Why did the gamma ray burst from GW170817 lag two seconds behind the gravitational wave?

Question

The ABC, reporting on the announcement of gravitational wave GW170817, explained that for the first time we could identify the precise source of a gravitational wave because we also observed the event in the electromagnetic spectrum. It notes however that the gamma ray burst detected by the FERMI space telescope was observed nearly two seconds later than the gravitational wave.

1. Did the gamma ray burst actually arrive at Earth two seconds after the gravitational wave, or is this time delay just some kind of observational artefact?

2. If the delay is real, what is its cause? Is the delay due to the gamma ray burst somehow being slowed, or was it 'emitted' at a different time in the merger event?

3. What does a delay even mean when the gravitational wave was detected for over 100 seconds? Is it a delay from peak GW to peak gamma ray?

Answer 1

This is addressed in section 4.1 "Speed of Gravity" of one of the GW170817 companion papers: Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A. General relativity predicts that GWs travel at the speed of light. The difference in time of arrival could come from a difference in speed or a difference in the time of emission, i.e. the gamma rays are emitted after the merger.

Under conservative assumptions described in the paper the fractional difference in the speed of light and the speed of gravity is bounded as:

$$-3\times 10^{-15} \le \frac{v_{GW}-c}{c} \le +7\times 10^{-16}$$

I think this is the strongest bound on the speed of gravity to date.

They also discuss dispersion through the intergalactic medium. The speed of light in a medium depends on the frequency of the light, with low frequencies traveling slower than high frequencies. Gamma-rays have very high frequencies and should not be slowed very much

The intergalactic medium dispersion has negligible impact on the gamma-ray photon speed, with an expected propagation delay many orders of magnitude smaller than our errors on ${v}_{\mathrm{GW}}$.

To answer your question 3, the delay is measured as the time from the merger of the two neutron stars to the start of the gamma-ray burst. The gravitational waves are emitted during the inspiral phase of the binary evolution too. They are detectable for about 100 sec before the merger.

During the merger, the material at the core of the event will be very dense. Even gamma rays won't be able to propagate through it. To answer question 2, the gamma rays that were observed are probably generated slightly after the merger, outside of the newly formed single body.

Answer 2

The delay is real. It is the delay between the merger event = the end of the GW signal, and the GRB event = a peak in gamma photons. See this video.

As mentioned in this article, such a delay is expected. The GW trace the exact time of the merger, but it may take some time for the GRB jet to form and propagate. The exact mechanics are still uncertain, and I expect we will see many theoreticians aiming to exactly explain the observed 1.7 s delay.

Answer 3

Gravitational waves are produced at ever increasing amplitudes as the NS come closer together, until they are disrupted by the tidal gravity---which determines the time the "chirp" ends. The electromagnetic radiation is produced by numerous different mechanisms. The gamma-ray emission, which was observed first as the SGRB emission, is produced by a collimated jet launched from the merger-remnant (either a more massive NS, or a newly formed BH).

What produces the jet is not entirely understood, but is powered by the accretion of material onto the central object (the 'engine'). The delay between the GW and the Gamma-rays is due to the time it takes for the NS to be shredded, and then for material to accrete onto the engine. This is determined by the 'viscous' time of the material, as viscosity in an accretion disk is what ultimately feeds material onto the central object. This timescale ends up being on the order of a second.

Answer 4

Both gravitational waves and neutrinos travel unimpeded by the extremely dense matter in a collapsing star's surface. Electromagnetic radiation, including gamma rays, are scattered by it