After the announcement of a neutron star merger on 16 October, I have been wondering how close such a merger would have to be in order to generate effects that could affect life on Earth.

For example, if one happened at the distance of Alpha Centauri, would we feel the gravitational wave? Would it be as bad as an earthquake? Could the $X$ and $\gamma$ radiation trigger mass extinctions?

After all, NGC 4993 is about $30\times10^6$ times further away than $\alpha \,{\rm Cen}$, so the signals should be about $10^{15}$ times stronger. If $\alpha\,{\rm Cen}$ is still too far, how close would a merger need to be?

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    $\begingroup$ Gravitational wave strain is an amplitude and scales as the reciprocal of distance, not distance squared. $\endgroup$ – Rob Jeffries Oct 17 '17 at 23:10

The gravitational wave strain varies as the reciprocal of distance.

The strain due to the neutron star merger was of order $10^{-21}$ (maybe $10^{-20}$ right at merger), with a wavelength of 1000 km.

Alpha Cen is about 30 million times closer, so the strain due to a NS merger would be more like $10^{-13}$.

Changing the lengths of objects by 1 part in $10^{13}$ on timescales of 0.01s is not something that could be felt.

The high energy radiation is something completely different. I will leave others to answer that in detail, But a paper by Troja et al. (2017) finds an X-ray luminosity of $10^{32}$ W for the afterglow of GW170817, which is about 12 orders of magnitude greater than the X-ray luminosity of the Sun. However, at the distance of Alpha Cen this means just a factor 10-100 increase in the X-ray flux at the Earth for a short period of time (a few weeks?), all of which would be absorbed in the atmosphere.

Gamma rays are more penetrating, but much would depend how the Earth was oriented with respect to any jet.

  • $\begingroup$ Thanks for setting me straight on gravitational wave scaling, Rob. All in all, it seems survivable. $\endgroup$ – hdhondt Oct 18 '17 at 2:09

We experience Gravitational Waves all the time with no effect to us, the Gamma Ray Burst that accompanies the GW can be lethal to a biomass.

  • $\begingroup$ That does not even come close to answering my question. $\endgroup$ – hdhondt Nov 14 '17 at 21:23

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