How is the redshift of gravitational wave events measured?

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SUMMARY

LIGO reported a gravitational wave event with a redshift range of 0.6 < z < 1.3, determined through a combination of pre-merger, chirp, and ring-down frequency measurements. The redshift estimation relies on the emitted frequency, which is not directly observable in gravitational waves (GWs). The methodology discussed includes fitting models to observations, as outlined in the paper by Messenger et al. (2014) and PRL 116, 061102 (2016). The degeneracy between redshift and black hole masses complicates accurate mass determination, highlighting the need for independent redshift measurements to improve precision.

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Jorrie
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LIGO reported the announced event with a redshift of 0.6 < z < 1.3. With no em radiation event reported
(AFAIK), does anyone know how they have determined the redshift?
I understand how they could measure the pre-merger, chirp and ring-down frequencies with good accuracy. In order to determine redshift one must also know the emitted frequency. What methods are available for GWs?

PS: I have found one paper so far (2014, Messenger et. al): http://journals.aps.org/prx/pdf/10.1103/PhysRevX.4.041004
 
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This is in Section VI of their paper. PRL 116, 061102 (2016)
 
You may have noticed that the range of redshifts is really broad. Same with the range of masses. This is because the redshift is pretty highly degenerate with the masses of the two black holes. But it isn't perfectly degenerate.

Basically they slapped the whole model together and fit it to observations, and showed that the observations only fit within a limited range of redshift and mass.

If we had an independent measurement of the redshift of the merger, the mass could probably have been nailed down to much greater accuracy.
 

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