Fermi Gamma-ray Space Telescope

Ephemeris Computation Utility (gtephem) Tutorial

The gtephem tool is used to present the user with the best ephemeris available in a pulsar database for a given pulsar and instant of time. Note, the default solar system ephemeris for the FSSC Fermitools is JPL DE405 but not all pulsar ephemerides use this, some use JPL DE200. As such, it may be helpful to try the following examples with the command line option 'solareph="JPL DE200" ' if no valid ephemeris is found for your pulsar.

Known Issues: Some differences have been noted between output on Windows and output on Linux. The values are not qualitatively far off, but it is not yet known whether the differences are significant, or indicate a problem in the code.

Prerequisites

Sample File. To try the examples in this section, you can download the following simulated data file. The simulated data is for demonstration purposes.

Also see:

Tutorial Fermitools References
Pulsar Analysis Tutorial: Collect Ephemeris Information of Your Pulsar gtephem
Ephemeris Data File gtpulsarDb

» Fermitools References

How To Find A Spin Ephemeris For A Particular Pulsar At A Particular Time

This tool can be used to check whether an ephemeris is available for a particular pulsar at a particular moment in time. This is accomplished by setting parameter strict=yes, which causes gtephem to consider only ephemerides whose interval of validity includes the given time. If an ephemeris is available, the tool will compute and display an extrapolated ephemeris for that given moment in time, in a manner similar to that shown in the next section. If no ephemeris is available, the tool will produce output similar to the following: 

prompt> gtephem strict=yes
Pulsar ephemerides database file name[] bogus_pulsardb.fits
Pulsar name[ANY] PSR J9999+9999
Reference time at which to find/compute ephemeris[0.] 54900.0
Time format for the reference time of this ephemeris computation (MJD|ISO|FERMI|GLAST) [MJD] MJD
Time system for the reference time of this ephemeris computation (TAI|TDB|TT|UTC) [TDB] TDB
gtephem: Spin ephemeris estimated is:
gtephem:              Reference Time : 54900 MJD (TDB)
gtephem:              Ephemeris Data : Not Available
prompt>

How To Compute (Extrapolate) A Spin Ephemeris For A Particular Pulsar At A Particular Time

To find an ephemeris for a particular pulsar at a particular time, ignoring the interval of validity of the candidate ephemerides, gtephem can be invoked with parameter strict=no. (This option can be omitted because no is the default value.)

prompt> gtephem
Pulsar ephemerides database file name[] bogus_pulsardb.fits
Pulsar name[ANY] PSR J9999+9999
Reference time at which to find/compute ephemeris[0.] 54900.0
Time format for the reference time of this ephemeris computation (MJD|ISO|FERMI|GLAST) [MJD] MJD
Time system for the reference time of this ephemeris computation (TAI|TDB|TT|UTC) [TDB] TDB
gtephem: Spin ephemeris estimated is:
gtephem:              Reference Time : 54900 MJD (TDB)
gtephem:    Right Ascension (degree) : 111.11
gtephem:        Declination (degree) : 22.22
gtephem:                 Pulse Phase : 0.360227987170219
gtephem:        Pulse Frequency (Hz) : 12.3443922918935
gtephem:       1st Derivative (Hz/s) : -2.34410576e-10
gtephem:     2nd Derivative (Hz/s/s) : 3.45e-20
prompt>

How To Compute (Extrapolate) A Binary Ephemeris For A Particular Pulsar At A Particular Time

If orbital parameters for a binary orbit of a pulsar of interest are included in an ephemerides database file, the tool will use them to compute orbital effects on photon arrival times (light-travel time, gravitational time dilation, etc.), apply them when it estimates a spin ephemeris at a given moment in time, as shown in the following example. Unlike spin parameters there is no validity interval associated with orbital parameters, so the strict=yes option does not have any effect on search and computation of orbital parameters.

prompt> gtephem
Pulsar ephemerides database file name[] bogus_pulsardb.fits
Pulsar name[ANY] PSR J1834-0010
Reference time at which to find/compute ephemeris[0.] 54369.3
Time format for the reference time of this ephemeris computation (MJD|ISO|FERMI|GLAST) [MJD] MJD
Time system for the reference time of this ephemeris computation (TAI|TDB|TT|UTC) [TDB] TDB
gtephem: Spin ephemeris estimated is:
gtephem:              Reference Time : 54369.3 MJD (TDB)
gtephem:    Right Ascension (degree) : 278.571942
gtephem:        Declination (degree) : -0.180347
gtephem:                 Pulse Phase : 0.369979202747345
gtephem:        Pulse Frequency (Hz) : 1.91955412967573
gtephem:       1st Derivative (Hz/s) : -5.27493e-17
gtephem:     2nd Derivative (Hz/s/s) : 0
gtephem: Orbital position estimated is:
gtephem:              Reference Time : 54369.3 MJD (TDB)
gtephem:               Orbital Phase : 0.61474512854852
gtephem:  Orbital Delay (TDB second) : -0.477417544956552
gtephem:     Barycenter Arrival Time : 54369.29999447433397 MJD (TDB)
prompt>

