I Merging Neutron Stars: What We Know So Far

[Vanadium 50]

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.

 

[DaveC426913]

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..

 

[phyzguy]

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.

 

[Spinnor]

Video of super computer models of a neutron star pair uniting.



From, http://splash.abc.net.au/web/splash#!/media/1590050/when-neutron-stars-collide

The physics behind the mergers, 54 min. talk, Dr. Brian Metzger, Colombia University,

 

[phyzguy]

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.

 

[mfb]

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.

 

[Vanadium 50]

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.

 

[mfb]

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.

 

[phyzguy]

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.

 

[Haelfix]

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.

 

[phyzguy]

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.

 

[phyzguy]

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.

 

[mfb]

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[/size].

Nature has a good new article, summarizing the different sources of the rumors.

 

[phyzguy]

@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.

 

[Vanadium 50]

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.

 

[Chronos]

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.

 

[George Jones]

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.

 

[Vanadium 50]

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.

 

[Jonathan Scott]

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!

 

[mfb]

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.

 

[Jonathan Scott]

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?

 

[jerromyjon]

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"...

 

[Jonathan Scott]

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.

 

[mfb]

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.

 

[Jonathan Scott]

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.

 

[George Jones]

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.

 

[Vanadium 50]

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.

 

[Jonathan Scott]

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!

 

[phyzguy]

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.

 

[Jonathan Scott]

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/.

 

[phyzguy]

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.

 

[DaveC426913]

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?

 

[mfb]

We can learn a lot about the interior of neutron stars. We can't do that for black holes.

 

[Jonathan Scott]

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.

 

[phyzguy]

The latest I'm hearing is that there will be a press conference on this event on Oct 16. Stay tuned.

 

[phyzguy]

And here it comes!

https://www.sciencealert.com/there-s-another-big-gravitational-wave-announcement-on-the-way

https://www.nsf.gov/news/news_summ.jsp?cntn_id=243379&org=NSF&from=news

 

[mfb]

From the panel:

* Vicky Kalogera is an expert for binary neutron stars and white dwarfs.
* Andy Howell and Ryan Foley use supernovae to study dark energy and work on quick optical follow-up observations for the supernovae.
* Marcelle Soares-Santos studies dark energy and cosmological gravitational waves
* David Sand is another supernova expert
* Nial Tanvir studies gamma ray bursts, and found evidence that short GRBs are caused by binary mergers
* Edo Berger studies GRBs and other supernovae
* Eleonora Troja studies GRBs
* Alessandra Corsi studies GRBs
* Various LIGO/Virgo people, of course.

We get a very consistent picture here. I predict that LIGO and Virgo measured a binary neutron star or neutron star black hole merger, which was also seen as short-duration gamma ray burst.

 

[Jonathan Scott]

It's difficult to keep secrets with so many people involved. A Google search already finds a two web pages containing abstracts of papers about this event from the electromagnetic point of view (X-ray and optical); the pages themselves have been removed, but Google has cached them!

Also, Francis Villatoro "emulenews" has tweeted a brief summary at https://twitter.com/emulenews/status/917302944990728193 as follows:

Non-official GW170817 = time coincidence with short GRB170817A (Fermi GBM, INTEGRAL); optical counterpart localization in NGC4993 (40 Mpc)
Official press-conference LIGO/Virgo on Oct 16 – keep tuned!

 

[mfb]

ESO announced a press conference "to present groundbreaking observations of an astronomical phenomenon that has never been witnessed before" at the same time as the LIGO press conference. Is this the same conference, or do they announce the same thing in separate conferences, or is this unrelated? The timing is very suspicious.

16:00 CEST, 14:00 UTC, 10:00 EDT, or 11:20 after this post.

 

[Borek]

16^th October, day of two important announcements: one astronomical and the other related to WiFi security (google KRACK).

 

[websterling]

The LIGO conference will be streamed on YouTube starting at 10:00 a.m. EDT

www.youtube.com/watch?v=mtLPKYl4AHs&feature=youtu.be

 

[mfb]

ESO spoiled it - yes , two neutron stars merging. About 70 telescopes looked at it in a giant observation campaign.

 

[Greg Bernhardt]

Press Conference is live!

 

[Hawksteinman]

It's been confirmed!

 

[Jonathan Scott]

Lots of very neat results. GW170817 was detected for more than 30 seconds in this case, gradually building up to the merger of two neutron stars (probably to form a combined neutron star). Then Fermi and Integral both saw a short GRB within the next couple of seconds, so everyone pointed their telescopes in the general direction to try to spot anything. A few hours afterwards a bright new electromagnetic source was identified in NGC 4993 which matched the expected characteristics of a kilonova, and provided a lot of interesting spectral features as it died away over a few days. Detected in just about everything except neutrinos!

 

[Jonathan Scott]

And now there's a Wikipedia entry: https://en.wikipedia.org/wiki/GW170817

 

[Haelfix]

It looks like there is a new standard candle source.

