spectrum - 1D spectrum analysis

Introduction

gammapy.spectrum holds functions and classes related to 1D region based spectral analysis. This includes also simulation tools.

The basic of 1D spectral analysis are explained in this talk. A good reference for the forward-folding on-off likelihood fitting methods is Section 7.5 “Spectra and Light Curves” in [Naurois2012], in publications usually the reference [Piron2001] is used. A standard reference for the unfolding method is [Albert2007].

Getting Started

The following code snippet demonstrates how to load an observation stored in OGIP format and fit a spectral model.

from gammapy.spectrum import SpectrumDatasetOnOff
from gammapy.utils.fitting import Fit
from gammapy.spectrum.models import PowerLaw

filename = '$GAMMAPY_DATA/joint-crab/spectra/hess/pha_obs23523.fits'
dataset = SpectrumDatasetOnOff.from_ogip_files(filename)

model = PowerLaw(
    index=2,
    amplitude='1e-12 cm-2 s-1 TeV-1',
    reference='1 TeV',
)

dataset.model = model

fit = Fit([dataset])
result = fit.run()
model.parameters.covariance = result.parameters.covariance
print(model)

It will print the following output to the console:

PowerLaw

Parameters:

       name     value     error        unit      min max frozen
    --------- --------- --------- -------------- --- --- ------
        index 2.817e+00 1.496e-01                nan nan  False
    amplitude 5.142e-11 6.423e-12 cm-2 s-1 TeV-1 nan nan  False
    reference 1.000e+00 0.000e+00            TeV nan nan   True

Covariance:

       name     index   amplitude reference
    --------- --------- --------- ---------
        index 2.239e-02 6.160e-13 0.000e+00
    amplitude 6.160e-13 4.126e-23 0.000e+00
    reference 0.000e+00 0.000e+00 0.000e+00

Using gammapy.spectrum

For more advanced use cases please go to the tutorial notebooks:

The following pages describe gammapy.spectrum in more detail:

Reference/API

gammapy.spectrum Package

1D spectrum analysis.

Functions

cosmic_ray_spectrum([particle]) Cosmic ray flux at Earth.
create_crab_spectral_model([reference]) Create the Crab nebula spectral model depending of the reference given.
integrate_spectrum(func, xmin, xmax[, …]) Integrate 1d function using the log-log trapezoidal rule.

Classes

CountsSpectrum(energy_lo, energy_hi[, data, …]) Generic counts spectrum.
FluxPoints(table) Flux points container.
FluxPointsDataset(model, data[, mask_fit, …]) Fit a set of flux points with a parametric model.
FluxPointsEstimator(datasets, e_edges[, …]) Flux points estimator.
SensitivityEstimator(arf, rmf, bkg, livetime) Estimate differential sensitivity.
SpectrumDataset([model, counts, livetime, …]) Spectrum dataset for likelihood fitting.
SpectrumDatasetOnOff([model, counts, …]) Spectrum dataset for on-off likelihood fitting.
SpectrumDatasetOnOffStacker(obs_list) Stack a list of homogeneous datasets.
SpectrumEvaluator(model[, aeff, edisp, …]) Calculate number of predicted counts (npred).
SpectrumExtraction(observations, bkg_estimate) Creating input data to 1D spectrum fitting.
SpectrumSimulation(livetime, source_model[, …]) Simulate SpectrumObservation.

gammapy.spectrum.models Module

Spectral models for Gammapy.

Classes

SpectralModel([params]) Spectral model base class.
ConstantModel(const) Constant model.
CompoundSpectralModel(model1, model2, operator) Arithmetic combination of two spectral models.
PowerLaw([index, amplitude, reference]) Spectral power-law model.
PowerLaw2([amplitude, index, emin, emax]) Spectral power-law model with integral as amplitude parameter.
ExponentialCutoffPowerLaw([index, …]) Spectral exponential cutoff power-law model.
ExponentialCutoffPowerLaw3FGL([index, …]) Spectral exponential cutoff power-law model used for 3FGL.
PLSuperExpCutoff3FGL([index_1, index_2, …]) Spectral super exponential cutoff power-law model used for 3FGL.
LogParabola([amplitude, reference, alpha, beta]) Spectral log parabola model.
TableModel(energy, values[, norm, …]) A model generated from a table of energy and value arrays.
AbsorbedSpectralModel(spectral_model, …[, …]) Spectral model with EBL absorption.
Absorption(energy_lo, energy_hi, param_lo, …) Gamma-ray absorption models.
NaimaModel(radiative_model[, distance, seed]) A wrapper for Naima models.
SpectralGaussian([norm, mean, sigma]) Gaussian spectral model.
SpectralLogGaussian([norm, mean, sigma]) Gaussian Log spectral model.
ScaleModel(model[, norm]) Wrapper to scale another spectral model by a norm factor.