Fermi Gamma-ray Space Telescope

Pulsar Ephemerides Databases


An ephemeris includes a time origin, which determines when the phase is 0 and when the instantaneous frequency is equal to the constant part of frequency (i.e., the frequency is a Taylor series around the time origin). The standard pulsar ephemeris for the Fermitools includes the pulsar frequency and its first two time derivatives.

Because the spin frequencies of most pulsars vary with time, an ephemeris is valid for a specific time period. Even when the ephemeris includes the derivatives of the frequency, the ephemeris validity is limited by 'glitches' and other noise in the pulsar rotation, and inaccuracies in the determination of the frequency and its derivatives. Because the LAT detection rate of pulsar photons is low and accumulation times of pulsar counts are long, analysis of LAT pulsar data is particularly sensitive to changes in the pulsar frequency; therefore you should ensure that the pulsar spin ephemeris is applicable to the data time range.

The pulsar ephemerides files may include multiple names for a given pulsar, such as a colloquial name (e.g., 'Crab'), the 'B-name' and the 'J-name.' However, the exact spelling (including spaces) of one of these names must be used. You can get pulsar ephemeris information from the FSSC Web server at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/ephems/index.html. The ATNF pulsar database also has some capability to generate an ephemeris

Querying a Pulsar Database

The primary purpose of the gtephem tool is calculating the spin ephemeris of a pulsar at a user-specified reference time. But as the Ephemeris Computation Utility analysis thread shows, this tool can serve many additional purposes.

When the gtephem tool is run, the tool prompts for the file with the pulsar ephemeris, the pulsar name and the reference time. The tool then calculates the spin ephemeris for this reference time; the spin ephemeris output to the screen includes the pulse phase, pulsar frequency (Hz), and the first and second frequency derivatives. If the ephemeris database file includes an ephemeris for a binary orbit, the effects of the binary orbit on the spin ephemeris at the reference time will be included.

The tool returns an error message if the database does not include a pulsar with the input name. By including 'strict=yes' on the command line, the tool will provide a spin ephemeris only if the spin ephemeris for the pulsar is valid at the reference time. Thus gtephem can be used to ensure that the pulsar ephemerides file includes valid data for a pulsar. Note that if the tool is not run with 'strict=yes' on the command line (the default is 'strict=no') then the tool will extrapolate the spin ephemeris beyond the valid time range. Orbital ephemerides are considered to be always valid.

Running gtephem with 'chatter=4' on the command line provides not only the spin ephemeris at the reference time, but the spin and orbit ephemerides in the database file. Thus using this option you can query the database.

Manipulating Pulsar Ephemerides Files

The Fermi tool gtpulsardb creates a new pulsar ephemerides file by extracting ephemerides from one or more pulsar ephemerides files; see the Ephemeris Data File analysis thread for examples.

The ephemeris for a specific pulsar can be extracted from the input pulsar ephemerides file and written out to the output pulsar ephemerides file by entering 'NAME' at the 'Filter ephemerides' query and then entering the pulsar name. Note that the input file may have multiple names for a given pulsar but a name must be entered exactly as spelled in the input file.

The ephemerides for all pulsars that have valid time ranges during some portion of a user-specified time range are extracted by entering 'TIME' at the 'Filter ephemerides' and then providing the time range. Note that the valid and user-specified time ranges must only overlap for the pulsar ephemeris to be extracted; one need not be a subset of the other.

As for most FTOOLS, when queried for an input file you may enter the name of an ASCII file with a list of input FITS files. Thus instead of entering the name of a specific FITS-format pulsar ephemerides file, you may enter '@xxxx' where 'xxxx' is the name of an ASCII file with the names of FITS-format pulsar ephemerides files, each on a separate line. The gtpulsardb tool creates a pulsar ephemerides file with ephemerides of all pulsars in all the pulsar ephemerides files that meet the filtering condition; no filtering is an option. Thus gtpulsardb can concatenate a group of pulsar ephemerides files into a single file, with and without any filtering.

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