 

[ohwilleke]

Some of the published papers (reportedly there are 40 in all):

B. P. Abbott et al. GW170817: Observation of gravitational waves from a binary neutron star inspiral. Physical Review Letters. Published online October 16, 2017. doi: 10.1103/PhysRevLett.119.161101.

B. P. Abbott et al. Multi-messenger observations of a binary neutron star merger. Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa91c9.

I. Arcavi et al. Optical follow-up of gravitational-wave events with Las Cumbres Observatory. Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa910f.

J. Hjorth et al. The distance to NGC 4993: The host galaxy of the gravitational-wave event GW170817. Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9110.

M.C. Diaz et al. Observations of the first electromagnetic counterpart to a gravitational-wave source by the TOROS collaboration. Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa9060.

B. P. Abbott et al. Gravitational waves and gamma-rays from a binary neutron star merger: GW170817 and GRB 170817A. Astrophysical Journal Letters. Published online October 16, 2017. doi: 10.3847/2041-8213/aa920c.

E. Troja et al. The X-ray counterpart to the gravitational-wave event GW170817. Nature. Published online October 16, 2017. doi: 10.1038/nature24290.

I. Arcavi et al. Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger. Nature. Published online October 16, 2017. doi: 10.1038/nature24291.

The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration. A gravitational-wave standard siren measurement of the Hubble constant. Nature. Published online October 16, 2017. doi: 10.1038/nature24471.

D. Kasen et al. Origin of the heavy elements in binary neutron star mergers from a gravitational wave event. Nature. Published online October 16, 2017. doi:1038/nature24453.

S.J. Smartt et al. A kilonova as the electromagnetic counterpart to a gravitational wave source. Nature. Published online October 16, 2017. doi: 10.1038/nature24303.

E. Pian et al. Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger. Nature. Published online October 16, 2017. doi: 10.1038/nature24298.

S. Covino et al. The unpolarized macronova associated with the gravitational wave event GW 170817. Nature Astronomy. Published online October 16, 2017. doi:10.1038/s41550-017-0285-z.

M.M. Kasliwal et al. Illuminating gravitational waves: A concordant picture of photons from a neutron star merger. Science. Published online October 16, 2017. doi: 10.1126/science.aap9455.

P.A. Evans et al. Swift and NuSTAR observations of GW170817: Detection of a blue kilonova. Science. Published online October 16. doi: 10.1126/science.aap9580.

G. Hallinan et al. A radio counterpart to a neutron star merger. Science. Published online October 16, 2017. doi: 10.1126/science.aap9855.

D.A. Coulter et al. Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave source. Science. Published online October 16, 2017. doi: 10.1126/science.aap9811.

M.R. Drout et al. Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis. Science Published online October 16, 2017. doi: 10.1126/science.aaq0049.

B.J. Shappee et al. Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger. Science. Published online October 16, 2017. doi: 10.1126/science.aaq0186.

C.D. Kilpatrick et al. Electromagnetic evidence that SSS17a is the result of a binary neutron star merger. Science. Published online October 16, 2017. doi: 10.1126/science.aaq0073.

 

[diogenesNY]

And now comes the confusing and perhaps non-meaningful mainstream reportage of this event.

This AM, this was breathlessly reported on NPR news, with some extremely confusing comments. I am abundantly cognizant that an NPR radio newsbrief is hardly a source that once can hang a hat on, however there were some strangely specific comments that I hope somebody here can help me make some sense (or nonsense as the case may be) of.

The matter of the observed collision was noted, followed by the comments (unsourced) that this resulted in the production of a large quantity of gold and platinum, and that consequently we now have a clear idea where heavy metals come from.

Again, please note, this was radio news reporting, which I tend to assume is both incomplete, misleading and confused.

I have some general knowledge of supernova based nucleosynthesis ... but this just left me 'alone in the dark' so to speak. Superficially, it sounds like an outrageously general and conclusive statement. I mean, I am sure that there may have been some observation of relevant spectral lines (my guess... I am sure someone can shed some light on this) but the general conclusion sounds like... well... mainstream science journalism overreach or over-summerization... I don't know which, if either. Either way, before I go semi-randomly punching up web pages, perhaps someone could offer some clarity, guidance and maybe a direction on some more meaningful sources online to at least get some more meaningful (albeit informed-layman digestible) information.

Is the wiki article a reasonable place to start?

diogenesNY

Edit: Okay, read the wiki page ... and the kilonova reference and the kilonova wiki page. Very cool and interesting. Would like to read more about this

 

[ohwilleke]

The Science News article is the best place to start before trying to dive into any of the scientific papers, many of which are closed access. https://www.sciencenews.org/article/neutron-star-collision-gravitational-waves?tgt=nr I'd recommend reading the abstracts to the papers next. The wiki article is too short to really tell the whole story or even a meaningful enough part of it and only cites to secondary sources so far.