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LAT Target of Opportunity Observations

Observers can request a Target-of-Opportunity (ToO) pointed observation of the Large Area Telescope (LAT). Requests will be evaluated by the Fermi project scientist in consultation with the Fermi Science Support Center (FSSC) planning team to assess the feasibility and impact of the observation.

Due to the large field of view of the LAT and the characteristics of the gamma-ray sky, ToO observations are not frequently needed. Sky survey observations typically provide coverage of the entire sky every three hours. These data meet the needs of most studies, including those that would typically require ToO observations at other facilities. The scientific motivation for ToO pointed observations with Fermi must be highly compelling and strongly justified.

ToO requests may be submitted through the Fermi ToO ARK Interface. Queries about submitting requests should be sent by email to the ToO planning team. General questions are welcome through the FSSC help desk. Guidelines are provided below.

See Approved ToOs and the Observatory Timeline for information about ToOs performed by Fermi, and The Observatory for general information about Fermi observations and observing types.

Guidelines for TOO Requestors

  • Requestors should provide the location of the target, the duration desired for the observation, and a scientific justification with the expected impact of the ToO.
  • Diverting from the planned survey requires a strong science justification with measurable improvement from ToO observations. Exposure can increase by up to a factor of 2. The ToO planning team expects to accept around 1-3 observations in a year.
  • Responses to TOO requests are only available during Eastern time business hours (9 am - 5 pm ET).
  • Approved TOOs can typically be implemented within 24-48 hours.
  • TOO observations can be requested for 2-3 days or up to more than a week depending on the behavior of the target and the science goals.
  • Communication between the requestor and the TOO planning team is important to optimize the observation and then monitor progress once begun.
  • The location and timing of an observation will be modified to optimize observations and respect observatory constraints. Requestors should provide enough information about the science measurement goals to enable planning flexibility, including estimations for the duration of interest for the observation and detail about the most critical times of interest within that window.
  • Locations close to the Sun direction are not accessible for targeted observations; those opposite to the Sun direction are favorable during partial sinusoidal survey observations.

  • Q: Can TOOs be requested through the GI program?
    A: Yes. This works well for events expected at known times or with some expected rate of occurrence and a duration of several days.

    Q: Do TOOs have to be requested through the GI program?
    A: No. Requests for TOO observations can be made without a GI award.

    Q: For how long can pointed observations be requested?
    A: There is no hard limit on the duration of a ToO observation. Practically, most observations are less than a week. Keep in mind that LAT pointed observations of most of the sky are interrupted by the Earth during each approximately 95-minute orbit.

    Q: How much will the exposure improve for my desired target?
    A: Pointed mode observations can provide approximately a factor of two increase in sensitivity within a given time interval. Fermi's low-Earth orbit imposes periodic interruptions by Earth occultation that limit continuous observation of most of the sky.

    LAT observes the entire sky typically within 3 hours during survey observations. A target will spend about 30 minutes of that time on average in the LAT field of view. The exposure increase from pointed observations depends on the location of the requested target, the time of the observation, and the details of the scheduled survey.

    During two-sided rocking mode observations, a factor of 2 increase in exposure may be possible.

    During partial sinusoidal survey observations, the cadence of the survey decreases to 90 minutes with coverage of about 85% of the sky. The remaining fraction of the sky is not observed for a few days to a week depending on the time of year.

    If a target is too close to the Sun, the exposure can't be improved.

    If a target is in the opposite direction to the Sun, and the observatory is in partial sinusoidal survey mode, exposure may be restored at times when no exposure was available in the planned survey. The June 2025 ToO is an example of this scenario.

    Q: How will requests be evaluated?
    A: The project scientist will consider observation feasibility, scientific impact and impact on the rest of the sky. Pointed observations reduce coverage of the rest of the sky, potentially affecting multiwavelength campaigns and the uniformity of time-monitoring studies.

    The science support center planning team will identify an optimal offset from the target position given observatory constraints. The science support center planning team will compute the expected time the proposed target will spend in the LAT field of view for a pointed observation at that offset. The time on the desired source can be compared to the planned survey. The expected outcome of the observation will be compared to the impacts on the rest of the sky. Activity of interest from other sources and multiwavelength campaigns will be considered in the decision. ToO observations must also pass a constraint review to be approved.

    Q: How can I assess the feasibility of a desired observation?
    A: Positions toward the Sun cannot be targets for pointed observations. ToO performance is best for sources further than 30 degrees from the Sun.

    Q: What observations are most likely to be accepted?
    A: Observations that allow a meaningful improvement in the gamma-ray measurements of a high interest event over a few days are good candidates for a ToO. A multi-day transient event that falls in a hole in the exposure for the scheduled survey but can be exposed through pointed observations is a good candidate for a ToO. Other scenarios may have merit and should be explored with project science and the ToO planning team.

  • For more information on coordinating observations and campaigns with the LAT, please contact the LAT Multiwavelength Coordinating Group.

  • ToO observations were suspended in 2018 following the anomaly with the -Y Solar Array Drive Assembly (SADA), which left the -Y solar panel unable to rotate and necessitated changes to observing program to maintain sufficient spacecraft power and thermal control.

    In 2025, the mission developed a restored ToO capability to address the operational constraints from the solar array drive failure. The new procedure enables the observatory to adjust the observation pattern while adhering to power and solar array constraints. The restored ToO capability was tested in January 2025, and the first ToO under the new system was successfully carried out in June 2025.