LIGO-Virgo: Mystery Object in "Mass Gap"

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bystander
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LIGO-Virgo: Mystery Object in "Mass Gap"

Post by bystander » Tue Jun 23, 2020 5:56 pm

LIGO-Virgo Finds Mystery Object in "Mass Gap"
International LIGO-Virgo Collaboration | LIGO Observatory | 2020 Jun 23

In August of 2019, the LIGO-Virgo gravitational-wave network witnessed the merger of a black hole with 23 times the mass of our sun and a mystery object 2.6 times the mass of the sun. Scientists do not know if the mystery object was a neutron star or black hole, but either way it set a record as being either the heaviest known neutron star or the lightest known black hole.

Click to play embedded YouTube video.
Visualization of the Binary Black Hole Merger GW190814
Credit: N. Fischer, S. Ossokine, H. Pfeiffer, A. Buonanno (MPG), SXS Collaboration

When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive die, they explode in a supernova and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: does anything lie in this so-called mass gap?

Now, in a new study from the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector in Europe, scientists have announced the discovery of an object of 2.6 solar masses, placing it firmly in the mass gap. The object was found on August 14, 2019, as it merged with a black hole of 23 solar masses, generating a splash of gravitational waves detected back on Earth by LIGO and Virgo. ...

The cosmic merger described in the study, an event dubbed GW190814, resulted in a final black hole about 25 times the mass of the sun (some of the merged mass was converted to a blast of energy in the form of gravitational waves). The newly formed black hole lies about 800 million light-years away from Earth. ...

LIGO-Virgo Finds Mystery Astronomical Object in "Mass Gap"
Northwestern University | 2020 Jun 23

A Black Hole with a Puzzling Companion
Albert Einstein Institute | Max Planck Institute for Gravitational Physics (MPG) | 2020 Jun 23

Find May Reshape Understanding of Biggest Stars in the Universe
Science & Technology Facilities Council, UK | 2020 Jun 23

Scientists Grapple with the Cosmic Mystery of GW190814
University of Birmingham, UK | 2020 Jun 23

Astronomers Find Mystery Object in "Mass Gap"
University of Portsmouth, UK | 2020 Jun 23

GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass
Black Hole with a 2.6 Solar Mass Compact Object
~ R. Abbott et al
A Deep CFHT Optical Search for a Counterpart to the Possible
Neutron Star-Black Hole Merger GW190814
~ Nicholas Vieira et al
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MarkBour
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Re: LIGO-Virgo: Mystery Object in "Mass Gap"

Post by MarkBour » Thu Jun 25, 2020 6:44 pm

Fantastic news item. Thanks, bystander!
  1. Is there any reason the "mystery object" could not have been a binary?
  2. Although it would be a lot harder, has anyone tried a search for a loss of signal, as opposed to a search for a corresponding direct signal of the merger? I wonder if enough data is available to perform a search for a loss of signal (e.g. a neutron star that's gone now.)
Mark Goldfain

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Re: LIGO-Virgo: Mystery Object in "Mass Gap"

Post by BDanielMayfield » Fri Jun 26, 2020 4:17 am

MarkBour wrote:
Thu Jun 25, 2020 6:44 pm
Fantastic news item. Thanks, bystander!
  1. Is there any reason the "mystery object" could not have been a binary?
  2. Although it would be a lot harder, has anyone tried a search for a loss of signal, as opposed to a search for a corresponding direct signal of the merger? I wonder if enough data is available to perform a search for a loss of signal (e.g. a neutron star that's gone now.)
1. If the swallowed object was a binary then no doubt the GW signal would have been different. And it's very hard to imagine two stellar mass objects being able to maintain an orbit of each other while being drawn down the gravity well of a 23 Sol massed BH.

2. This happened in a galaxy far, far away. Well maybe not that far, just 800 million light-years :ssmile: But at such distances NS's probably would be way too dim to detect.

