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Comment by zymhan | Hacker News Reader

The Gravitational wave was detected 2 seconds before the Gamma Ray Burst. I wonder why it is faster?

> On 17 August 2017 the NSF's Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, working with the Virgo Interferometer in Italy, detected gravitational waves passing the Earth. This event, the fifth ever detected, was named GW170817. About two seconds later, two space observatories, NASA’s Fermi Gamma-ray Space Telescope and ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL), detected a short gamma-ray burst from the same area of the sky.

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The gravitational waves here were measured for about 100 seconds (as opposed to the much shorter 'chirp' from the first gravitational wave detection made in 2016) so the whole event took some time to unfold.

The 2 second delay was due to the fact that gamma rays were generated by matter slowing down after it was ejected from the system and which collided with galactic gas. That matter was ejected from the black hole's axis of rotation and the black hole formed some time after the gravity waves were detected.

Take a look at this article: http://physicsworld.com/cws/article/news/2017/oct/16/spectac...

"As this material was sucked into the black hole, a fast-moving jet of material blasted outward along the black hole's axis of rotation. When this jet collided with gas in the galaxy, it started slowing down and the lost kinetic energy was broadcast as gamma rays"

Is that essentially cherenkov radiation? That does seem to require charged particles, so it seems unlikely.

If not, is it black body radiation due to the heat, or just excess energy from nuclear reactions that are caused by the high-speed neutrons.

It seems to me it's Bremsstrahlung turned to 11 - https://en.wikipedia.org/wiki/Bremsstrahlung

>the black hole formed some time after the gravity waves were detected

You mean some time after the gravity waves were emitted, right? Just checking my sanity here.

IANAAstronomer but my a-little-bit educated guess is gravity waves are emited during the "very close dance" phase of the merger and the GRB is the explosion that happens right after that.

I would assume it's for a similar reason to why neutrinos from a supernova arrive before the photons. Photons won't be traveling at the speed of light (in a vacuum) until the're actually in a vacuum. Neutrinos and the propagation of gravitational waves wouldn't be slowed down as much by the matter between the center of the event and open space.

That's the fun science bit where we say "Oh that's odd" :)

Is there some matter along the path that interacts with the gamma ray burst and slows it by two seconds? Why is it two seconds? Does it tell us something about the source?

All very exciting.

Yeah my first thought was that space isn't quite a vacuum, which maybe adds up over 130M light years to something measurable. Meanwhile, the gravitational wave is a fluctuation in space itself, so always moves at SoL?

The sibling comment about one simply happening after another also seems very plausible though.

Is it faster, or did the gravitational waves simply precede the GRB?

You are correct. Waves come from the final in spiral, before the black hole forms.

The GRB is from the rapid in fall, and friction of neutron star matter falling into the black hole.

I think the simplest explanation is that the gamma ray burst takes 2 seconds to form after the gravitational event.

Gravitational wave is caused by the abrupt change of movement of massive object through spacetime. I would imagine the two neutron stars are traveling to each other in way slower speed than the speed of light. When the edges of the two neutron stars first touched, their movement was abruptly stopped and the gravitational wave propagated out. The light only went out after the merging of the two objects, the compression, and the ignition of the kilonova, which took some time.

The gravity waves are caused by the high-speed orbiting of the neutron stars around each other before the explosive collision and merger.

My guess would be resolution. Gravitational waves are not only caused by the actual merger but also from the two masses losing energy while spinning closer and closer to each other. The GRB happens when they finally merge.

I suppose it would be weirder if the gamma rays arrived first.

I wonder if the gravitation waves stretches the Space-time in such a way that it takes 2 seconds longer for the photons of GRB to travel "on top of it" and reach earth.

By davidhydes comment [1] it is because the gamma burst actually happens after the merger. It is caused by the ejected mass slowing down due to friction.

My layman's interpretation is that this is cherenkov radiation on an astronomical scale, though that suggests it is caused by charged particles, which neutrons aren't.

[1] https://news.ycombinator.com/item?id=15484357

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