Merging Neutron Stars: What We Know So Far

In summary, rumors are circulating that LIGO/VIRGO has detected a signal of merging neutron stars, accompanied by an optical counterpart. These rumors were sparked by the sudden pointing of multiple telescopes at the same patch of sky. While the science behind the rumors cannot be discussed pre-publication, it is reasonable to use this as an opportunity to discuss the science behind merging neutron stars. The leading hypothesis for the origin of short gamma ray bursts is that they are caused by neutron star mergers, so if LIGO has truly detected such an event, we can expect a gamma ray counterpart as well. The gravitational wave signal from such an event would be very informative, as it would help refine models of neutron stars. However, it is uncertain if
  • #1
Vanadium 50
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Rumors are starting to fly that LIGO/VIRGO sees a signal of merging neutron stars, with an optical counterpart. Indeed, the thing that seems to have triggered the rumors was having a number of telescopes suddenly pointing at the same patch of sky.

It's difficult to discuss the science behind the rumors pre-publication, but it might be reasonable to use this thread to discuss the science behind merging neutron stars.
 
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  • #2
Vanadium 50 said:
Indeed, the thing that seems to have triggered the rumors was having a number of telescopes suddenly pointing at the same patch of sky.
It's that age-old prank, writ large.

"Hey Carl! Let's point our scope at a random spot in the sky and act really intent, and see how many others we can get to do the same..."

Soon enough, there's two dozen observatories gathered on a busy street corner, all looking up and wondering what everyone else is looking at..
 
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  • #3
It sounds like these tweets are the primary source of the rumor:

J Craig Wheeler‏ @ast309 8h8 hours ago
Right or wrong, I should not have sent that tweet. LIGO deserves to announce when they deem appropriate. Mea culpa.

J Craig Wheeler‏ @ast309 Aug 18
New LIGO. Source with optical counterpart. Blow your sox off!

J Craig Wheeler‏ @ast309 Aug 15
Rumor of exciting new LIGO source.
 
  • #5
The leading hypothesis for the origin of short gamma ray bursts (GRBs) is that they are due to neutron star - neutron star mergers. So if LIGO really has seen such an event, we should expect not just an optical counterpart, but a gamma ray counterpart as well. I'm dying to find out whether a such a gamma ray event coincident with the gravitational wave event has been seen.
 
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  • #6
The gravitational wave signal from such an event would be really interesting. For merging black holes, we know how it should happen - GR makes accurate predictions. For neutron stars, you need to describe the matter as well, and we don't know much about the interior of neutron stars. The GW signal would help finding the best models.
 
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  • #7
mfb said:
The GW signal would help finding the best models.

I'm not so sure one signal tells you much. I think you need an ensemble. As I understand it, an important part of the dynamic is resonances between the core and the crust as one neutron star is being perturbed by the other. As you say, a BH is very simple object: you know it's mass, charge and spin, and that's all there is. A neutron star is much more complicated, with a lot of internal dynamics, a response that depends on the dynamics, and on top of all this an unclear EOS. A single observation can remove the outlier models, but I think you're going to need several such events to sort out the neutron star EOS.

phyzguy said:
So if LIGO really has seen such an event, we should expect not just an optical counterpart, but a gamma ray counterpart as well.

I don't think this is necessarily true. There's the logic argument, "all short GRBs are caused by NS mergers" does not imply "all NS mergers cause GRBs", but even if that were true, there's no guarantee that the Earth is aligned along the right axis. I don't know the relationship between the best axis for GW observation and the best axis for GRB detection. It's even possible they are anti-correlated.
 
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  • #8
Every ensemble starts with the first signal.
Sure, larger samples will be better, and we can expect them - after further upgrades (2018++). For now even a single signal would be great.
 
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  • #9
More on this. This site gives info on a Chandra (orbiting X-Ray telescope) pointing apparently searching for an X-ray counterpart of a GW event. The information on the Chandra pointing says:

"Gravitational wave source detected by aLIGO, VIRGO, or both. Single EM counterpart identified by Dark Energy Camera for Chandra follow-up at a distance of <~400 Mpc. "

Apparently there was also an observation by Fermi of a short gamma ray burst (SGRB170817A), which may have been coincident with the GW event. If you put all this together, it sounds like the event may have been seen in gravitational waves, gamma rays, X-rays, and optical light. It sure sounds real. I can't wait for the announcement.
 
