calc_kcorr

sherpa.astro.utils.calc_kcorr(data, model, z, obslo, obshi, restlo=None, resthi=None)[source] [edit on github]

Calculate the K correction for a model.

The K correction ([1], [2], [3], [4]) is the numeric factor applied to measured energy fluxes to convert values in an observed energy band to that they are in a rest-frame energy band (that is, correct for the change in spectral shape between the rest-frame and observed-frame bands). This is often used when converting a flux into a luminosity.

Parameters:

  • data – The data object to use.

  • model – The source expression: this should not include any instrument responses.

  • z (number or array, >= 0) – The redshift, or redshifts, of the source.

  • obslo (number) – The minimum energy of the observed band.

  • obshi (number) – The maximum energy of the observed band, which must be larger than obslo.

  • restlo (number or None) – The minimum energy of the rest-frame band. If None then use obslo.

  • restlo – The maximum energy of the rest-frame band. It must be larger than restlo. If None then use obshi.

Returns:

kz

Return type:

number or array of numbers

Notes

This is only defined when the analysis is in ‘energy’ units.

If the model contains a redshift parameter then it should be set to 0, rather than the source redshift.

If the source model is at zero redshift, the observed energy band is olo to ohi, and the rest frame band is rlo to rhi (which need not match the observed band), then the K correction at a redshift z can be calculated as:

frest = calc_energy_flux(data, model, rlo, rhi)
fobs  = calc_energy_flux(data, model, olo*(1+z), ohi*(1+z))
kz    = frest / fobs

The energy ranges used - rlo to rhi and olo*(1+z) to ohi*(1+z) - should be fully covered by the data grid, otherwise the flux calculation will be truncated at the grid boundaries, leading to incorrect results.

References