In this tutorial we want to make an image of the sources in the Galactic plane, using only photons above 10 GeV.
The Fermi LAT observes the whole gamma-ray sky in the energy band of roughly 100 MeV to 100 GeV, with some exposure below and above that range. The angular resolution (called the PSF) varies by almost two orders of magnitude from ~ 15 deg at 100 MeV to ~ 0.2 deg at 100 GeV. (Check out the Fermi LAT Performance page for further information.)
Because of the high source density in the Galactic plane and the vert poor Fermi LAT angular resolution at low energies it makes sense to only use the high-energy photons (we use E > 10 GeV here).
One point to keep in mind though is that the Fermi LAT only detects very few photons at high energies, because the Galactic diffuse and source emission roughly follows a differential power-law spectrum
which corresponds to an integral power law spectrum of
with a power law spectral index of \(\Gamma \sim 2.6\) for the Galactic diffuse emission and often harder \(\Gamma \sim 1.8\) to \(\Gamma \sim 2.6\) source emission.
Note that for \(\Gamma = 2.6\) there are more than two orders of magnitude difference in the number of photons above a given energy when moving up one decade in energy, i.e. for the Galactic diffuse emission the number of photons above 10 GeV in only 0.25% compared to the number of photons above 1 GeV, and above 100 GeV it’s only 0.00063% compared to 1 GeV.
With this background knowledge you should be able to understand the basic facts about the following Fermi LAT count images in energy bands:
In this tutorial we would like to produce an image showing only the Galactic source emission above 10 GeV, with the diffuse Galactic and isotropic emission substracted.
We will do this via the formula excess = (total counts) - (diffuse model counts), where the diffuse model consists of a Galactic and isotropic part. (For the Galactic plane the Galactic diffuse emission is much brighter than the isotropic diffuse emission, so you can’t see the isotropic diffuse emission in the images above).