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  • #10
It would be a bit odd to have a coincident GW and GRB event attributed to a neutron star merger. From what I've heard the best guess estimates have it at about a one in fifty chance for having the proper alignment for a coincident detection. So not impossible, but it wasn't expected to be detected this early. Perhaps this means some sort of modeling error, as some of the numerical hydrodynamic simulations seem extremely difficult.
 
  • #11
We'll have to wait for the announcements to know for sure. If this SGRB170817A is really in NGC 4993 at 40 MPc distance, I think that would make it the closest SGRB detected. Just detecting a SGRB with clear optical counterpart in a galaxy this close would be a major find, even if it isn't associated with a GW event. As to Haelfix's objection, I think that most of the gamma rays are beamed, but there are still a significant number of gamma rays emitted outside the narrow beam, so if it is close enough, we could see a GW event and a GRB at the same time even if we aren't in the narrow beam. Or perhaps the rumors are wrong or there were two different events. We'll have to wait and see.

I did find these two images about SGRB170817A at the Fermi website. I added the X at the approximate location of NGC 4993.
 

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  • #12
Interesting that this paper just appeared on the arXiv today, basically saying that if the SGRB is close enough, it can be detected by Fermi even if we are significantly off-axis.
 
  • #13
LIGO news:
Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.
Candidates.

Nature has a good new article, summarizing the different sources of the rumors.
 
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  • #14
@mfb - Thanks for posting this. Not only is it exciting that there appear to be multiple candidates, but it also confirms that both LIGO and Virgo have seen events.
 
  • #15
I knew there was going to be a public statement today. Here it is:

25 August 2017 -- The Virgo and LIGO Scientific Collaborations have been observing since November 30, 2016 in the second Advanced Detector Observing Run ‘O2’ , searching for gravitational-wave signals, first with the two LIGO detectors, then with both LIGO and Virgo instruments operating together since August 1, 2017. Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.

So there is some confirmation, although it's not iron-clad, and certainly does not confirm any particular interpretation.
 
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  • #16
The low mass of NS vs BH binary systems raises questions in my mind about the GW detectability distance of binary NS mergers. The abundance of NS binary systems detected by EM based detectors appears quite low compared to their projected life expectancy. It will be interesting to see what researchers conclude about the nature of GW events thus far detected.
 
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  • #17
Vanadium 50 said:
It's difficult to discuss the science behind the rumors pre-publication, but it might be reasonable to use this thread to discuss the science behind merging neutron stars.

As we find gravitational wave events that have optical/electromagnetic counterparts, it will be interesting to compare the standard candle optical distances to the distances from the gravitational wave signal analysis.
 
  • #18
George Jones said:
it will be interesting to compare the standard candle optical distances to the distances from the gravitational wave signal analysis.

True. You can do this even without optical counterparts, since you get simultaneous redshifts and luminosities from BH mergers. Unfortunately, the precision is not very good - you'd want much more sensitivity to a) improve the precision, and b) to extend the reach (and thereby the rate of detection and redshift lever arm) to larger distances.
 
  • #19
As a black hole sceptic, I'm hoping that the gravitational wave signal will indicate masses far too heavy to be a neutron star according to standard theory, in which case any EM emissions will require a lot of explaining!
 
  • #20
Black holes can have accretion disks around them, and a merger certainly disrupts them. I don't see how "any EM signal" would be very surprising. The details of the signal would be more interesting.
 
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  • #21
mfb said:
Black holes can have accretion disks around them, and a merger certainly disrupts them. I don't see how "any EM signal" would be very surprising. The details of the signal would be more interesting.
Certainly it's the details that matter, and I agree "any" may be an exaggeration depending on the distance. However, when Fermi saw evidence of a possible GRB at exactly the same time as the first GW detection it was dismissed as a coincidence by most, as even with accretion disks involved the theoretical expected amount of EM energy emitted by a black hole merger was orders of magnitude too small to have triggered the apparent GRB detection, which would have needed a significant amount of the collision energy to be radiated in the EM spectrum. So the theory was felt to be stronger than the apparent observation in that case.
If we have evidence this time of a significant amount of energy being emitted in the EM spectrum but the masses turn out to be too large not to be black holes according to standard theory then that would again suggest that something is wrong with the standard theory, which is always interesting.
It would be interesting to know why this is being described as a binary neutron star merger; is this because the initial analysis of the GW signal shows relatively light masses (in which case it seems surprising that anything was detected at all) or because the SGRB and other EM emissions suggest that it wasn't a black hole, regardless of the masses?
 
