4.3 Binary pulsars4 Pulsar Timing4.1 Observing basics

4.2 The Timing Model 

Ideally, in order to model the rotational behaviour of the neutron star, we require TOAs measured by an inertial observer. Due to the Earth's orbit around the Sun, an observatory located on Earth experiences accelerations with respect to the neutron star. The observatory is therefore not in an inertial frame. To a very good approximation, the centre-of-mass of the solar system, the solar system barycentre, can be regarded as an inertial frame. It is standard practice [79] to transform the observed TOAs to this frame using a planetary ephemeris such as the JPL DE200 [144]. The transformation is summarised in the following equation as the difference between barycentric (tex2html_wrap_inline2173) and observed (t) TOAs:


Here tex2html_wrap_inline2177 is the position of the Earth with respect to the barycentre, tex2html_wrap_inline2179 is a unit vector in the direction towards the pulsar at a distance d, and c is the speed of light. The first term on the right hand side of this expression is the light travel time from the Earth to the solar system barycentre. For all but the nearest pulsars, the incoming pulses can be approximated by plane wavefronts. The second term, which represents the delay due to spherical wavefronts and which yields the trigonometric parallax and hence d, is only presently measurable for four nearby millisecond pulsars [87Jump To The Next Citation Point In The Article, 41Jump To The Next Citation Point In The Article, 136Jump To The Next Citation Point In The Article]. The term tex2html_wrap_inline2187 represents the Einstein and Shapiro corrections due to general relativistic effects within the solar system [19Jump To The Next Citation Point In The Article]. Since measurements are often carried out at different observing frequencies, each with its own dispersive delays, the TOAs are generally shifted back to the equivalent time that would be observed at infinite frequency. This shift corresponds to the term tex2html_wrap_inline2189 and may be calculated from equation 1Popup Equation .

Following the accumulation of about ten to twenty barycentric TOAs from observations spaced over at least several months, a surprisingly simple model can be applied to the TOAs and optimised so that it is sufficient to account for the arrival time of any pulse emitted during the time span of the observations and predict the arrival times of subsequent pulses. The model is based on a Taylor expansion of the angular rotational frequency tex2html_wrap_inline2191 about a model value tex2html_wrap_inline2193 at some reference epoch tex2html_wrap_inline2195 . The model pulse phase tex2html_wrap_inline2197 as a function of barycentric time is thus given by:


where tex2html_wrap_inline2199 is the pulse phase at tex2html_wrap_inline2201 . Based on this simple model, and using initial estimates of the position, dispersion measure and pulse period, a ``timing residual'' is calculated for each TOA as the difference between the observed and predicted pulse phases.

Early sets of residuals will exhibit a number of trends indicating a systematic error in one or more of the model parameters, or a parameter not initially incorporated into the model (e.g.   tex2html_wrap_inline2203). From equation 9Popup Equation, an error in the assumed tex2html_wrap_inline2193 results in a linear slope with time. A parabolic trend results from an error tex2html_wrap_inline2203 . Additional effects will arise if the assumed position of the pulsar (the unit vector tex2html_wrap_inline2179 in equation 8Popup Equation) is incorrect. A position error of just one arcsecond results in an annual sinusoid with a peak-to-peak amplitude of about 5 ms for a pulsar on the ecliptic; this is easily measurable for typical TOA uncertainties of order one milliperiod or better. Similarly, the effect of a proper motion produces an annual sinusoid of linearly increasing magnitude.

After a number of iterations, and with the benefit of a modicum of experience, it is possible to identify and account for each of these various effects to produce a ``timing solution'' which is phase coherent over the whole data span. The resulting model parameters provide spin and astrometric information about the neutron star to a precision which improves as the length of the data span increases. The latest observations of the original millisecond pulsar, B1937+21, spanning almost 9 yr (exactly 165,711,423,279 rotations!) measure a period of tex2html_wrap_inline2211 ms [87Jump To The Next Citation Point In The Article, 85] defined at midnight UT on December 5 1988! Astrometric measurements based on these data are no less impressive, with position errors of tex2html_wrap_inline2213 arcsec being presently possible.

4.3 Binary pulsars4 Pulsar Timing4.1 Observing basics

image Binary and Millisecond Pulsars
D. R. Lorimer (dunc@mpifr-bonn.mpg.de)
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to livrev@aei-potsdam.mpg.de