to be counterparts of black holes in the binary systems is not as strong. The first binary, called GW200105 has a black hole of mass of about \(8.9\ \mathrm{M}_\odot\) and a neutron star of \(1.9\ \mathrm{M}_\odot\). It is found that the black hole spin for GW200105 could lie between 0 and as high as 30% of the limit of rotation rate of black holes. The second one, GW200115, is a system of \(5.7\ \mathrm{M}_\odot\) black hole and \(1.5\ \mathrm{M}_\odot\) neutron star and the black hole spin lies between 0 and 80% of the maximum rate.
The below video is a numerical simulation representing the coalescence of a black hole-neutron star system (NSBH). It represents a system consistent with the observed parameters for GW200115, one of the first two confirmed NSBH systems. Gravitational waves are represented in blue. The density of the neutron star is represented in yellow to orange (low to high densities).
(Courtesy: S.V.Chaurasia, T. Dietrich, N. Fischer, S. Ossokine, H. Pfeiffer)
Let’s Talk About The LSC
The LIGO Scientific Collaboration (LSC) was established by Barry Barish, an American Nobel Laureate and emeritus of Caltech. It is a scientific collaboration of international physics institutes and research groups dedicated to the search of gravitational waves. India has also joined this collaboration in the form of LIGO-India Scientific Collaboration (LISC). We are proud to say that the present LISC institutions include our very own IIT Madras. Horizon’s ex-head Pranav Satheesh is also working under this collaboration!
LSC members can access the US-based Advanced LIGO detectors in Hanford, Washington and in Livingston, Louisiana, as well as the GEO 600 detector in Sarstedt, Germany. LSC members can also access the data from the Virgo detector in Pisa, Italy after an agreement with European Gravitational Observatory (EGO). The LSC and the Virgo Collaboration work and cooperate closely and are referred to collectively as LVC (Now Kagra from Japan and Korea has joined in as well to form the LVK collaboration).
About LIGO & Other Detectors
(Image credit: Pinterest)
LIGO, Laser Interferometer Gravitational-wave Observatory, in a nutshell, is based on laser interferometry, a technique that superimposes waves and analyses the interference pattern to extract information about the nature of the waves and predict the astronomical objects and events that produce them.
LIGO is the world’s largest gravitational wave observatory and an engineering marvel made with precision. It is a combination of two ginormous laser interferometers located 3000 kilometers apart that exploit characteristics of light and space to detect and understand gravitational waves coming from any direction! You don’t have to point the interferometers at a specific region of the sky to observe the waves as you’d do for a telescope.
LIGO (and other detectors like it) is unlike any other observatory on Earth. Ask someone to draw a picture of an observatory and odds are they will draw a gleaming white telescope dome perched on a mountain-top. As a gravitational wave observatory, LIGO bears no resemblance to this whatsoever.
Each detector has two arms extending a whopping 4000 meters and comprising 1.2 m-wide steel vacuum tubes arranged in an "L" shape. (Courtesy: Kim Fetrow/Imageworks)
An exposed segment of LIGO Livingston's vacuum tube. These tubes are protected from the outside environment by 10-foot wide and 12-foot tall shelter made out of concrete. (Courtesy: Caltech/MIT/LIGO Lab)
The detectors used something known as “matched filtering”, which compares the observed noise with the predictions of signals from Einstein’s General Relativity and picks up the required needle from the haystack.
It is highly unlikely that GW200115 is some random noise, with a probability of occurring once in 100,000 years! However the astrophysical nature of GW200105 is a little difficult to establish with the chance of it being merely noise to be less than once in 2.8 years.
Why Is This Event So Special?
The scientific team compared the data obtained with the models to examine the prediction of how NSBHs form. Although there was very little data available, various formation channels have been proposed, with the main channels either isolated binary evolution or some sort of dynamical capture.
- In isolated binary evolution, the original stars are born and evolve together, orbiting around each other till they consume their fuel and become stellar remnants, which eventually merge.
- In the dynamical channel, the stars form within a cluster environment, and while they evolve into stellar remnants one captures the other with its gravitational influence, forming an orbiting pair of compact objects that eventually merge.
It’s likely that NSBHs in general form via various formation channels, rather than any single method. But comparing how the objects are spinning with respect to each other provides more clues. Interestingly, the primary object (black hole) of GW200115 had a spin opposite of the secondary object (neutron star). This hints at dynamical capture since these binary cousins tend to have random spin orientations while isolated sibling binaries are usually well aligned.
Another thing that makes this merger special is that the event was a rather quiet one, involving no tidal disruption. A tidal disruption event, as described by a standard wiki page, is an astronomical phenomenon that occurs when a star approaches sufficiently close to a supermassive black hole and is pulled apart by the black hole’s tidal force, experiencing spaghettification.
Image from a numerical simulation in which the neutron star gets tidally disrupted during the merger process. This is similar to the above embedded video, but for a system with different masses and for which the tidal disruption of the neutron star is visible. This illustrates the tidal disruption process, which is believed to have not occurred in the two observed systems reported here. (Courtesy: S.V.Chaurasia, T. Dietrich, N. Fischer, S. Ossokine, H. Pfeiffer, T. Vu)
Rarity Of The Event
After observing so many mergers and other events involving Binary Neutron Stars (BNS) and Binary BLack Holes (BBH), this is the very first time that we observed an NSBH system. These events occurred 900 million years and almost 1 billion years ago respectively, but it took so long to reach the earth!
This is the so-called stellar graveyard, a graphical catalog of black holes and neutron stars observed so far. The numbers on the vertical axis denote the mass of the stellar objects in factors of solar masses. Connected points indicate mergers. LSA has detected numerous BBHs (blue) and a couple of BNSs (orange). But there are only two systems connecting blue and orange points (as highlighted in the image), our first discovery of NSBH mergers! Read more about it
here. (Courtesy: LIGO-Virgo Collaboration; Frank Elavsky, Aaron Geller)
To know how rare these are, they use two methods. First one by assuming these events to be representative of the whole population, and second by constructing a broader population and considering less significant events that could’ve gone undetected. This produced a range of merger rate estimates, generally supporting the conclusions that NSBHs are rarer than BNSs, and much rarer than BBHs — for every 10 or so BNSs, we can expect 1 NSBH. This result is broadly consistent with the fact that no NSBHs have been detected previously.
What The Future Holds
The data LIGO collects may have far-reaching effects on many areas of physics including gravitation, relativity, astrophysics, cosmology, particle physics, and nuclear physics.
Collaborations like the LSC detect gravitational waves from some of the most violent and energetic processes in the Universe, such as these mergers. Although they aren’t sure about the secondary object for GW200105, these are most likely NSBHs. Since there were no electromagnetic signals as proof, they can’t fully confirm the presence of neutron stars. These first direct measurements of NSBHs and their merger rates will help scientists to reverse engineer and test theoretical predictions of how these objects form. And there’s plenty more to learn from these detections and future detections of NSBHs, especially with more sensitive gravitational wave detectors in the future.
With that, the tale of the two sisters comes to an end — or does it?
References
- LIGO page for information on GW200105 and GW200115
- A new source of gravitational waves: neutron star–black hole binaries
- Gotta catch ’em all: the first neutron star-black hole merger detection!
- Scientists Confirm Black Hole And Neutron Star Collisions in World-First Discovery
Edited by Aniket Kukreti
