How to generate a light curve?
What you will learn
You will learn how to generate a light curve for a source to study the temporal variation of the flux and spectrum.
You can generate a light curve using the :ref:`cslightcrv` script which takes on input an :ref:`event list <glossary_eventlist>` or an :ref:`observation definition file <glossary_obsdef>`. Light curves cannot be generated from counts cubes since the time information is dropped in the generation of the counts cube.
:ref:`cslightcrv` divides the event list into a number of intervals and performs a maximum likelihood analyse for each of the intervals. The analysis results are writted into a light curve FITS file.
The following example illustrates how you can generate a light curve for Src002 for the time interval from 59235.5 to 59236.0 (MJD), corresponding to the dates from 2021-01-21 (12:00:00) to 2021-01-22 (00:00:00) (to convert dates to MJD and vice versa we suggest to use the xTime Date and Time Conversion Utility). The time interval is linearly divided into 5 time bins.
$ cslightcrv
Input event list or observation definition XML file [events.fits] obs_selected.xml
Input model definition XML file [$CTOOLS/share/models/crab.xml] $CTOOLS/share/models/1dc_howto_unbinned.xml
Algorithm for defining time bins (FILE|LIN|GTI) [GTI] LIN
Lightcurve start time (UTC string, JD, MJD or MET in seconds) [MJD 51544.5] MJD 59235.5
Lightcurve stop time (UTC string, JD, MJD or MET in seconds) [MJD 51544.6] MJD 59236.0
Number of time bins (1-10000) [5]
Analysis method (3D|ONOFF) [3D]
Source name [Crab] Src002
Lower energy limit of events (TeV) [0.1]
Upper energy limit of events (TeV) [100.0]
Number of energy bins for binned (0=unbinned for 3D analysis only) (0-100) [0]
Output light curve file [lightcurve.fits]
The resulting light curve is shown in the figure below.
Light curve from 59235.5 to 59236.0 (MJD) for Src002
Note
The plot was created using the show_lightcurve.py script that is located in the ctools example folder. The example script requires the matplotlib Python module for display. You may reproduce the plot by typing
$ $CTOOLS/share/examples/python/show_lightcurve.py lightcurve.fits