Background

One method of background modeling for IACTs is to construct spatial and / or spectral model templates of the irreducible cosmic ray background for a given reconstruction and gamma-hadron separation from Off Observation. These templates can then be used as an ingredient to model the background in observations that contain gamma-ray emission of interest, or to compute the sensitivity for that set of cuts.

Here we specify two formats for such background template models:

  • bkg_2d models depend on ENERGY and THETA, i.e. are radially symmetric.
  • bkg_3d models depend on ENERGY and field of view coordinates DETX and DETY.

Note

Generating background models requires the construction of several intermediate products (counts and livetime histograms, both filled by cutting out exclusion regions around sources like AGN) to arrive at the models containing an absolute rate described here. At this time we don’t specify a format for those intermediate formats.

Note

Background models are sometimes considered an instrument response function (IRF) and sometimes not (e.g. when the background is estimated from different parts of the field of view for the same observation).

Here we have the background format specifications listed under IRFs, simply because the storage format is very similar to the other IRFs (e.g. effective area) and we didn’t want to introduce a new top-level section besides IRFs.

bkg_2d format

The bkg_2d format contains a 2-dimensional array of post-select background rate, stored in the BINTABLE HDU format.

Header keywords:

  • HDU_CLASS = bkg_2d
  • HDU_DOC = TODO

Columns:

  • THETA_LO, THETA_HI – ndim: 1, unit: deg
    • Field of view offset axis
    • Binning is often chosen with a square root scale, so that each THETA bin has equal solid angle, which means bins at the center of the field of view have smaller width THETA_HI - THETA_LO.
  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • Energy axis
  • BKG ndim: 2, unit: s^-1 MeV^-1 sr^-1
    • Absolute post-select background rate (expected background per time, energy and solid angle).
    • Note that this is not a “flux” or “surface brightness”. This is already a count rate, it doesn’t need to be multiplied with effective area to obtain predicted counts, like gamma-ray flux and surface brightness models do.

Recommended axis order: ENERGY, THETA

Example data file: TODO

bkg_3d format

The bkg_3d format contains a 3-dimensional array of post-select background rate, stored in the BINTABLE HDU format.

TODO: maybe we should we use TeV as units, since we use this for IACTs and also store energy in TeV?

Columns:

  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • Energy axis
  • DETX_LO, DETX_HI, DETY_LO, DETY_HI – ndim: 1, unit: deg
  • BKG – ndim: 3, unit: s^-1 MeV^-1 sr^-1
    • Absolute post-select background rate (expected background per time, energy and solid angle).
    • Note that this is not a “flux” or “surface brightness”. This is already a count rate, it doesn’t need to be multiplied with effective area to obtain predicted counts, like gamma-ray flux and surface brightness models do.

Recommended axis order for BKG: ENERGY, DETX, DETY

Header keywords:

  • HDU_CLASS = bkg_3d
  • HDU_DOC = TODO

Example data file: TODO