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  • #22
Jonathan Scott said:
If we have evidence this time of a significant amount of energy being emitted in the EM spectrum but the masses turn out to be too large to be black holes according to standard theory then that would again suggest that something is wrong with the standard theory, which is always interesting.
I think you meant to say "binary neutron stars" rather than "black holes"...
 
  • #23
jerromyjon said:
I think you meant to say "binary neutron stars" rather than "black holes"...
Actually I lost the word "not" when reordering my words, which I've now edited to correct, thanks.
 
  • #24
Jonathan Scott said:
It would be interesting to know why this is being described as a binary neutron star merger; is this because the initial analysis of the GW signal shows relatively light masses (in which case it seems surprising that anything was detected at all) or because the SGRB and other EM emissions suggest that it wasn't a black hole, regardless of the masses?
Binary neutron star mergers are among the signals LIGO and VIRGO want to find. The range is not as good as for large black holes, but there is a huge volume in space where the sensitivity is sufficient. It wouldn't be surprising to find such an event.
 
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  • #25
mfb said:
Binary neutron star mergers are among the signals LIGO and VIRGO want to find. The range is not as good as for large black holes, but there is a huge volume in space where the sensitivity is sufficient. It wouldn't be surprising to find such an event.
If they really have GW and EM detections of the same binary neutron star merger event, that's pretty amazing regardless. I thought the chances of that with current sensitivities were considered quite small, although obviously they were hoping for it.
 
  • #26
Vanadium 50 said:
True. You can do this even without optical counterparts, since you get simultaneous redshifts and luminosities from BH mergers. Unfortunately, the precision is not very good - you'd want much more sensitivity to a) improve the precision, and b) to extend the reach (and thereby the rate of detection and redshift lever arm) to larger distances.

From "Using Gravitational-Wave Standard Sirens" by Holz and Hughes (The Astrophysical Journal)
http://iopscience.iop.org/article/10.1086/431341/pdf

Since GWs do not provide the redshift of the source, BBH GW measurements alone do not probe the distance-redshift relation. However, as first noted by Bernard Schutz, should some kind of ‘‘electromagnetic’’ (EM) counterpart to a BBH GW event be identified, the situation changes drastically (Schutz 1986, 2002). ...

In-spiral GWs encode the luminosity distance to a binary, its position on the sky, its orientation, and information about certain combinations of masses and spins (see Arun et al. 2004 and Blanchet et al. 2004 for up-to-date discussion and details). The in-spiral does not encode a source’s cosmological redshift. Redshift is instead entangled with the binary’s evolution. For example, the masses ##\left(m_1, m_2\right)## impact orbital evolution as timescales ##(Gm_1/c^3; Gm_2 /c^3)##. These timescales redshift, so the measured masses redshift; a binary with masses ##\left(m_1, m_2\right)## at redshift z is indistinguishable from a local binary with masses ##\left[\left(1+z\right)m_1, \left(1+z\right)m_2\right]## (modulo amplitude). This reflects the fact that general relativity has no absolute scale.
 
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  • #27
I just read the paper - I'm not sure I completely buy their argument, but will check with a GR expert Tuesday. It seems to be that even if the signals are scale independent, the transitions won't be (except by accident) - i.e. the inspiral to merge transition and the merge to ringdown.
 
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  • #28
I see there are mentions on the web of the gravitational wave event being provisionally referred to as GW170818, which isn't even the same day as GRB 170817A. Someone has created a Wikipedia page for it under that name, saying it occurred on 18th and referring to the Nature article, which as far as I can see doesn't say when the event occurred. I don't know what that date is based on - perhaps simply the day of J Craig Wheeler's "Blow your sox off!" tweet? But he posted about "Rumor of exciting new LIGO source" on 15th August, suggesting more than one event in that case. I wish we could get some more detail!
 
  • #29
As @mfb pointed out in post #13, LIGO has confirmed that they are investigating more than one event. The fact that the Chandra archive specifically refers to SGR170817A, the Dark Energy Camera, and aLIGO/Virgo strongly suggests that they all saw this same event, so I think Wheeler's post on 15-Aug must refer to a different event. But we'll have to wait and see.
 
