Detecting candidate sources

What you will learn

You will learn how to detect candidate sources in a sky map.

This will provide you with an initial model definition file in XML format that can be directly used for maximum likelihood fitting. In subsequent steps you will iteratively improve this model to obtain a reliable description of the observed gamma-ray sky.

As next step you need to identify the location of candidate sources in the sky map. You do this using the cssrcdetect script which implements a peak finding algorithm that is applied to a smoothed version of a sky map. By default, the smoothing kernel is a radial disk, and we apply here a disk radius of 0.05 degree for the analysis. Only peaks more significant than 5 sigma are retained.

$ cssrcdetect
Input sky map file [skymap.fits] skymap_irf.fits
Source model type (POINT) [POINT]
Background model type (NONE|IRF|AEFF|CUBE|RACC) [NONE] IRF
Detection threshold (Gaussian sigma) [5.0]
Correlation kernel radius (deg) [0.1] 0.05
Output model definition XML file [models.xml]
Output DS9 region file [ds9.reg]

This generates a model definition file models.xml that contains point source model components for all sources that were detected in the sky map. All model components have a power law as spectral component with parameters that correspond to 1% of the very-high-energy spectrum of the Crab, which is a reasonable starting condition for a subsequent model fitting using the ctlike tool.

The script generated also a ds9 region file that can be overlaid on the sky map. As indicated in the figure below, cssrcdetect detected four sources.

IRF background subtracted sky map of the events recorded around the Galactic Centre during the Galactic Plane Survey with the detected sources overlaid

Below is the content of the model definition file that was generated by cssrcdetect. There are five model components in the file and each model component is identified using a unique name attribute. The first four components, "Src001" to "Src004", are the four point sources detected by cssrcdetect, the fifth component, "Background", is a background model component of type "CTAIrfBackground" that was added because IRF was specified for the Background model type when running the script. Having a background model component added allows to directly use the model definition file for a model fitting with ctlike.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<source_library title="source library">
  <source name="Src001" type="PointSource">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="5.7e-18" min="0" free="1" />
      <parameter name="Index" value="1" error="-0" scale="-2.48" min="-4.03225806451613" max="4.03225806451613" free="1" />
      <parameter name="PivotEnergy" value="1" scale="300000" free="0" />
    </spectrum>
    <spatialModel type="PointSource">
      <parameter name="RA" value="266.424004498437" error="0" scale="1" free="1" />
      <parameter name="DEC" value="-29.0049010253548" error="0" scale="1" free="1" />
    </spatialModel>
  </source>
  <source name="Src002" type="PointSource">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="5.7e-18" min="0" free="1" />
      <parameter name="Index" value="1" error="-0" scale="-2.48" min="-4.03225806451613" max="4.03225806451613" free="1" />
      <parameter name="PivotEnergy" value="1" scale="300000" free="0" />
    </spectrum>
    <spatialModel type="PointSource">
      <parameter name="RA" value="264.802012181975" error="0" scale="1" free="1" />
      <parameter name="DEC" value="-31.0050720432975" error="0" scale="1" free="1" />
    </spatialModel>
  </source>
  <source name="Src003" type="PointSource">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="5.7e-18" min="0" free="1" />
      <parameter name="Index" value="1" error="-0" scale="-2.48" min="-4.03225806451613" max="4.03225806451613" free="1" />
      <parameter name="PivotEnergy" value="1" scale="300000" free="0" />
    </spectrum>
    <spatialModel type="PointSource">
      <parameter name="RA" value="266.831945177213" error="0" scale="1" free="1" />
      <parameter name="DEC" value="-28.1460284439951" error="0" scale="1" free="1" />
    </spatialModel>
  </source>
  <source name="Src004" type="PointSource">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="5.7e-18" min="0" free="1" />
      <parameter name="Index" value="1" error="-0" scale="-2.48" min="-4.03225806451613" max="4.03225806451613" free="1" />
      <parameter name="PivotEnergy" value="1" scale="300000" free="0" />
    </spectrum>
    <spatialModel type="PointSource">
      <parameter name="RA" value="268.244867737546" error="0" scale="1" free="1" />
      <parameter name="DEC" value="-30.3728286753247" error="0" scale="1" free="1" />
    </spatialModel>
  </source>
  <source name="Background" type="CTAIrfBackground">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="1" error="0" scale="1" min="0" free="1" />
      <parameter name="Index" value="0" error="0" scale="1" min="-10" max="10" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
  </source>
</source_library>