<p>The Fermi instruments have a <a href="https://www.nasa.gov/content/goddard/fermi-spacecraft-and-instruments" rel="nofollow noreferrer">very wide field of view</a>; 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.</p>

CC BY-SA 4.0
2020-11-21T03:28:14.813Z
it may be possible to augment this answer using sources found in this [comment](https://astronomy.stackexchange.com/questions/39961/how-did-fermi-manage-to-see-neutron-star-merger#comment80995_39961).
uhoh
CC BY-SA 4.0
2020-11-20T15:00:38.093Z
2020-11-20T15:00:38.093Z
antlersoft
<p>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?</p>

CC BY-SA 4.0
2020-11-21T03:27:18.187Z
related: [“Who saw” the binary neutron star merger first? What was the sequence of events? (GRB/GW170817)](https://astronomy.stackexchange.com/q/23187/7982) and [Is there a standardized “Astronomical Alert” system?](https://astronomy.stackexchange.com/q/32778/7982) (also [1](https://astronomy.stackexchange.com/q/38733/7982), [2](https://astronomy.stackexchange.com/q/28404/7982))
uhoh
<gravitational-waves><neutron-star>
How did Fermi manage to see neutron star merger?
Elephantik
# How did Fermi manage to see neutron star merger?

(Asked by: username_0 on 2020-11-20)

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?

**Tags:** gravitational-waves, neutron-star

> **Comments:**
> **username_1**: related: [“Who saw” the binary neutron star merger first? What was the sequence of events? (GRB/GW170817)](https://astronomy.stackexchange.com/q/23187/7982) and [Is there a standardized “Astronomical Alert” system?](https://astronomy.stackexchange.com/q/32778/7982) (also [1](https://astronomy.stackexchange.com/q/38733/7982), [2](https://astronomy.stackexchange.com/q/28404/7982))  

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## 1 answers

### Answer by username_2 on 2020-11-20. Score: 2 | Accepted answer

The Fermi instruments have a [very wide field of view](https://www.nasa.gov/content/goddard/fermi-spacecraft-and-instruments); 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.

> **Comments:**
> **username_1**: it may be possible to augment this answer using sources found in this [comment](https://astronomy.stackexchange.com/questions/39961/how-did-fermi-manage-to-see-neutron-star-merger#comment80995_39961).  

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