  • #30
I was very excited to hear about the new LIGO/Virgo announcement today, but it wasn't the neutron star event I was hoping for. It was a new black hole merger event, GW170814, which for the first time was seen by Virgo as well as both LIGO locations. Lots more details, including papers, on both www.virgo-gw.eu and https://www.ligo.caltech.edu/.
 
  • #31
I was disappointed as well. Since this announcement was about GW170814, I think the announcement about the 170817 event must still be forthcoming. Recalling the J Craig Wheeler tweets (below), today's announcement is probably about the event that triggered the Aug 15 tweet. I'm still expecting a merging NS announcement.

J Craig Wheeler‏ @ast309 Aug 18
New LIGO. Source with optical counterpart. Blow your sox off!

J Craig Wheeler‏ @ast309 Aug 15
Rumor of exciting new LIGO source.
 
  • #32
Not the first time I've heard an expression of disappointment about this.
Why is a neutron star merger more exciting than a black hole merger?
 
  • #33
We can learn a lot about the interior of neutron stars. We can't do that for black holes.
 
  • #34
DaveC426913 said:
Not the first time I've heard an expression of disappointment about this.
Why is a neutron star merger more exciting than a black hole merger?
A neutron star merger is expected to be visible in various parts of the electromagnetic spectrum, providing a huge amount of additional information compared with a black hole merger.
 
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  • #35
The latest I'm hearing is that there will be a press conference on this event on Oct 16. Stay tuned.
 
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<h2>What are neutron stars?</h2><p>Neutron stars are incredibly dense and compact objects that are formed when a massive star runs out of fuel and undergoes a supernova explosion. They are made up almost entirely of neutrons and have a diameter of about 20 kilometers.</p><h2>How do neutron stars merge?</h2><p>Neutron stars can merge in a variety of ways, but the most common is through the emission of gravitational waves. As the two stars orbit each other, they lose energy through the emission of these waves, causing them to spiral closer and eventually merge.</p><h2>What happens when neutron stars merge?</h2><p>When neutron stars merge, a cataclysmic event known as a kilonova occurs. This results in the release of a huge amount of energy in the form of light and other electromagnetic radiation. It also creates heavy elements, such as gold and platinum, through a process called r-process nucleosynthesis.</p><h2>What have we learned from observing merging neutron stars?</h2><p>Observing the merger of neutron stars has provided valuable insights into various areas of astrophysics, including the origin of heavy elements, the behavior of matter under extreme conditions, and the nature of gravity. It has also confirmed the existence of gravitational waves, which were predicted by Einstein's theory of general relativity.</p><h2>What are the potential implications of merging neutron stars?</h2><p>Merging neutron stars can have a wide range of implications, from helping us understand the evolution of the universe to providing new avenues for studying fundamental physics. It may also have practical applications, such as improving our understanding of nuclear fusion and developing new technologies for detecting gravitational waves.</p>

What are neutron stars?

Neutron stars are incredibly dense and compact objects that are formed when a massive star runs out of fuel and undergoes a supernova explosion. They are made up almost entirely of neutrons and have a diameter of about 20 kilometers.

How do neutron stars merge?

Neutron stars can merge in a variety of ways, but the most common is through the emission of gravitational waves. As the two stars orbit each other, they lose energy through the emission of these waves, causing them to spiral closer and eventually merge.

What happens when neutron stars merge?

When neutron stars merge, a cataclysmic event known as a kilonova occurs. This results in the release of a huge amount of energy in the form of light and other electromagnetic radiation. It also creates heavy elements, such as gold and platinum, through a process called r-process nucleosynthesis.

What have we learned from observing merging neutron stars?

Observing the merger of neutron stars has provided valuable insights into various areas of astrophysics, including the origin of heavy elements, the behavior of matter under extreme conditions, and the nature of gravity. It has also confirmed the existence of gravitational waves, which were predicted by Einstein's theory of general relativity.

What are the potential implications of merging neutron stars?

Merging neutron stars can have a wide range of implications, from helping us understand the evolution of the universe to providing new avenues for studying fundamental physics. It may also have practical applications, such as improving our understanding of nuclear fusion and developing new technologies for detecting gravitational waves.

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