- Determine the pulse period:
- If you know the approximate period, then the xronos routine efsearch can be used to determine the best period from the data. Type "fhelp efsearch" for more details.
- If not, the xronos routine powspec can be used to generate a power density spectrum to help you find the period. Type "fhelp powspec" for more details.
- Divide the pulse profile and speficy epoch of phase zero:
Once you know the period, you can generate a pulse-profile using the xronos routine efold. Type "fhelp efold" for more details. [Note, however, that for some binaries the motion of the pulsar around its companion can smear the pulse, while for others the motion of the Earth around the Sun can have a similar, though usually smaller, effect. The ftool fxbary can correct for the latter but not the former.]
Looking at the pulse profile, and noting the count rate, you should decide how to divide up the pulse to meet your scientific objectives while retaining a reasonable signal-to-noise ratio. This entails determining how many pulse-phase bins to use as well as deciding whether to put certain features, such as a notch, wholly in one phase bin. For the latter, you need to determine the epoch of phase zero:
- Before running efold, use fdump or fkeyprint to get the values of TIMEZERO and TSTART.
- In efold, when prompted for the epoch, give TSTART (divided by 86,400 because efold wants the epoch in days).
- Continue through efold and examine the resulting plot. Decide where on the x-axis you want phase zero to be. For example, it could be at phase 0.16, in which case:
- Calculate the epoch of phase zero, ephem, according to:
ephem = TSTART + TIMEZERO + period (in seconds) * 0.16
- Extract the pulse-phase spectra:
In the current release of the extractors, the parameters used for phase extraction, namely, period, ephem and phaseint, are hidden, i.e. you're not prompted for them. To unhide them, you can edit the
corresponding parameter file (e.g. saextrct.par in your pfile directory). Or you can accomplish the same thing by setting the parameters at the command line, like this:
saextrct ephem=685023425.0 period=41.7 phaseint="0.0-0.25"
In this example, you'll be prompted for the rest of the parameters. And, once the program has finished, you should rerun the extractor with everything the same except the value of phaseint.
Alternatively, you can create a simple command file to drive your extractions. Here, we take advantage of the "mode=h" option of the ftools which turns all parameters into hidden ones, leaving the user to change specific ones at the command line. For example, the ASCII file:
saextrct mode=h outfile="ph00_02" phaseint="0.0-0.2"
saextrct mode=h outfile="ph02_04" phaseint="0.2-0.4"
saextrct mode=h outfile="ph04_06" phaseint="0.4-0.6"
saextrct mode=h outfile="ph06_08" phaseint="0.6-0.8"
saextrct mode=h outfile="ph08_10" phaseint="0.8-1.0"
will extract five equally spaced pulse-phase spectra. Important note: the reason all the other parameters don't appear on the command line is that they have already been set by a previous "dummy" extractor run with the desired settings. To verify parameter settings, type, e.g. "plist saextrct".
- Correct the exposures in the PHA files:
As mentioned above, the exposure must be corrected for spectra extracted through a phase filter. In practice, this entails running the ftool fmodhead to change the value of the the EXPOSURE keyword. For example, if you have extracted five equally spaced phase-resolved spectra called ph00_02.pha etc., then:
- Use the ftool fkeyprint to obtain the uncorrected exposure from the header of the PHA file:
fkeyprint ph00_02.pha EXPOSURE
Let's say the value is 15,030 seconds.
- Divide the value obained by five to get the correct exposure, i.e. 3,006 seconds.
- Create an ASCII file, exposure.dat, say, containing the line:
EXPOSURE 3006.0
- Correct the value of the EXPOSURE keyword with fmodhead:
fmodhead ph00_02.pha exposure.dat
- Verify the change was made with:
fkeyprint ph00_02.pha EXPOSURE
- Repeat for the rest of the PHA files.
