Background format

Here we specify two formats for the background template models (see Background) of a full-enclosure IRF:

  • 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.

BKG_2D

The BKG_2D format contains a 2-dimensional array of post-select background rate, stored in the BINTABLE HDU format.

Required columns:

  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • Reconstructed energy axis
  • THETA_LO, THETA_HI – ndim: 1, unit: deg
  • BKG – ndim: 2, unit: s^-1 MeV^-1 sr^-1
    • Absolute post-select background rate (expected background per time, energy and solid angle).

Recommended axis order: ENERGY, THETA

Header keywords:

As explained in HDU classes, the following header keyword should be used to declare the type of HDU:

Example data file: here.

BKG_3D

The BKG_3D format contains a 3-dimensional array of post-select background rate, stored in the BINTABLE HDU format.

Required columns:

  • ENERG_LO, ENERG_HI – ndim: 1, unit: TeV
    • Reconstructed 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).

Recommended axis order: ENERGY, DETX, DETY

Header keywords:

As explained in HDU classes, the following header keyword should be used to declare the type of HDU:

Further header keywords:

  • FOVALIGN = ‘ALTAZ’ / ‘RADEC’
    • Alignment of the field-of-view coordinate system (see Field of view)

Example data file: here.

Notes

The background rate is not a “flux” or “surface brightness”. It 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. The rate is computed per observation time (without any dead-time correction, don’t use livetime when computing or using the background rate).