Fermi Gamma-ray Space Telescope

Pulsar Analysis Overview

Many of the sources that the LAT detects, particularly close to the Galactic Plane, may be gamma-ray pulsars. Determining whether a source is a pulsar entails searching for a periodicity in the times of arrival of events from the source. The photon flux detected by the LAT from most pulsars is low; for example, one photon is expected to be detected from every 500 rotations of the Crab pulsar. Therefore the analysis of most suspected pulsars requires long accumulations of the counts from the region surrounding the source. This analysis is very sensitive to the pulsar ephemeris (sky position and timing information).

The analysis of a candidate pulsar begins with the extraction of events from the region surrounding the LAT source. In contrast to the likelihood analysis, only the events from close to the source direction (typically 1 degree or less) are required because the surrounding sources and background do not have to be modeled at this stage of the analysis.

The pulsar community generally relies on non-Fermi specific tools like TEMPO2 with the Fermi plug-in, PINT, and PRESTO for the bulk of pulsar analysis and only use the Fermitools for initial data reduction. This is the current recommendation for pulsar analysis.

The Fermitools only offer basic pulsar functionality. The Fermi observatory is not an inertial frame, and changes in its position during the observation will affect the photons' arrival time. The 'barycentric correction' compensates for this motion. The tool gtpphase calculates this correction and assigns a rotational phase to each of the selected events.

In order to allow a blind search for periodicity using software outside the Fermitools, the gtbary tool applies the barycentric correction to arrival times for a selected data set and replaces the original event arrival time with an equivalent barycentric arrival time. CAUTION: running gtbary changes the event time and does not retain the original time; therefore a dataset of this type cannot be used for spectral analysis. The barycenter-corrected events can now be searched for pulsations.

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