GW170729 was gravitational wave signal's detected by the Virgo and LIGO detectors. I read the following in a news report:
Known as GW170729, in accordance with the date of its discovery (in US format), the gravitational wave that astronomers can listen in to was caused by the formation of a black hole more than 80 times as massive as our sun and about nine billion light years away. (Source)
I am specifically interested in the claim that it comes from 9-billion light years away. I have been unable to find a good source to back this claim up.
How accurate is this claim? Please provide a source for your answer and where applicable, a simple explanation of how the data backs this up. More information about this event can be found here.