Fermi Gamma-ray Space Telescope

LAT Pointed Observations

Fermi includes pointed observations as part of its regular observing program. During the initial, somewhat more than 5 years, of operations Fermi operated primarily in a sky-survey mode with only occasional pointed observations. In December 2013 Fermi transitioned to a new observing strategy where sky survey was interspersed with pointed observations to increase exposure around the Galactic center. For normally scheduled LAT observations, Fermi's attitude is controlled by a weekly timeline that is generated by the Fermi Science Support Center (FSSC) and then included in uploads to the Fermi spacecraft by the Mission Operations Center (MOC).

Pointed observations are performed for several reasons that include:

  • Regular monitoring covering the Galactic center.
  • Accepted proposals from the annual proposal cycle that included requests for pointed observations.
  • Observations in response to accepted Target of Opportunity (ToO) requests. These include both observations that are motivated by already accepted proposals, and completely new observations.
  • Autonomous observations of gamma-ray burst locations that have durations of a few hours.

Pointed observations using the LAT are affected by two factors that are particular to this instrument. These are that (i) the LAT has a very wide field-of-view (FoV) and the Earth is a bright source of gamma rays (ii) the optimum LAT response is obtained by off-setting the pointing direction by a few degrees from the center of the FoV. However, there are no solar constraints on LAT observations, and the LAT can be pointed directly at the Sun. Because of the wide FoV, observations of a target will generally not be undertaken when the source is too close to the Earth limb. During times when a target is not visible because of Earth angle constraints, during regular observations Fermi will undertake other pre-planned observations. Typically these will be sky-survey observations, although pointed observations of another target are possible.

Pointed observations performed for ToOs can be undertaken in two different ways. For the fastest changes to Fermi observations a "ToO order" is sent by the FSSC to the MOC who upload this to the spacecraft. In this mode Fermi points directly at the commanded location with the spacecraft automatically executing "limb-tracing" when the target is too close to the Earth, with observations of the target resuming when the Earth angle constraint is no longer violated. When sufficient time is available, ToO observations are preferably obtained by uploading a modified timeline to the spacecraft. In this case, typically pointed observations of the target will be interspersed with, for example, sky survey observations.

Exceptional Types of Pointed Observations

In exceptional cases the constraints on making observations close to the Earth limb may be overridden for special purposes. For example, the Earth limb has been deliberately observed in order to obtain a high flux of gamma rays for calibrations purposes. At some times in the mission, short weekly observations of the orbit poles were made to include Earth limb data to also aid in instrument calibration. Also, Fermi has at times been pointed directly at the Earth to look for Terrestrial Gamma-ray Flashes.