Fermi Gamma-ray Space Telescope

Spin Pulse Phase

The gtpphase tool calculates the pulsar phase for each count in an event file and adds a column containing that information. This is a standard operation, but this Fermi-specific tool is provided to interact with the Fermi-specific files.

The timing ephemeris can be input manually (e.g., from a published result or the result of a periodicity search), or may be extracted from the Fermi pulsar ephemerides database.

The tool adds the rotation phase to each count in the event file; each count is a row in the FITS file, and gtpphase adds a PULSE_PHASE column. If the pulsar database file includes a binary ephemeris for the pulsar, the spin phase includes the effect of the binary motion on the spin phase. No information is lost, but the input and output file names are the same. If the tool runs successfully, no additional output results.

The standard Fermitools assume that the pulsar ephemeris can be described by three timing terms: pulsar period, period first derivative, and period second derivative (or the equivalent terms in frequency space). Some pulsars have sufficiently irregular timing that this three-term timing solution is not good enough to produce the best results. An alternative tool, based on the TEMPO2 software used by the pulsar timing community, is also available for such cases.

Standard FITS tools can now be used to manipulate and display the pulse data. For example, fhisto can bin the pulses, and the resulting pulse profile can be plotted with fplot. Similarly, cuts on the data could be made with fselect.

A number of statistics have been created to test whether a time series is periodic with a specified frequency or period:

  • Chi-squared test (Leahy et al. 1983, ApJ, 266, 160; Vaughan et al. 1994, ApJ, 435, 362)
  • Zn2 test (Buccheri et al. 1983, A&A, 128, 245)
  • Rayleigh test (equivalent to Zn2 test for n = 1)
  • H test (De Jager et al. 1989, A&A, 221, 180)
  • Bayesian approach (Gregory and Loredo 1992, ApJ, 398, 146; Gregory and Loredo 1996, ApJ, 473, 1059)

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