(this page is work in progress, but it will be a more uniform version of the recipes which are currently available at the Timing tool page of the wiki)
A light curve is a FITS file that contains the information about the brightness of a source as a function of time. Typically, light curves are evenly spaced, that is, the binning of the data does not change with time. The brightness of the source is typically encoded either as a count rate or by the number of events detected in the bin.
This means that FITS light curves contain the following rows:
- TIME: This is the time of the bin. Whether this time is at the start, in the center, or at the end of the bin depends on the value of the header keyword TIMEPIXR, which states the relative time (in units of the binsize - TIMEDEL
- RATE or COUNTS: This is the count rate of the bin (typically in units of counts/s), or the number of counts.
- FRACEXP: This optional column designates the fractional time that the detector was sensitive in a given time bin. Reasons why FRACEXP might be less than 1 are, e.g., telemetry drop outs during the time bins or a detector intrinsic dead time. Note that depending on the instrument, the count rate in the RATE column may or may not be corrected for the FRACEXP. You need to consult the FITS file and/or the documentation of the instrument to find out (often there is a header keyword for this).
In addition to the header keywords TIMEPIXR and TIMEDEL, the following header keywords are often also used:
- MJDREF: Reference time for the TIME-column. This means that the MJD of the events can be obtained from by the equation MJD=MJDREF+TIME/86400, where it was assumed that the TIME-column is in units of seconds. For high precision lightcurves, the reference MJD is generally encoded in an integer and a fractional part, MJDREFI and MJDREFF. Use these if they are present. If you need the MJD, use
Note the parentheses - in pretty much all computing languages derived from C (including S-LANG, python and so on) this enforces that the contents of the parenthesis are computed first, which will reduce the roundoff error.
- TSTART and TSTOP: Start and end of the observations, in the same units as the TIME-column.
There is unfortunately much less standardization going on with time series than with spectra. This means that all of what is said in the following needs to be taken with a grain of salt, and it is strongly recommended that you take a look at the FITS-file and do not blindly copy the recipes here. One thing that is very important for timing is the question whether the time series contains gaps or not. How these are encoded is mission specific, but in general, a light curve FITS-file will have a Good Time Interval (GTI) extension that lists the valid time intervals. The light curve may or may not be limited to the GTI - for example, some light curves produced by the XMM SAS do NOT limit themselves to the GTI but zero pad the lightcurve. This means that before you work with the time series you have to remove these gaps.