XML model definition format

Sky models can be defined using an XML file format that is inspired by the format used for the Large Area Telescope (LAT) aboard NASA’s Fermi satellite. To illustrate the format, the definition of a point source at the position of the Crab nebula with a power spectral shape is shown below. The tags <spatialModel>, <spectrum> and <temporalModel> describe the spatial, spectral and temporal model components defined above. The <temporalModel> tag can be omitted, which will automatically allocate a constant temporal component for the model.

Each source model needs to have a unique name specified by the name attributes. The type attribute specifies the type of the model and needs to be one of PointSource, ExtendedSource, DiffuseSource and CompositeSource. Optional attributes include:

• id, specifying an identifier string for a model that can be used to tie the source model to one or several observations;,
• ts, specifying the Test Statistic value of the source model component, and
• tscalc, specifying whether the Test Statistic value should be computed in a maximum-likelihood computation.

<?xml version="1.0" standalone="no"?>
<source_library title="source library">
  <source name="Crab" type="PointSource">
    <spatialModel type="PointSource">
      <parameter name="RA"  scale="1.0" value="83.6331" min="-360" max="360" free="1"/>
      <parameter name="DEC" scale="1.0" value="22.0145" min="-90"  max="90"  free="1"/>
    </spatialModel>
    <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>
    <temporalModel type="Constant">
      <parameter name="Normalization" scale="1.0" value="1.0" min="0.1" max="10.0" free="0"/>
    </temporalModel>
  </source>
</source_library>

To accomodate for possible calibration biases for different instruments, instrument specific scaling factors can be specified as part of a model. In the example below, the model will be multiplied by a factor of 1 for LAT and a factor of 0.5 for CTA.

<?xml version="1.0" standalone="no"?>
<source_library title="source library">
  <source name="Crab" type="PointSource">
    ...
    <scaling>
      <instrument name="LAT" scale="1.0" min="0.1" max="10.0" value="1.0" free="0"/>
      <instrument name="CTA" scale="1.0" min="0.1" max="10.0" value="0.5" free="0"/>
    </scaling>
  </source>
</source_library>

A source model can be associated to a known gamma-ray source. Associations a listed in an additional <associations> tags that may contain one or several associations, defined by the <association> tag. Each association has a name attribute and contains an arbitrary number of properties, defined by the ``property``tag. The XML format of the association information is illustrated below.

<?xml version="1.0" standalone="no"?>
<source_library title="source library">
  <source name="Crab" type="PointSource">
    <spatialModel type="PointSource">
      <parameter name="RA"  scale="1.0" value="83.6331" min="-360" max="360" free="1"/>
      <parameter name="DEC" scale="1.0" value="22.0145" min="-90"  max="90"  free="1"/>
    </spatialModel>
    <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>
    <temporalModel type="Constant">
      <parameter name="Normalization" scale="1.0" value="1.0" min="0.1" max="10.0" free="0"/>
    </temporalModel>
    <associations>
      <association name="Crab">
        <property name="spatial"  value="RadialDisk" />
        <property name="ra"       value="83.6331" />
        <property name="dec"      value="22.0145" />
        <property name="extent"   value="0.11" />
        <property name="spectral" value="FileFunction" />
        <property name="flux"     value="2.177e-11" />
        <property name="eflux"    value="1.278e-11" />
        <property name="prob_pos" value="0.92" />
        <property name="prob_ext" value="0.75" />
        <property name="prob_flx" value="0.00" />
        <property name="overlap"  value="1.0" />
      </association>
    </associations>
  </source>
</source_library>