Since the start of science operations of the Fermi mission in August 2008, the default observation mode has been an all-sky survey, optimized to provide relatively uniform coverage of the entire sky every three hours. However, Fermi is capable of flexible survey mode patterns, and inertially pointed observations both of which allow increased coverage of selected parts of the sky. The survey and observation modes can be combined or modified to execute alternate observing strategies that more effectively accomodate specific science drivers, such as a desire for increased sensitivity to flaring sources in the northern hemisphere, or monitoring sources near the galactic center. As we approach the 5-year anniversary of the Fermi launch, it is time to consider new approaches that will continue Fermi's success for many more years.
To help guide future observation plans, the Fermi Project is soliciting "white papers" from the scientific community. These should detail specific alternative observing strategies and discuss, in a quantitative way, the scientific benefits. We emphasize that this solicitation differs from a call for proposals, in that it is not a competitive peer review. Multiple ideas, or combinations thereof may ultimately be implemented. Continuing dialogs between any groups involved and the Fermi Project are also encouraged.
Current Fermi observing modes, including the predominant rocking survey mode, are describe below.
After exceeding a pre-determined threshold for a bright GBM-detected GRB a 2.5 hour pointed Autonomous repoint observation (ARR) at location the GRB is initiated onboard the spacecraft as an inertial pointing. A limb following mode is used while the target location is occulted by the Earth.
A Target of Opportunity (TOO) observation is an unplanned pointed observation deemed immediately significant for Fermi. Typically the TOO interupts the scheduled observing mode as less than a 1 week long observation. The TOO transitions to a planned pointed observation if the duration is more than a few days. A limb following mode is used while target location is occulted.
This mode is simply a planned inertial pointing for a specific target. Survey mode or a secondary target pointing is employed when the primary target location is occulted in lieu of limb following mode.
During the pointing, the target is placed at least 10 degrees from the LAT boresight to avoid possible systematics. The default planned pointed mode adopts a target offset of 10 degrees in Dec and 5 degrees in RA. The offset direction in Dec is chosen to move closer to the orbit equator, and an offset of 5 degrees in RA to mitigate an acquiring target issue.
In survey mode the LAT's pointing is currently offset 50 degrees from the zenith perpendicular to the orbital plane. Once per orbit the pointing is rocked to the opposite side of the orbital plane according to a predefined rocking profile. Between rocking once per orbit and the ~55 day precession of the orbit, the sky exposure is uniform. A list of all survey profiles used in the mission along with additional discussion can be found here.
The rocking profile is defined as a 17-point profile of time, rocking angle pairs as illustrated in the figure.
The allowed rocking angles are between -60 and 60 deg, but thermal issues can become important if substantial time is spent less than 50 degrees from the zenith. The timing of the slew between +-50 deg must avoid orbit noon/midnight (i.e. cannot slew only w.r.t. celestial coordinates).
Modified rocking profiles can be utilized to emphasize coverage of selected regions of the sky. For example, rocking north only would increase overlap with VERITAS (or south only for HESS). Fermi has also been operated to include the nadir direction in the rocking profile to increase LAT detections of TGFs. Pointed mode and survey mode observations can also be combined.
For Fermi the minimum Earth Avoidance Angle (EAA) is 5 degrees. Further, the LAT boresight does not get closer than 1 EAA to the Earth limb during pointed mode observations.
The Limb Following Angle (LFA) is 45 degrees. When in limb following mode, the spacecraft tracks the limb starting 1 EAA from the limb, reaching EAA+LFF at the mid-point and then going back to 1 EAA. Maneuver speed is designed to have the spacecraft get back to 1 EAA, just as the target emerges from occultation.
Examples of potential observing strategies are given below. They include the spacecraft (FT2) files used to generate the all sky exposure maps and a quantitative evaluation of the results. When appropriate, observing strategies can be evaluated according to the following considerations.
It's also helpful to make a histogram of exposure in equal area bins to get a sense of what fraction of the sky wins and what fraction looses (and by how much). Integrating the exposure on the sky (with suitable zenith cut) and comparing with standard survey mode provides a measure of the efficiency of the observation (i.e. how much exposure is lost due to zenith cuts).