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When the GW170817 neutron stars merger was observed by LIGO/Virgo, the Fermi gamma-ray telescope observed the event 2s after the merge. How did it know where and when to look? It must take some time to rotate the satellite and I guess that evaluation of data from LIGO/Virgo also isn't instantaneous. Or is it? And does it automatically send suspicious signals to the Fermi operation centre to point it to the right location?

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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.

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  • $\begingroup$ it may be possible to augment this answer using sources found in this comment. $\endgroup$
    – uhoh
    Commented Nov 21, 2020 at 3:28

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