Merging neutron stars

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Vanadium 50
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Main Question or Discussion Point

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
DaveC426913
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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..
 
  • #3
phyzguy
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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.
 
  • #4
Spinnor
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  • #5
phyzguy
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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.
 
  • #6
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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.
 
  • #7
Vanadium 50
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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.

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
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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.
 
  • #9
phyzguy
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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.
 
  • #10
Haelfix
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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
phyzguy
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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
phyzguy
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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
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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.
 
  • #14
phyzguy
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@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
Vanadium 50
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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.
 
  • #16
Chronos
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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.
 
  • #17
George Jones
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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
Vanadium 50
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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
Jonathan Scott
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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
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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.
 
  • #21
Jonathan Scott
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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?
 
  • #22
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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
Jonathan Scott
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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
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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.
 
  • #25
Jonathan Scott
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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.
 

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