You can simulate a CTA observation with the ctobssim tool.
To invoke the tool, type ctobssim at the console prompt (which is denoted by $). ctobssim will query for a number of parameters:
$ ctobssim
RA of pointing (degrees) (0-360) [83.63]
Dec of pointing (degrees) (-90-90) [22.01]
Radius of FOV (degrees) (0-180) [5.0]
Start time (MET in s) [0.0]
End time (MET in s) [1800.0]
Lower energy limit (TeV) [0.1]
Upper energy limit (TeV) [100.0]
Calibration database [prod2]
Instrument response function [South_0.5h]
Input model XML file [$CTOOLS/share/models/crab.xml]
Output event data file or observation definition XML file [events.fits]
Each line represents a query for one parameter value. The line starts with a short description of the parameter, followed by the default value proposed by ctobssim in squared brackets [ ].
If no parameter is entered the default value will be used (which is the case for all parameters shown here). Otherwise, the specified value will overwrite the default value. The round brackets ( ) indicate the range of possible parameter values (if applicable).
You may have recognised that the environment variable $CTOOLS has been used in the path name of the model. ctools will automatically expand the environment variables in parameter inputs.
The CTA instrument properties (effective area, PSF width) are taken from the prod2 database. The response for the southern array using the cuts optimised for 50 hours of observing time are used.
Note
ctools comes bundled with CTA response functions for the northern and the southern array. The response functions are based on a Prod2 analysis with cuts optimised for 0.5 hours, 5 hours and 50 hours of observing time. The following Instrument Response Functions (IRFs) are available: North_0.5h, North_5h, North_50h, South_0.5h, South_5h, and South_50h.
Events are simulated based on the instrument properties and based on a source and background model. Only events that fall within the specified region of interest (ROI), defined as a circle around a sky position in Right Ascension and Declination (in degrees), will be stored in the output event data file. The duration of the simulation is taken here to 30 minutes (or 1800 seconds). Events are simulated for energies between 0.1 and 100 TeV.
The source and background model is defined by the XML file $CTOOLS/share/models/crab.xml:
<?xml version="1.0" standalone="no"?>
<source_library title="source library">
<source name="Crab" type="PointSource">
<spectrum type="PowerLaw">
<parameter name="Prefactor" scale="1e-16" value="5.7" min="1e-07" max="1000.0" free="1"/>
<parameter name="Index" scale="-1" value="2.48" min="0.0" max="+5.0" free="1"/>
<parameter name="PivotEnergy" scale="1e6" value="0.3" min="0.01" max="1000.0" free="0"/>
</spectrum>
<spatialModel type="SkyDirFunction">
<parameter name="RA" scale="1.0" value="83.6331" min="-360" max="360" free="0"/>
<parameter name="DEC" scale="1.0" value="22.0145" min="-90" max="90" free="0"/>
</spatialModel>
</source>
<source name="CTABackgroundModel" type="CTAIrfBackground" instrument="CTA">
<spectrum type="PowerLaw">
<parameter name="Prefactor" scale="1.0" value="1.0" min="1e-3" max="1e+3" free="1"/>
<parameter name="Index" scale="1.0" value="0.0" min="-5.0" max="+5.0" free="1"/>
<parameter name="PivotEnergy" scale="1e6" value="1.0" min="0.01" max="1000.0" free="0"/>
</spectrum>
</source>
</source_library>
The model consists of a source library that contains two components: the Crab nebula and an instrumental background model.
The Crab nebula is modelled by a factorized sky model that has a spectral and a spatial component (tags <spectrum> and <spatialModel>, respectively). The spectrum is modelled by a power law, which is defined by three parameters: the Prefactor, the Index and the Scale. The spatial model has two parameters: Right Ascension in degrees (RA), and Declination in degrees (DEC). Each parameter has a value and a scale factor, the real value of the parameter being the product value * scale. Typically, scale is chosen so that value is of the order of 1 (this is relevant for model fitting). In addition, value is bound by a minimum (min) and maximum (max) value, and a parameter may be free (free="1") or fixed (free="0"). The min, max, and free attributes are not relevant here for the simulations, but they will be important for the model fitting later.
