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
- Field of view offset axis (see Field of view).
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:

HDUDOC = ‘https://github.com/open-gamma-ray-astro/gamma-astro-data-formats’
HDUVERS = ‘0.2’
HDUCLASS = ‘GADF’
HDUCLAS1 = ‘RESPONSE’
HDUCLAS2 = ‘BKG’
HDUCLAS3 = ‘FULL-ENCLOSURE’
HDUCLAS4 = ‘BKG_2D’

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
- Field of view coordinates binning (see Field of view)
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:

HDUDOC = ‘https://github.com/open-gamma-ray-astro/gamma-astro-data-formats’
HDUVERS = ‘0.2’
HDUCLASS = ‘GADF’
HDUCLAS1 = ‘RESPONSE’
HDUCLAS2 = ‘BKG’
HDUCLAS3 = ‘FULL-ENCLOSURE’
HDUCLAS4 = ‘BKG_3D’

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).