How did Fermi manage to see neutron star merger?
The Fermi instruments have a very wide field of view; the gamma ray burst detector covers the whole sky not occulted by the earth with low angular precision, and the Large Area Telescope covers about 1/5 of the entire sky with arc-minute precision.  I don't know if the LAT happened to be pointing in the right direction (I guess ~20% chance?) but the burst detector would at least be able to identify the timing of the gamma ray signal from the collision from nearly anywhere in the sky.