APOD: GW170817: A Spectacular Merger Event... (2017 Oct 16)

[APOD Robot]

GW170817: A Spectacular Multi-Radiation Merger Event Detected

Explanation: Both gravitational and electromagnetic radiations have been detected in rapid succession for an explosive merging event for the first time. Data from the outburst fit well with a spectacular binary neutron-star death-spiral. The explosive episode was seen on August 17 in nearby NGC 4993, an elliptical galaxy only 130 million light years distant. Gravitational waves were seen first by the ground based LIGO and Virgo observatories, while seconds later the Earth-orbiting Fermi observatory detected gamma-rays, and hours after that Hubble and other observatories detected light throughout the electromagnetic spectrum. Pictured is an animated illustrative movie of the event's likely progenitors. The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova. This first coincident detection confirms that LIGO events can be associated with short-duration gamma-ray bursts. Such powerful neutron star mergers are thought to have seeded the universe with many heavy nuclei including the iodine needed for life and the uranium and plutonium needed for nuclear fission power. You may already own a souvenir of one of these explosions -- they are also thought to be the original creators of gold.

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[neufer]

HOLY SMUGGLETUGGLES

[owlice]

YAY!!!!

[bystander]

http://asterisk.apod.com/viewtopic.php?t=37665

[SimonASK]

Just watched the double news conference, however they didn't say much about what was the result of the neutron star merger. At one point it was hinted that this is as yet unknown, elsewhere it was suggested that it was the smallest black hole known. Can anyone offer any further knowledge or insights?

[Chris Peterson]

Just watched the double news conference, however they didn't say much about what was the result of the neutron star merger. At one point it was hinted that this is as yet unknown, elsewhere it was suggested that it was the smallest black hole known. Can anyone offer any further knowledge or insights?

Nobody knows at this point. There's enough data here to refine the sort of models used to understand these things, so we may see some reasonable scenarios show up in papers over then next months.

[SimonASK]

Thanks Chris Peterson for your reply.

[dstarrman]

Apparently, the gamma rays from this event traveled faster than light, or the light from the event traveled at less than light speed. Which is it and how?

[Chris Peterson]

Apparently, the gamma rays from this event traveled faster than light, or the light from the event traveled at less than light speed. Which is it and how?

There are other possibilities. The gamma rays may have started being produced earlier than the light. The observation of the light may have been delayed by the notification times and instrument limitations.

Nobody is suggesting that any of the radiation that reached us was traveling at anything other than c.

[dstarrman]

Thanx Chris!

[De58te]

Apparently, the gamma rays from this event traveled faster than light, or the light from the event traveled at less than light speed. Which is it and how?

If I may submit a layman's armchair science theory. Gamma Rays are thousands of times smaller than light rays, and like x-rays aren't hindered much by dense matter in its way. Now to light photons. In the diagram the collision was two dimensional, completely visible to us. But what if we seen the collision with one of the neutron stars in front of the other. Like watching a car accident from directly behind one car.
Or even if the neutron star was just 1degree off so that 1 degree of its limb was blocking our view. Now if I recall a photon created in the center of our Sun takes up to a year to reach the surface, because the medium is so dense it is constantly colliding with some matter. Likewise the light created in the neutron star collision couldn't travel a direct route through the neutron star because neutron stars are thousands of times more dense than our sun. So more than likely the light photon shot out sideways to us and then through gravitational lensing it was bent around to our direction. This then would create a time delay between the photons and the gamma rays.

[Spif]

As I recall from school, neutron stars are in fact not composed of pure neutrons. There is expected to be some complexity in their structure. Some ratio of protons are typically mixed in with the degenerate neutron matter and to balance the charge there is also a soup of electrons typically flowing in a loop of current. Some divergence of these flowing and spinning charges are what create the magnetic field? And around the surface is an outer blanket or shell of non-degenerate or normal atomic matter of some relatively small depth.

I wonder then, when a merge event like this happens, does any fraction of the expelled matter come from the degenerate cores of the two stars or does nearly all of the remnant cloud come only from the two outer shells of atomic matter?

Also, I wonder if there is any confidence regarding what the typical composition of the outer shell of a neutron star is? I imagine that since these shells were formed from gravitationally collapsing and fusing matter at the bottom layers of a supernova, they are probably made (to a fair degree) out of atoms heavier than iron?

Anyway, the NY Times has a nice video about this event as well.

[Chris Peterson]

Apparently, the gamma rays from this event traveled faster than light, or the light from the event traveled at less than light speed. Which is it and how?

If I may submit a layman's armchair science theory.

I think the short answer is that everything traveled towards us at the same speed. The event was fast and small. There were probably delays in when specific kinds of radiation were produced as a result of the process, but I doubt any photons were delayed by interactions with their surrounds (not enough for us to measure, anyway).

[Ann]

Fantastic event. Very much a milestone.

Thanks for the additional info, Chris!

Ann

[neufer]

As I recall from school, neutron stars are in fact not composed of pure neutrons. There is expected to be some complexity in their structure. Some ratio of protons are typically mixed in with the degenerate neutron matter and to balance the charge there is also a soup of electrons typically flowing in a loop of current. Some divergence of these flowing and spinning charges are what create the magnetic field? And around the surface is an outer blanket or shell of non-degenerate or normal atomic matter of some relatively small depth.

I wonder then, when a merge event like this happens, does any fraction of the expelled matter come from the degenerate cores of the two stars or does nearly all of the remnant cloud come only from the two outer shells of atomic matter?

