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PSRCHIVE user documentation: rmfit1.0 PurposeThe Faraday Rotation Measure (RM) fitting code (rmfit) implements a number of different algorithms for RM estimation.2.0 UsagePrerequisite: Data should be calibrated.Initialization: Data are tscrunched and dedipsersed. rmfit implements the following three methods of RM estimation
2.1 Brute Force SearchThe brute force search is executed by specifying the t command line option. In this case, rmfit automatically chooses the minimum and maximum RM and the RM step size as follows:
rmfit m min_RM,max_RM,num_steps D K /xs filenameFor each trial RM, rmfit corrects Faraday rotation and computes the total linear polarization. The resulting "RM spectrum" is plotted when the D K /xs options are used. rmfit fits a Gaussian to the peak of the "RM spectrum" and uses the centroid of this Gaussian to determine the best RM. If this fit fails, then the best RM is set to the trial RM at which the linear polarization reaches its maximum value. 2.2 Iterative differential position angle refinementThe RMrefinement algorithm is executed with the r command line option.This algorithm corrects the data for Faraday rotation using the current RM, splits the bandwidth in two and integrates each half over frequency, then computes the weighted differential polarization angle, deltaPA, between the two halves of the band. By default, only those pulse phase bins with linearly polarized flux that is 3 sigma above the offpulse noise in both halves of the band are included in the estimate of differential position angle. This threshold can be set with the T command line option. If the deltaPA estimate is larger than its uncertainty, then the data are corrected for Faraday rotation with the derived RM and the deltaPA is estimated again. This process is repeated until deltaPA is smaller than its uncertainty, at which point the final RM is reported. By default, the current RM used in the first iteration is that stored in the header of the data file. RMrefinement can also be combined with the brute force search; e.g. as follows: rmfit t r D K /xs filenameIn this case, the best RM output by the brute force search is the first guess used by the RMrefinement algorithm. 2.3 Quadratic Fitting (Noutsos et al. 2008)The quadratic fitting algorithm is executed with the w command line option and can be run as:rmfit w start_pulse,end_pulse archive D K /xsIn this case, the PA is computed per each frequency channel as an average on the pulse window identified by start_pulse and end_pulse. The RM is evaluated as a fit on the PAs. 3.0 AlgorithmsThe iterative differential position angle refinement algorithm is based on the derivation presented here.4.0 Testing and examplesThe special commandline optionJ is equivalent to running
rmfit r u 2000 U 6 a 1 L D K rmsearch.ps/cpswith the additional constraint that the maximum RM will never be less than 100. That is, the maximum RM will be set to 6 times the DM ( U 6 ), but it will never be greater than 2000 (u
2000 ), and never less than 100.
5.0 Known bugs and features that require implementation