Getting More Information on Computation

If gtephem is invoked with chatter parameter set to 4 or larger, it will display a brief summary of loaded ephemerides, and spin and orbital ephemerides that are used to compute an extrapolated spin ephemeris at a particular moment in time, together with an extrapolated ephemeris at the given time. This feature shows you additional information on computations being performed, which may help you understand the tool. Also, the extra information printed may contain a clue to diagnose certain types of problem. So, try a high chatter when you are unsure on the tool's behavior.

prompt> gtephem chatter=4
Pulsar ephemerides database file name[bogus_pulsardb.fits] 
Pulsar name[PSR J1834-0010] 
Reference time at which to find/compute ephemeris[54369.3] 
Time format for the reference time of this ephemeris computation (MJD|ISO|FERMI|GLAST) [MJD] 
Time system for the reference time of this ephemeris computation (TAI|TDB|TT|UTC) [TDB] 
gtephem: ==========================
gtephem: Pulsar ephemerides are loaded and/or filtered as follows:
gtephem:    Load FITSDB AUTHOR='Anonymous Tester' DATE='2009-02-11T19:12:04'
gtephem:    Filter by pulsar name 'PSR J1834-0010'
gtephem: ==========================
gtephem: Pulsar ephemerides FITS files loaded are:
gtephem: [1] PULSARDB AUTHOR='Anonymous Tester' DATE='2009-02-11T19:12:04'
gtephem:    * Load TEXTDB SPIN_PARAMETERS(FREQ) FILENAME='bogus_pulsardb_spin.txt'
gtephem:    * Load TEXTDB OBSERVERS FILENAME='bogus_pulsardb_obs.txt'
gtephem:    * Load FITSDB AUTHOR='Masaharu Hirayama' DATE='2006-03-24T15:40:16'
gtephem: ==========================
gtephem: Spin ephemerides in the database are summarized as follows:
gtephem:    1144 spin ephemeri(de)s in total
gtephem:    1 spin ephemeri(de)s for pulsar "PSR J1834-0010"
gtephem:    (Sub-selection by solar system ephemeris not requested)
gtephem:    1 spin ephemeri(de)s loaded into memory
gtephem: ==========================
gtephem: Orbital ephemerides in the database are summarized as follows:
gtephem:    19 orbital ephemeri(de)s in total
gtephem:    1 orbital ephemeri(de)s for pulsar "PSR J1834-0010"
gtephem:    (Sub-selection by solar system ephemeris not requested)
gtephem:    1 orbital ephemeri(de)s loaded into memory
gtephem: ==========================
gtephem: --------------------------
gtephem: Spin ephemeris chosen from database is:
gtephem:      Valid Since : 51671 MJD (TDB)
gtephem:      Valid Until : 52532 MJD (TDB)
gtephem:            Epoch = 52101 MJD (TDB)
gtephem:               RA = 278.571942 degrees
gtephem:              Dec = -0.180347 degrees
gtephem:             Phi0 = 0.122490413499755
gtephem:               F0 = 1.9195541400136 s**(-1)
gtephem:               F1 = -5.27493e-17 s**(-2)
gtephem:               F2 = 0 s**(-3)
gtephem: Orbital ephemeris chosen from database is:
gtephem:               PB = 156479.326538 s
gtephem:            PBDOT = 0
gtephem:               A1 = 0.7233272 lt-s
gtephem:             XDOT = 0 lt-s/s
gtephem:              ECC = 0.0001612
gtephem:           ECCDOT = 0 s**(-1)
gtephem:               OM = 0 radians
gtephem:            OMDOT = 0 radians/s
gtephem:               T0 = 52060.841007950002677 MJD (TDB)
gtephem:            GAMMA = 0 s
gtephem:        SHAPIRO_R = 0 s
gtephem:        SHAPIRO_S = 0
gtephem: --------------------------
gtephem: Spin ephemeris estimated is:
gtephem:              Reference Time : 54369.3 MJD (TDB)
gtephem:    Right Ascension (degree) : 278.571942
gtephem:        Declination (degree) : -0.180347
gtephem:     Distance (light-second) : Unknown
gtephem:                 Pulse Phase : 0.369979202747345
gtephem:        Pulse Frequency (Hz) : 1.91955412967573
gtephem:       1st Derivative (Hz/s) : -5.27493e-17
gtephem:     2nd Derivative (Hz/s/s) : 0
gtephem: Orbital position estimated is:
gtephem:              Reference Time : 54369.3 MJD (TDB)
gtephem:               Orbital Phase : 0.61474512854852
gtephem:  Orbital Delay (TDB second) : -0.477417544956552
gtephem:     Barycenter Arrival Time : 54369.29999447433397 MJD (TDB)
prompt>

Last updated: 11/09/2017