How should I interpret the 'Location area (deg^2)' table on Wiki's 'List of gravitational wave observations' page? Is it in 'square degrees'?

Question

Wikipedia's page for 'List of gravitational wave observations' has a location table called 'Location area (deg^2)'. Just to be sure, if you click on 'deg^2' it takes you to the 'square degree' page....

The values range from 16 to 1651, with two different events at 1033.....

But if gravitational waves can reach us from any direction, and there are 41,253 square degrees in a sphere, why aren't any numbers higher than 1651? Out of a possible 41253? It doesn't say anything about dividing Earth's sphere into quadrants, or anything...

Also, isn't it a weird coincidence that two events occurred in the exact same direction (1033)?

Answer 1

It is quite hard to determine precisely the direction from which the GW signal is coming from, so instead of a single point in the sky we give a confidence region: check out figure 8 in https://arxiv.org/abs/1811.12907, there they show what kind of shape and size these regions have.

Note that we are not 100% certain that the signal has come from that region. Instead, we draw a 90% confidence region: we are 90% sure that the signal has come from that region.

The figure reported in Wikipedia is the size of that region: it is heavily dependent on the percentage we choose --- if we were to draw a 95% confidence region it would be larger!

These regions are quite large! For comparison, the Moon's angular size from the Earth is of the order of 0.2 square degrees. So, we are quite uncertain about the direction the GW signals come from. You can see in the figure that many regions are tall and skinny --- this reflects the fact that they were computed using only data from the two LIGO detectors; when Virgo joined it enabled the regions to be shrunk by a lot along the long axis.

Answer 2

The numbers give the size of the localization area (in square degrees), not the direction.