Tobias C Hinse edited untitled.tex  over 8 years ago

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As a test the CURVEFIT routine has been used in a similar manner. The resulting reduced chi2 was also 95.22 matching and confirming the result from the previous section. The /NODERIVATIVE keyword does not change anything and expressions for the partial derivative has been included. The RMS also agrees with the results obtained from LINFIT. However, the formal $1\sigma$ uncertainties in the best-fit parameters (TZERO and PERIOD) are one magnitude smaller compared to the equivalent values obtained from LINFIT. The data and the best-fit line (obtained from LINFIT) is shown in Fig.~\ref{linearfit} with the residuals plotted in Fig.~\ref{linearfit_res}. There is absolutely no difference when using the results from CURVEFIT.  \subsection{Linear ephemeris - conclusion}  After fitting a straight line and visually inspecting the residual plots I cannot see any convincing trend that should justify a quadratic ephemeris (linear + a quadratic term). What I see is a sinusoidal variation around the best-fit line. Relative to the linear linear line  the first timing measurement arrives 20s earlier than expected. Then the trend goes down and increases again to 40s at E=0, then decreases again to a minimum to around 20s and increases again thereafter. There is no obvious quadratic trend from looking at the residuals in Fig.~\ref{linearfit_res}. \section{Quadratic ephemeris}  Although there is no obvious reason to include a quadratic term I will nevertheless consider a quadratic model. I will do this by again using IDL's CURVEFIT procedure and the MPFIT package (also IDL) which is a more sophisticated fitting tool utilizing the Levenberg-Marquardt least-squares minimisation algorithm developed by Marwardt. 

\end{table}  \section{New dataset: Linear ephemeris using MPFIT}  We have determined the following linear ephemeris using MPFIT. We followed the monte-carlo approach and determined a best-fit model by generating 10 million random initial guesses. We used best-fit parameters from LINFIT to obtain a first estimate of the initial epoch and period. Then initial guesses were drawn from a Gaussian distribution centered at the LINFIT values with standard deviation given by five times the formal LINFIT uncertainties. The linear ephemeris is shown in Fig.~\ref{Linearfit_NEW}  \section{Figures:}