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 Fermi mission has initiated a monitoring campaign of the radio and X-ray pulsars that are candidates for pulsed gamma-ray emission. Ephemerides resulting from this campaign are provided in a database (D4) available from the FSSC website at http://fermi.gsfc.nasa.gov/ssc/data/p7rep/access/lat/ephems/index.html. The user is prompted for the location of this database by the Science Tools routines requiring it.

The Science Tools package includes a suite of pulsar analysis tools to determine whether a LAT source is a pulsar known at other wavelengths, refine the ephemeris, and assign spin and orbital phases to every event. Users can then use other Fermi science tools and standard FTOOLs to analyze the pulse-dependent spectrum, plot the pulse profile, etc.

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 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 FSSC Science Tool gtpphase calculates this correction and assigns a rotational phase to each of the selected events. The gtptest routine can then be used to evaluate the significance of possible periodicity.

Although the typical pulsar analysis uses gtpphase with a known pulsar ephemeris, there may be cases in which the pulsar ephemeris is unknown or is insufficiently well known. Other FSSC Science Tools can be used for pulsar analysis in such cases. In some cases the source will be suspected of being a known pulsar, with a candidate spin ephemeris. Thus gtpsearch performs a limited search of ephemerides around the candidate ephemeris, which can be entered by the user or extracted from the Fermi-provided database. Note that even when the LAT source has been definitively identified as a known pulsar, the ephemeris can be refined using gtpsearch. A 'blind search' tool, gtpspec, is also provided. Note that gtpspec uses a Fast Fourier Transform technique, not the time-differencing technique used by the LAT instrument team (Atwood et al. 2006, ApJL, 652, L49).

In order to allow a blind search for periodicity using software outside the FSSC Science Tools, 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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