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Predictive

Chapter 6: Running tempo2 in predictive mode

6.1 Introduction

On-line pulsar observing software and offline folding procedures require knowledge of the functional form of the pulsar's phase and pulse-period over a given time interval. Tempo1 produces "polyco" files that contain the pulsar parameters in the form of a simple polynomial expansion. The pulse phase and frequency at time T are then calculated as

 DT = 1440(T-T0)
 PHASE = RPHASE + 60F_0 DT + c_1 + DTc_2 + DT^2c_3 + ...
 FREQ = F0 + (1/60)(c_2 + 2DTc_3 + 3DT^2c_4 + ... 

Tempo2 allows this old-style "polyco" format to be produced, but also provides for a new technique ("predictor" files) that should be used for high precision timing.

6.2 Tempo1-style polycos

Tempo1 style polycos can be produced using:

 tempo2 -f 0437-4715.par -polyco "53000 53001 300 12 8 pks 1400.0" -tempo1

(use tempo2 -h for a complete list of the required parameters). The example above will request that tempo2 makes a polyco file for the pulsar PSR J0437-4715 from MJDs 53000 to 53001 with the file divided into segments each of nspan = 300 minutes. The number of coefficients for use in the fitting ncoeff = 12. The maximum absolute hour angle range for the prediction, maxha = 8 (note this is effectively a one-sided hour angle, which means that maxha = 12 implies a polyco for a full 24h period). The observatory site for the prediction is site_code = pks at an observing frequency of freq = 1400 MHz. Therefore, the format used in defining the prediction is

 -polyco "mjd1 mjd2 nspan ncoeff maxha site_code freq"

(note the use of quotation marks around the parameters). Tempo2 should produce a file (polyco.dat) that has an identical format to tempo1 polyco files. ( note: the tempo1 software switches off clock corrections in predictive mode. To emulate this the CLK flag in the parameter file should be set to CLK UNCORR).

6.3 Tempo2 style predictor files

Owing to interplanetary and interstellar dispersion, the polynomial coefficients produced by tempo1 are specific to a given observing frequency. For observations at radio wavelengths, there is usually a significant variation of instantaneous pulse phase across the observing band. If high precision is required, the form of this variation is dependent on many parts of the full timing model. Therefore, tempo2 predictor files are time and frequency dependent predictive polynomials.

The polynomial is expressed in terms of a two-dimensional adaptation of the conventional Chebyshev basis functions. These are written into an ascii text file and various pieces of software are available (click here) to process these predictor files.

The following command is run to create predictor files:

 tempo2 -f mypar.par -pred "sitename mjd1 mjd2 freq1 freq2 ntimecoeff nfreqcoeff seg_length"

where the segment length is given in seconds.

• Chapter 1: Introduction, obtaining and installing tempo2 and basic examples
• Chapter 2: Required files
• Chapter 3: Parameter files
• Chapter 4: Observation files
• Chapter 5: Running tempo2 and command-line arguments
• Chapter 6: Running tempo2 in predictive mode (polycos and predictor files)
• Chapter 7: Processing multiple pulsars simultaneously
• Chapter 8: The additional graphical interfaces and output formats available
• Appendix A: Developing the software
• Appendix B: Tempo2 error and warning messages
• Appendix C: Frequently asked questions