Also, I wonder if there is any confidence regarding what the typical composition of the outer shell of a neutron star is? I imagine that since these shells were formed from gravitationally collapsing and fusing matter at the bottom layers of a supernova, they are probably made (to a fair degree) out of atoms heavier than iron?

• ~16,000 Earth masses (= ~2% of the star masses) got ejected
  (predominantly from fastest moving outer equatorial regions).

[b][color=#0000FF]Cross-section of neutron star. Densities are in terms of ρ[sub]0[/sub] the saturation nuclear matter density, where nucleons begin to touch.[/color][/b]

---

<<Current models indicate that matter at the surface of a neutron star is composed of ordinary atomic nuclei crushed into a solid lattice with a sea of electrons flowing through the gaps between them. It is possible that the nuclei at the surface are iron, due to iron's high binding energy per nucleon. It is also possible that heavy elements, such as iron, simply sink beneath the surface, leaving only light nuclei like helium and hydrogen. If the surface temperature exceeds 10⁶ kelvin (as in the case of a young pulsar), the surface should be fluid instead of the solid phase that might exist in cooler neutron stars.

The "atmosphere" of a neutron star is hypothesized to be at most several micrometers thick, and its dynamics are fully controlled by the neutron star's magnetic field. Below the atmosphere one encounters a solid "crust". This crust is extremely hard and very smooth (with maximum surface irregularities of ~5 mm), due to the extreme gravitational field. The expected hierarchy of phases of nuclear matter in the inner crust has been characterized as nuclear pasta.

Proceeding inward, one encounters nuclei with ever-increasing numbers of neutrons; such nuclei would decay quickly on Earth, but are kept stable by tremendous pressures. As this process continues at increasing depths, the neutron drip becomes overwhelming, and the concentration of free neutrons increases rapidly. In that region, there are nuclei, free electrons, and free neutrons. The nuclei become increasingly small (gravity and pressure overwhelming the strong force) until the core is reached, by definition the point where mostly neutrons exist.

The composition of the superdense matter in the core remains uncertain. One model describes the core as superfluid neutron-degenerate matter (mostly neutrons, with some protons and electrons). More exotic forms of matter are possible, including degenerate strange matter (containing strange quarks in addition to up and down quarks), matter containing high-energy pions and kaons in addition to neutrons, or ultra-dense quark-degenerate matter.>>

[Uncle Jeff]

Any word yet on spectral analysis of the visible light? Do we see the heavy elements (Gold, Thorium and Uranium) in the ejecta? Are there spikes in the gamma spectrum corresponding to known decays of actinides (or even transactinides)?

[neufer]

Any word yet on spectral analysis of the visible light? Do we see the heavy elements (Gold, Thorium and Uranium) in the ejecta? Are there spikes in the gamma spectrum corresponding to known decays of actinides (or even transactinides)?

Due to Doppler broadening from the rapid ejecta it was difficult to pinpoint the elements precisely.

[Boomer12k]

Nothing seems to excite, like GOLD... There's Gold in them thar skies!!!

"I've got...Gooooold Fever.....can't get enough of the yellow stuff...." Clint Eastwood...Paint Your Wagon... watched it a couple weeks ago.

Given detected on August 17th... and the distance...130 or so million LY.... it happened a long time ago... in a Galaxy Far, Far, Away..... Roll Into....

Maybe it was not Neutron Stars, but a DEATH STAR....lol....Go Rebels...

:---[===] *

[Ann]

APOD Robot wrote:

The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova.

Why would this event lead to an optical supernova over time?

Did we see a supernova here?

Ann

[alter-ego]

Locating the merger had an interesting twist. It was first thought VIRGO missed the event but a closer look revealed a weak data stream that was not statistically significant. Based on the incoming broad-area localization data from the other observatories (LIGO & Fermi/INTEGRAL), VIRGO scientists realized that the event could be located in a well characterized, instrument partial blind spot (due to VIRGO's location on Earth). It was that blind spot that provided the smaller, overlapping localization oval to then permit a tractable sky search.

https://journals.aps.org/prl/pdf/10.110 ... 119.161101                                                                S&T

[EricP]

The APOD for August 16 says that "The explosive episode was seen on August 17". Was a temporal anomaly caused by the event?

[neufer]

The APOD for August 16 says that "The explosive episode was seen on August 17". Was a temporal anomaly caused by the event?

Was a temporal anomaly causing you to read October as August?

[neufer]

APOD Robot wrote:

The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova.

Why would this event lead to an optical supernova over time?

As I understand it the emitted gamma rays had to first impinge upon NGC 4993 gas/dust to produce the X-ray, UV, light, IR & radio cascade observed here. While the X-rays occurred too quickly to be observed in time the radio may still be observable for months to come. If the kilonova remnant had been a hot heavy new neutron star (rather than a cold light black hole) X-rays should have been observable for some time.

[EricP]

read October as August?

Yep, I'm an idiot. I'd delete it if I could.

[BDanielMayfield]

APOD Robot wrote:

The video depicts hot neutron stars as they spiral in toward each other and emit gravitational radiation. As they merge, a powerful jet extends that drives the short-duration gamma-ray burst, followed by clouds of ejecta and, over time, an optical supernova-type episode called a kilonova.

...

Did we see a supernova here?

Ann

Sort of Ann. What was caught by em observatories was/is a “supernova-type episode called a kilonova.” Apparently this is the first direct evidence of the heretofore hypothetical kilonova class of explosive event. Kilonovas aren’t near as bright as the many other well observered types of SNs, therefore they are much harder to detect, and fade to undetectably very quickly.

Bruce