Perhaps there is something exotic as yet to be discovered in the mass gap between the most massive NSs and the least massive BHs. Quark stars :!: :?:

Bruce
"Happy are the peaceable ... "

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Re: LIGO-Virgo: Mystery Object in "Mass Gap"

Post by MarkBour » Fri Jun 26, 2020 7:00 pm

BDanielMayfield wrote:
Fri Jun 26, 2020 4:17 am
MarkBour wrote:
Thu Jun 25, 2020 6:44 pm
Fantastic news item. Thanks, bystander!
  1. Is there any reason the "mystery object" could not have been a binary?
  2. Although it would be a lot harder, has anyone tried a search for a loss of signal, as opposed to a search for a corresponding direct signal of the merger? I wonder if enough data is available to perform a search for a loss of signal (e.g. a neutron star that's gone now.)
1. If the swallowed object was a binary then no doubt the GW signal would have been different. And it's very hard to imagine two stellar mass objects being able to maintain an orbit of each other while being drawn down the gravity well of a 23 Sol massed BH.

2. This happened in a galaxy far, far away. Well maybe not that far, just 800 million light-years :ssmile: But at such distances NS's probably would be way too dim to detect.

Perhaps there is something exotic as yet to be discovered in the mass gap between the most massive NSs and the least massive BHs. Quark stars :!: :?:

Bruce
Bruce, thanks for the reply. I didn't realize your point #2; thanks for that ... I did not realize that individual neutron stars in other galaxies are basically unobservable with current technology and equipment. In fact, I have now learned that there are expected to be vastly more of them in the Milky Way than we've ever been able to detect.

As to your point #1, I'm really unsure about it. I agree with the second sentence. It's the first I'm still wondering about. It may be that in a 3-body encounter, one of them will generally get ejected. Could it ever result in a "hasty" merger, or a smearing of the two smaller bodies, as both are captured? I guess a case in point is in another Breaking Science News post by @bystander just today. The posting "Caltech: Black Hole Collision May Have Exploded with Light", regarding the observation S190521g, would be a conjectured merger of two objects, which we would expect one future day to merge with the larger BH.

I guess my whole motivation for introducing the possibility is that there must be some eventual resolution of the "mass-gap" that is being mentioned. It was stated in a way to strongly hint that there is some theoretical reason to not expect to find a BH that is just slightly more massive than a neutron star. I really would wonder why such a mass gap would exist. If a neutron star that exceeds 2.5 solar mass will collapse into a black hole, then why would it be unexpected to find that a black hole exists with just slightly more mass than the limit? Interestingly, as I read about the GW170817 observations, it seems that the estimates are that this event created a 2.74 solar mass BH. If the mass gap is not a theoretical issue at all, but merely a statement that we haven't yet run across and BH's that are in the 2.5-4 solar mass range, then I guess that is just a temporary observational issue which is rapidly vanishing as our observations grow. Headlines can be far more trouble than the story, no?
Mark Goldfain

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Don't be a strange star!

Post by neufer » Sat Jun 27, 2020 2:58 pm

bystander wrote:
Tue Jun 23, 2020 5:56 pm
LIGO-Virgo Finds Mystery Object in "Mass Gap"
International LIGO-Virgo Collaboration | LIGO Observatory | 2020 Jun 23
In August of 2019, the LIGO-Virgo gravitational-wave network witnessed the merger of a black hole with 23 times the mass of our sun and a mystery object 2.6 times the mass of the sun. Scientists do not know if the mystery object was a neutron star or black hole, but either way it set a record as being either the heaviest known neutron star or the lightest known black hole.
https://en.wikipedia.org/wiki/Neutron_star wrote:
<<A neutron star has a mass of at least 1.1 solar masses (M). The upper limit of mass for a neutron star is called the Tolman–Oppenheimer–Volkoff limit and is generally held to be around 2.1 M, but a recent estimate puts the upper limit at 2.16 M. The maximum observed mass of neutron stars is about 2.14 M for PSR J0740+6620 discovered in September, 2019. Compact stars below the Chandrasekhar limit of 1.39 M are generally white dwarfs whereas compact stars with a mass between 1.4 M and 2.16 M are expected to be neutron stars, but there is an interval of a few tenths of a solar mass where the masses of low-mass neutron stars and high-mass white dwarfs can overlap. It is thought that beyond 2.16 M the stellar remnant will overcome the strong force repulsion and neutron degeneracy pressure so that gravitational collapse will occur to produce a black hole, but the smallest observed mass of a stellar black hole is about 5 M. Between 2.16 M and 5 M, hypothetical intermediate-mass stars such as quark stars and electroweak stars have been proposed, but none have been shown to exist.>>
https://en.wikipedia.org/wiki/Quark_star wrote:
<<A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks. If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than by the strong force that binds ordinary hadrons. The analysis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. Their existence has not been confirmed.

