Fermi Gamma-ray Space Telescope

Temporal Effects of the LAT Observing Strategy

When searching for periodic modulation in a LAT light curve, it is important to be aware of artifacts that may be present. We list the main periods known, give some comments on their origins and, where possible, information on how to minimize these. In any case, if a period appears to be present in a light curve, we recommend that the same analysis be carried out on other sources, preferably at similar sky locations.

  • ~ 96 minutes

    This is the orbital period of the Fermi satellite. It is slowly decreasing as the spacecraft orbit gradually decays.

  • ~ 3.2 hours

    This is the standard LAT sky survey repeat period. It is equal to 2 spacecraft orbits. As the satellite orbit slowly decays, small changes are occasionally made to the survey period.

  • ~ 1 day

    The underlying cause of modulation at a period of 1 day has not yet been investigated in detail. However, modulation on timescales near 1 day was also seen in RXTE All-Sky Monitor data (e.g. Farrell et al., 2005, PASA, 22, 276). The modulation may not be at exactly 1 day, and can be shifted by an amount that reflects low-frequency modulation of the light curve.

  • 27.3 days and/or its harmonics

    The Moon is a fairly bright gamma-ray source. If a light curve is obtained for a position close to the path of the Moon, then it is possible for lunar emission to affect the light curve. This will occur on the Moon's sidereal period of 27.32 days. Since the Moon will only be close to a source briefly, any modulation will appear as brief flaring, and a power spectrum will likely show modulation at high-order harmonics of 27.3 days. (e.g. 5.46, 4.55, 3.90, 3.03 days) There is user contributed software, available from the FSSC that can add lunar coordinates to a spacecraft file. With these, it is possible to use gtmktime to select data only when the Moon is not close to a source. It is recommended to exclude times when the Moon was closer than 5 degrees to the region being analyzed.

  • 53.4 days

    This is the precession period of the orbit of the Fermi spacecraft.

  • 91 Days (Quarter of a Year)

    For aperture photometry for certain positions close to the position of a bright source, such as the Vela pulsar, it is possible to see modulation at a period of 1/4 of a year (i.e. ~91 days). This results from the 4-fold symmetry of the LAT point spread function, and a systematic change in spacecraft orientation during the course of a year. The "arms" of the PSF can systematically sweep in and out of an aperture at certain locations. Modulation on this period is not expected to be seen in light curves derived from likelihood analysis. It will also not be a major effect unless there is a very bright nearby source.

  • 1 year and/or its harmonics

    For light curves obtained for sources close to the path of the Sun, solar gamma-rays may affect a light curve. This may reveal itself as apparent modulation on a period of 1 year. In a power spectrum, harmonics of this period are likely to be seen. The spacecraft files already contain the coordinates of the Sun, and these enable times possibly affected to be excluded by using gtmktime. It is recommended to exclude times when the Sun was closer than 5 degrees to the region being analyzed.

Last updated by: Elizabeth Ferrara - 11/06/2012