Fermi previously included taget-of-opportunity observations as part of its observing program. However, in early 2018 the Fermi spacecraft experienced an anomaly with its -Y Solar Array Drive Assembly (SADA), which left the -Y solar panel unable to rotate a nd necessitated the creation of a new observing strategy. Please see this page for more information regarding the event and Fermi's current survey observation strategy. Due to this anomaly target-of-opportunity observations are no longer performed. Below we give information on how TOOs were previously undertaken, and this should therefore be considered for historical purposes/reference only.
The Fermi Project Scientist (or their designee) could declare a Target-of-Opportunity (TOO) pointed observation if warranted, prompted by observer request. Because of the large field of view of the instruments and the characteristics of the gamma-ray sky, this happened infrequently (see below) when in the standard sky survey observing mode.
The Project Scientist would consult with the FSSC regarding the feasibility and impact of a TOO observation. The TOO observation was implemented within 6 hours after the Project Scientist authorized the observation; the actual time was typically much shorter. Not all TOO observations needed to be implemented as soon as possible, and implementation could be requested during the next business day.
In the process of evaluating the impact of a TOO, the Project Scientist reviewed any scheduled or ongoing multiwavelength observations that had been reported to the FSSC multiwavelength reporting page, and it was attempted to minimize the impact. If the Project Scientist deemed those scheduled observations to be of higher priority than the TOO, they generally denied the request.
Survey mode observations provided coverage of the entire sky every 3 hours. This ensured continuous monitoring of all sources on timescales greater than 3 hours, guaranteeing gamma-ray data for a wide range of studies. During this three-hour period, each region of the sky was observed for around 30 minutes, as a result of the very wide field of view of the LAT (>2.5 sr). Sky survey data from the LAT were likely to address the needs of the great majority of studies, including those that would commonly require TOO observations at other types of facilities.
The scientific motivation for TOO pointed mode observations with Fermi therefore had to be very compelling and strongly justified.
Pointed mode observations could provide around a factor of two increase in sensitivity for a given time interval. Due to Fermi's low-Earth orbit, nearly all pointed observations were interrupted by Earth occultation. Pointed observations also had a detrimental effect on coverage of the rest of the sky, which could hurt multiwavelength campaigns and uniformity of time monitoring studies,