compute_fvar

gammapy.stats.compute_fvar(flux, flux_err, axis=0)

Calculate the fractional excess variance.

This method accesses the FLUX and FLUX_ERR columns from the lightcurve data.

The fractional excess variance \(F_{var}\), an intrinsic variability estimator, is given by:

\[F_{var} = \sqrt{ \frac{S^{2} - \bar{ \sigma^{2}}}{ \bar{x}^{2}}}\]

It is the excess variance after accounting for the measurement errors on the light curve \(\sigma\). \(S\) is the variance.

It is important to note that the errors on the flux must be gaussian.

Parameters

fluxQuantity

The measured fluxes.

flux_errQuantity

The error on measured fluxes.

axisint, optional

Axis along which the excess variance is computed. Default is 0.

Returns

fvar, fvar_errndarray

Fractional excess variance.

References

Vaughan2003

“On characterizing the variability properties of X-ray light curves from active galaxies”, Vaughan et al. (2003) https://ui.adsabs.harvard.edu/abs/2003MNRAS.345.1271V