ParametersChapter 3: The parameter files3.1 The basic timing model parametersThe parameter files (that contain the pulsar timing model and various instructions for the tempo2 fitting routines) have the same form as in the earlier tempo implementation. Each of the pulsar parameters has a label, a value and may have an uncertainty on the value and a flag indicating whether tempo2 should fit for this parameter or whether the parameter should be held constant (0 = default = hold constant; 1 = fit). The parameters labels are described in Table 3.1 at the bottom of this page. An example of a parameter file for PSR J04374715 taken from the ATNF pulsar catalogue (by selecting the ephemeris option): PSRJ J04374715
RAJ 04:37:15.7865145 1 7.000e07
DECJ 47:15:08.461584 1 8.000e06
DM 2.6469 1.000e04
PEPOCH 51194.000
F0 173.6879489990983 1 3.000e13
F1 1.728314E15 1 1.600e20
PMRA 121.438 6.000e03
PMDEC 71.438 7.000e03
BINARY DD
PB 5.741046 1 3.000e06
ECC 1.9186E5 1 5.000e09
A1 3.36669157 1 1.400e07
T0 51194.6239 1 8.000e04
OM 1.20 1 5.000e02
OMDOT 0.016 1.000e02
START 50640.928
FINISH 52088.897
CLK UTC(NIST)
EPHEM DE200
PBDOT 3.64E12 2.000e13
TZRMJD 51204.64376750220085
TZRFRQ 1413.400
TZRSITE 7
RM +1.5 5.000e01
PX 7.19 1.400e01
SINI 0.6788 1.200e03
M2 0.236 1.700e02
This indicates to tempo2 that all parameters should be held fixed except for the astrometric parameters (RAJ, DECJ), pulse parameters (F0, F1) and the Keplerian orbital parameters (PB, ECC, A1, T0 and OM). Uncertainties are given on many of the parameters. In more detail, a pulsar which has a spin period of P0=1.23456 s and no fitting is required then use: P0 1.23456 To request that tempo2 fits for this parameter: P0 1.23456 1 or to include an uncertainty on the measurement (which is ignored by the main tempo2 software) P0 1.23456 1 0.00003 Other commands may be given in parameter files that control the algorithms used by tempo2. Tempo2 only required the following parameters: PSRJ, DM, F0, PEPOCH, RAJ and DECJ. It is also possible to provide the pulsar parameters in the oldstyle tempo format where the arrival times and the parameters are given in the same file. Details of this mode (which we do not recommend) are given here. 3.2 Phase jumpsIt is often necessary to fit for a constant offset between two sets of arrival times. For instance, the templates used to determine arrival times at different observatories may be perfectly aligned. The command "JUMP" in the parameter file can be used to define such jumps:
JUMP i PKS_DFB 0.234 1 JUMP FREQ 1400 1500 0 1 JUMP TEL PKS 0.342 which would initialise a jump for all observations with the "i" flag set to "PKS_DFB" with the value of 0.234 and then fit for the jump. A jump would be included (and fitted) for all observations with frequencies between 1400 and 1500 MHz and a set (i.e. not fitted) jump would be included on all data observed using the "PKS" telescope. 3.3 Removing timing noiseEven with accurate spin and positional parameters, the timing residuals for some (particularly the young) pulsars contain remnant structures. Some of these structures are understood; cusps, for instance, signify sudden changes in the pulsar's spin rate during a glitch, sinusoidal oscillations can represent unmodelled companions (such as planets) or the pulsar precessing. However, many of the structures seen in the residuals are still not understood and are known as "timing noise". To obtain the most accurate pulsar's positional and proper motion parameters (and dispersion measure) it is essential to remove this timing noise. Traditionally this has been carried out by fitting higher order pulsar rotational derivative terms. However, Hobbs et al. (2004) described an improved method that used the fitting of harmonically related sinusoids. Such sinusoidal terms can be included in tempo2 parameter files. WAVE_OM 0 1 WAVE1 0 0 WAVE2 0 0 WAVE3 0 0 would allow tempo2 to select the fundamental frequency for the sinuosoids based on the dataspan (see the algorithm described in Hobbs et al. 2004) and subsequently fit for 3 sine and cosine waves. 3.4 Coping with badly wrong ephemeridesUnder normal circumstances, tempo2 should be able to predict exactly the number of turns between any two observations, so that the residual phase error is considerably less than one turn. In early stages of timing a pulsar, during the process of "phase connecting" the data, this may not be true, and phases may "wrap". tempo2 provides several tools to help in this situation. Most simply, one may add the PHASE command to an observation file. Adding PHASE n tells tempo2 that at that point, n extra phase turns occurred compared to what tempo2 would otherwise assume. Thus PHASE 1 has the effect of adding one full turn to all following arrival times. Naturally as the ephemeris changes the number of turns tempo2 would assume will change, and these will frequently need to be added or removed (hopefully removed). A more automated approach is to use one of the modes of the TRACK command. TRACK 0 is the default mode of tempo2. TRACK 1 tells tempo2 to assume that the phase error of each residual is within onehalf turn of the previous residual, rather than within onehalf turn of zero. This only makes sense if the residuals are provided to tempo2 in time order, though tempo2 only warns about this condition. TRACK 1 uses several data points to guess the correct number of turns. TRACK 2 allows a number of turns to be specified for each TOA using the "pn" flag; it behaves similarly to the PHASE command. Table 3.1: Pulsar parameters than can be entered in a parameter file
Note that PBBOT, XPBDOT, and A1DOT have special handling: the units are natively s/s or lts/s, but if the value is larger than 1e7, it is scaled down by 1e12; you may therefore see values like 100 to mean 1e10. When PBDOT or other derivatives or orbital quantities are specified, PB, A1 and so on are the binary parameters at the time T0 (not PEPOCH).
