Chapter 6: Running tempo2 in predictive mode
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
-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.