The spectral intensity I(E) (in units of photons/cm2/s/MeV) of the power law is given by
where the parameters in the XML definition have the following mappings:
Warning
Energies are given in the XML file in MeV units. This is a GammaLib convention that can not be modified. So make sure you always use MeV as energy unit in an XML file.
The instrumental background of CTA is modelled using the background information provided in the IRF (CTAIrfBackground) multipled by a power law. As it is defined here, the power law represents a constant of 1, hence the background IRF will be used without any modification. The power law will become active when fitting the data later and allows a spectral adjustment of the background model that may account for uncertainties in the background information provided in the IRF.
ctobssim has a couple of hidden parameters, the most important one being certainly seed. seed is an integer that specifies the seed value for the random number generator, and changing this parameter will allow to generate statistically independent Monte Carlo samples of CTA event data. To use for example a seed value of 41 you should type:
$ ctobssim seed=41
ctobssim will write 2 files in the working directory: events.fits and ctobssim.log. The first file contains the simulated events in FITS format and can be inspected using fv or ds9. The FITS file will contain three extensions: an empty primary image, a binary table named EVENTS that holds the events (one row per event), and a binary table named GTI holding the Good Time Intervals (for the moment a single row with 2 columns providing the start and the stop time of the simulated time interval).
The second file produced by ctobssim is a human readable log file that contains information about the job execution. As example, the last lines from this file are shown here:
2016-06-29T10:21:57: +======================+
2016-06-29T10:21:57: | Simulate observation |
2016-06-29T10:21:57: +======================+
2016-06-29T10:21:57: === CTA observation ===
2016-06-29T10:21:57: Simulation cone ...........: RA=83.63 deg, Dec=22.01 deg, radius=5.5 deg
2016-06-29T10:21:57: Time interval .............: 0 - 1800 s
2016-06-29T10:21:57: Photon energy range .......: 100 GeV - 199.526 GeV
2016-06-29T10:21:57: Event energy range ........: 100 GeV - 199.526 GeV
2016-06-29T10:21:57: Simulation area ..........: 5.75561e+09 cm2
2016-06-29T10:21:57: Use model ................: Crab
2016-06-29T10:21:57: Normalization ............: 1 [Crab]
2016-06-29T10:21:57: Flux .....................: 3.76031e-10 [Crab] photons/cm2/s
2016-06-29T10:21:57: Normalized flux ..........: 3.76031e-10 [Crab] photons/cm2/s
2016-06-29T10:21:57: Photon rate ..............: 2.16429 photons/s [Crab]
2016-06-29T10:21:57: MC source photons ........: 3889 [Crab]
2016-06-29T10:21:57: MC source events .........: 1226 [Crab]
2016-06-29T10:21:57: MC source events .........: 1226 (all source models)
2016-06-29T10:21:57: Photon energy range .......: 199.526 GeV - 398.107 GeV
...
2016-06-29T10:21:57: MC source photons .........: 10759 [Crab]
2016-06-29T10:21:57: MC source events ..........: 3686 [Crab]
2016-06-29T10:21:57: MC events outside ROI .....: 0
2016-06-29T10:21:57: MC background events ......: 19413
2016-06-29T10:21:57: MC events .................: 23099 (all models)
Each line starts with the UTC time at which the line has been written. In this run, 10759 Crab photons have been thrown during a time interval of 1800 seconds. 3686 of these photons have been registered by CTA as events. In the same time interval, 19413 background events have been registred by CTA.
Note
ctobssim will split the simulated energy range into a number of slices, controlled via the hidden eslices parameter (ten energy slices are used by default). For each energy slice, the simulation area will be adapted to the effective area of the array in that energy slice, which helps to keep the computing time low. The log file will provide information about the simulation in each slice. In the example above, the simulation results for the first energy slice are shown, followed by a summary of the results for all slices.
You may change the name of the log file using the hidden parameter logfile:
$ ctobssim logfile=my-private-log-file
Furthermore, you may decide on the amount of information provided in the log file (the chattiness of the executable) using the hidden parameter chatter:
$ ctobssim chatter=4
chatter can vary between 0 and 4, 0 providing no information while 4 provides the most detailed information.
By default, all ctools have a chatter level of 2.
You may also duplicate the log file information into the console by setting the hidden debug parameter to yes:
$ ctobssim debug=yes