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PSRCHIVE user documentation: pvt
1.0 PurposeOne of the foundations of pulsar timing is the assumption that the mean pulse profile (obtained from several thousand integrated single pulses) has a stable morphological form in a given frequency range. Although this is true in the majority of cases, there are notable exceptions. Some pulsars have two (or possibly more) quasi-stable modes of emission with different profile morphologies. The dominant mode can suddenly "switch", causing significant profile evolution in a very short time. Other pulsars can "turn off" for extended periods of time (known as "nulling"). Assuming the pulsed emission is intrinsically stable, there are a number of external influences that can alter the morphology of the profile. One of the most interesting is precession of the pulsar rotation axis, which changes our line of sight to the emission cone (and hence the shape of the observed profile). Because precession timescales are usually measured in tens or hundreds of years, it is necessary to search for small changes in morphology in order to study the effects of precession on pulse profiles. pvt was designed to detect differences between integrated pulse profiles, using a number of comparison methods.
2.0 Usagepvt has a number of options that govern the type of analysis performed. In general, it is invoked along with a list of file names, specifying Archives on disk that contain the Profiles the user wishes to compare. Most methods of comparison produce graphical output that can be analysed by eye. The simplest visual method of comparison is to plot the Profiles in superposition with each other, after normalisation.
pvt -N filenamesPulse amplitude and position angle can be compared in this manner. It is also possible to zoom in on a particular phase range (with the -z option).
Another useful comparison method is the construction of "difference profiles", which result from bin-by-bin subtraction of the amplitudes of one profile from another (again, after normalisation). To compute difference Profiles, it is necessary to have a common template Profile against which to compare all other Profiles. The location of this Profile must also be specified on the command line as follows:
pvt -s template -D filenamesFor a full list of options, use:
pvt -hMultiple filenames can be specified on the command line either individually or using wildcard characters.
3.0 Algorithmspvt uses simple flux normalisation methods and the Profile::morphological_difference method. It can also implement more specific analytical techniques related to the comparison of individual features in the integrated Profile, as described by Kramer et al. in the Astrophysical Journal, 520, 324, 1999.
4.0 Testing and examplesThis program is still somewhat experimental and the algorithms it employs could change. Often, the program makes hidden assumptions about the state of the Archives it is reading from (for example, in some cases it is assumed that full polarimetric information is available). Users are warned that until the code matures, this program may require some experimentation and even editing to produce the desired output.
5.0 Known bugs and features that require implementation