Describing CTA observations using XML

CTA observations can be described in GammaLib using an ASCII file in XML format (see Describing observations using XML). The CTA specific section of this file has the format:

<observation name="..." id="..." instrument="...">
    <parameter name="EventList"           file="..."/>
    <parameter name="EffectiveArea"       file="..."/>
    <parameter name="PointSpreadFunction" file="..."/>
    <parameter name="EnergyDispersion"    file="..."/>
    <parameter name="Background"          file="..."/>
</observation>

for an unbinned observation and

<observation name="..." id="..." instrument="...">
    <parameter name="CountsCube"          file="..."/>
    <parameter name="EffectiveArea"       file="..."/>
    <parameter name="PointSpreadFunction" file="..."/>
    <parameter name="EnergyDispersion"    file="..."/>
    <parameter name="Background"          file="..."/>
</observation>

for a binned observation. Each parameter within the <observation> tag specifies the filename for a specific file that is needed for the analysis. The difference between an unbinned and a binned observation is that the first uses an event list (requiring a parameter with name EventList) while the latter uses an event cube (requiring a parameter with name CountsCube). The EffectiveArea, PointSpreadFunction, EnergyDispersion and Background parameters provide the filenames of the instrument specific response function components for each observation (see Handling the instrument response). Alternatively, the calibration database and response name can be specified, e.g.

<observation name="..." id="..." instrument="...">
    <parameter name="EventList" file="..."/>
    <parameter name="Calibration" database="..." response="..."/>
</observation>

A variant of the binned analysis is the stacked analysis. While in a binned analysis an event cube is generated for each observation, event cubes will be summed in a stacked analysis. Summing the events requires computation of the total exposure, the average point spread function and the average background rate (energy dispersion is not yet handled for stacked analysis). Information for a stacked observation is provided in the following format:

<observation name="..." id="..." instrument="...">
    <parameter name="CountsCube"   file="..."/>
    <parameter name="ExposureCube" file="..."/>
    <parameter name="PsfCube"      file="..."/>
    <parameter name="BkgCube"      file="..."/>
</observation>

The stacked analysis uses also an event cube but now requires the pre-computed response cubes. The ExposureCube, PsfCube, and BkgCube parameters provide the filenames of the required components.

The instrument attribute of CTA observations can be one of CTA, HESS, MAGIC or VERITAS. This allows mixing of observations from difference IACTs within a single analysis. Note that no code that is specific to any of these four instruments is implemented in GammaLib, but the instrument attribute is used to tie models to instruments, allowing thus to provide specific background models for each of the instruments in a combined analysis.

The id attribute specifies an identifier for the observation that needs to be unique for a given instrument. The identifier allows to connect an observation to a specific model component.

Optionally, an observation can also have user defined minimum and maximum energy boundaries. These boundaries are specified as optional attributes in the XML file, e.g.

<observation name="..." id="..." instrument="..." emin="0.1" emax="100.0">
    ...
</observation>

Note

Energy boundaries in the observation XML file are specified in units of TeV.

In case that neither an event list nor an event cube is available, a CTA observation can be defined by specifying the pointing direction, the Good Time Intervals, the region of interest, and optionally the deadtime correction factor. Here an example of the expected XML format:

<observation name="GPS" id="000001" instrument="CTA">
    <parameter name="Pointing" ra="186.721" dec="-61.4328" />
    <parameter name="GoodTimeIntervals" tmin="0" tmax="35100" />
    <parameter name="TimeReference" mjdrefi="51544" mjdreff="0.5" timeunit="s" timesys="TT" timeref="LOCAL" />
    <parameter name="RegionOfInterest" ra="186.721" dec="-61.4328" rad="5" />
    <parameter name="Deadtime" deadc="0.95" />
    <parameter name="Calibration" database="prod2" response="South_50h" />
</observation>

Note

The time reference for the Good Time Intervals is specified using the TimeReference parameter.