Appendix C. Temporal Analysis with Non-Fermi Analysis Tools

1. Photon Arrival Time Corrections

In high precision temporal analysis, photon arrival times are converted into times at which the photons would have arrived if the instrument were at a different location. When a source of interest is in a binary system, the converted times are further converted into times at which the photons would have arrived if the photons were emitted at the center of gravity of the binary system. The following table summarizes photon arrival times that are frequently used in pulsar analysis. It also shows the time system normally used for each of these arrival times.

2. HEASARC Convention on Barycentric Corrections

By default, the Fermi pulsar analysis tools automatically perform those photon arrival time corrections on the fly, based on the information available (such as solar system ephemerides and binary orbital parameters). This is to preserve original photon arrival times in event files (stored in TIME column), such that other Fermi or non-Fermi analysis tools can use the original times, for example, in order to correctly refer an instrument response for a particular photon without an extra task in an instrument response computations.

Some of existing temporal analysis tools, however, expect the converted photon arrival times to be written back to the event file, overwriting the original photon arrival times. In fact, this is a commonly-employed convention in various high-energy astrophysics missions, as HEASARC of NASA/GSFC defines and recommends an event file format for temporal analysis in which original photon arrival times are overwritten when barycentric corrections are applied. As a result, if a Fermi event file is given to a non-Fermi temporal analysis tool, the tool may not run at all. Even worse the tool may appear to run properly, but only to produce a result that is scientifically incorrect.

To use such a temporal analysis tool to analyze Fermi data, a Fermi event file must be converted in a conventional way. Namely, on barycentric corrections photon arrival times in a Fermi event file must be overwritten by the results of barycentric corrections, as expected by the tool.

3. Exporting Fermi Data for Non-Fermi Temporal Analysis Tools

One of the Fermi pulsar tools, gtbary, performs barycentric corrections on all times in a given event file, creates an output file identical to the input file, and overwrites them in the output file with their barycentric times. The times converted by gtbary include all photon arrival times (i.e., the contents of TIME column), good time intervals (in GTI extension), and FITS header keywords storing some kind of time (e.g., TSTART keyword which stores a start time of an observation). Shown below is an example on how to run gtbary to create a converted event file for use with non-Fermi temporal analysis tools.

In the following example, gtbary processes the event file named fakepulsar_event.fits using the spacecraft data file named simscdata_1week.fits for a pulsar located at the right ascension of 111.11 and the declination of 22.22, which is the location of the simulated pulsar.

When successful, gtbary creates an output file (fakepulsar_event_bary.fits in the above example) that is ready to be given to a non-Fermi temporal analysis tool that is designed to analyze an event list in the HEASARC standard temporal file format.

Sample Files

To try the examples above yourself, you can download fake data files here:

• fakepulsar_event.fits (372 kB)
• simscdata_1week.fits (2.5 MB)

The output file in the example (below) is also available for download for your comparison.

• fakepulsar_event_bary.fits (380 kB)