GW170818

GW170818: two black holes merging about 1080 Mpc away, localized to 39 square degrees at 90%.

Arrived

2017-08-18

What merged

two black holes

Distance

1080 ± 430 Mpc

50% credible region

10.8 square degrees

90% credible region

39.2 square degrees

Found afterwards. It arrived the day after GW170817, while every telescope that could was on the kilonova, and no alert went out for it: too quiet at Hanford and Virgo to be caught as it happened, it was found in the archived data by an analysis run later. Two black holes of about thirty-five and twenty-seven solar masses, 1,080 Mpc away — and, with all three detectors contributing, the best-pinned pair of black holes of the first two runs, at 39 square degrees.

Those two areas are credible regions rather than a boundary: the smaller holds half the probability and the larger nine tenths, and the tenth left over really is outside it. There is no right ascension and declination here because there is none to give — open it in the app to see the patch itself.

GWTC-1 sky localization for GW170818 — LIGO Scientific Collaboration and Virgo Collaboration

Open it in Bhagol — opens the app if you have it, and this page if you do not.