Plotting Fermi 2FHL and 3FGL spectra¶

In the following we will show how to plot spectra for Fermi 2FHL and 3FGL sources, by using the SpectralModel, SpectrumButterfly and DifferentialFluxPoints classes.

As a first example we plot the spectral energy distribution for the source PKS 2155-304, namely '3FGL J2158.8-3013' and '2FHL J2158.8-3013', including best fit model, butterfly and flux points. First we load the corresponding catalog from catalog and access the data for the crab:

import matplotlib.pyplot as plt
from gammapy.catalog import SourceCatalog3FGL, SourceCatalog2FHL

plt.style.use('ggplot')

# load catalogs
fermi_3fgl = SourceCatalog3FGL()
fermi_2fhl = SourceCatalog2FHL()

# access crab data by corresponding identifier
crab_3fgl = fermi_3fgl['3FGL J2158.8-3013']
crab_2fhl = fermi_2fhl['2FHL J2158.8-3013']

First we plot the best fit model for the 3FGL model:

ax = crab_3fgl.spectral_model.plot(crab_3fgl.energy_range, energy_power=2,
                                   label='Fermi 3FGL', color='r',
                                   flux_unit='erg-1 cm-2 s-1')
ax.set_ylim(1e-12, 1E-9)

The crab_3fgl.energy_range attribute specifies the energy range of the 3FGL model. By setting the argument energy_power=2 we can plot the actual energy distribution instead of the differential flux density. The plot method returns an Axes object that can be reused later to plot additional information on it. Here we just modify the y-limits of the plot.

As the next step we add the butterfly for the best fit model by calling plot_error:

crab_3fgl.spectral_model.plot_error(crab_3fgl.energy_range, ax=ax, energy_power=2, color='r',
                                    flux_unit='erg-1 cm-2 s-1')

Finally we add the flux points by calling:

crab_3fgl.flux_points.plot(ax=ax, energy_power=2, color='r',
                           flux_unit='erg-1 cm-2 s-1')

The same can be done with the 2FHL best fit model:

crab_2fhl.spectral_model.plot(crab_2fhl.energy_range, ax=ax, energy_power=2,
                              c='g', label='Fermi 2FHL', flux_unit='erg-1 cm-2 s-1')

# plot butterfly and flux points
crab_2fhl.spectral_model.plot(crab_2fhl.energy_range, ax=ax, energy_power=2, color='g',
                              flux_unit='erg-1 cm-2 s-1')
crab_2fhl.flux_points.plot(ax=ax, energy_power=2, color='g',
                           flux_unit='erg-1 cm-2 s-1')

The final plot looks as following:

#.. plot:: spectrum/plot_fermi_spectra.py # :include-source:

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Plotting Fermi 2FHL and 3FGL spectra¶