Using agnpy radiative processes to fit a MWL SED
agnpy
includes sherpa and gammapy wrappers that allow the user to fit the broad-band emission of jetted AGN.
The wrappers consider the combination of several radiative processes and return a model
object that can be used by the fitting routine of the two packages.
In this documentation page, we gather several examples showing how to obtain the best-fit parameters for these models via a \(\chi^2\) minimisation.
The data to be fitted are flux points, representing the flux measurement of an instrument in one or more energy bins.
Prerequisites
Note that the agnpy.fit
module, containing the wrappers, needs gammapy
and sherpa
to be installed.
These packages are not among the basic dependencies of agnpy
and will not be installed automatically if agnpy
is installed via conda
or pip
.
Check the documentation of the relative packages for their installation instructions.
The following warning message will be printed by agnpy
if the packages have not been installed:
WARNING:root:sherpa and gammapy are not installed, the agnpy.fit module cannot be used
In this case, all agnpy
modules except agnpy.fit
, can be used normally.
Fit using the sherpa
wrapper
Fit using the Gammapy
wrapper
For Gammapy we show, beside the simple \(\chi^2\) minimisation, how to use a Monte Carlo Markov Chain (MCMC) to perform the fit.
Loading MWL SED data with Gammapy
and sherpa
We provide, in agnpy.fit
, functions that can directly load flux points in sherpa
and Gammapy
data objects.
The SED format that these functions can read follows the one proposed by the Data Formats for Gamma-ray Astronomy.
More specifically, the flux point format that can be currently read by the load_sherpa_flux_points
and load_gammapy_flux_points
functions has the following mandatory columns:
e_ref
, representing the energy of the center of the flux points (eV
);e2dnde
, representing the energy flux (erg / (cm2 s)
);e2dnde_errn
, representing the lower error on the energy flux (erg / (cm2 s)
);e2dnde_errp
, representing the higher error on the energy flux (erg / (cm2 s)
);instrument
, representing the instrument that performed the measurement.