Some massive stars collapse to form neutron stars at the end of their life cycle, as has been both observed and explained theoretically. Under the extreme temperatures and pressures inside neutron stars, the neutrons are normally kept apart by a degeneracy pressure, stabilizing the star and hindering further gravitational collapse. However, it is hypothesized that under even more extreme temperature and pressure, the degeneracy pressure of the neutrons is overcome, and the neutrons are forced to merge and dissolve into their constituent quarks, creating an ultra-dense phase of quark matter based on densely packed quarks. In this state, a new equilibrium is supposed to emerge, as a new degeneracy pressure between the quarks, as well as repulsive electromagnetic forces, will occur and hinder gravitational collapse.

If these ideas are correct, quark stars might occur, and be observable, somewhere in the universe. Theoretically, such a scenario is seen as scientifically plausible, but it has been impossible to prove both observationally and experimentally, because the very extreme conditions needed for stabilizing quark matter cannot be created in any laboratory nor observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among the unsolved problems in physics.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy – the point at which neutrons break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or not have typical sizes, electromagnetic fields, or surface temperatures, compared to neutron stars.

The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012 K) quark–gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012 K cannot be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.

It is theorized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion might be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.

Ordinary quark matter consisting of up and down quarks (also referred to as u and d quarks) has a very high Fermi energy compared to ordinary atomic matter and is stable only under extreme temperatures and/or pressures. This suggests that the only stable quark stars will be neutron stars with a quark matter core, while quark stars consisting entirely of ordinary quark matter will be highly unstable and dissolve spontaneously.

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of up and down quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle. This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter. Quark stars made of strange quark matter are known as strange stars, and they form a subgroup under the quark star category.

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, they could also be created in the early cosmic phase separations following the Big Bang. If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day.

Quark stars have some special characteristics that separate them from ordinary neutron stars. Under the physical conditions found inside neutron stars, with extremely high densities but temperatures well below 1012 K, quark matter is predicted to exhibit some peculiar characteristics. It is expected to behave as a Fermi liquid and enter a so-called color-flavor-locked (CFL) phase of color superconductivity, where "color" refers to the six "charges" exhibited in the strong interaction, instead of the positive and the negative charges in electromagnetism. At slightly lower densities, corresponding to higher layers closer to the surface of the compact star, the quark matter will behave as a non-CFL quark liquid, a phase that is even more mysterious than CFL and might include color conductivity and/or several additional yet undiscovered phases. None of these extreme conditions can currently be recreated in laboratories so nothing can be inferred about these phases from direct experiments.

At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low, so in the Milky Way there would only be a small population of quark stars. If it is correct, however, that overdense neutron stars can turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as observers would be looking for the wrong type of star.

Another star, XTE J1739-285, has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible quark star candidate. In 2006, You-Ling Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star. It was reported in 2008 that observations of supernovae SN 2006gy, SN 2005gj and SN 2005ap also suggest the existence of quark stars. It has been suggested that the collapsed core of supernova SN 1987A may be a quark star. In 2015, Zi-Gao Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.>>
Art Neuendorffer