# Super Exponential Cutoff Power Law Model used for 4FGL-DR1 (and DR2)#

This model parametrises super exponential cutoff power-law model spectrum used for 4FGL.

It is defined by the following equation:

$\phi(E) = \phi_0 \cdot \left(\frac{E}{E_0}\right)^{-\Gamma_1} \exp \left( a \left( E_0 ^{\Gamma_2} - E^{\Gamma_2} \right) \right)$

See Equation (4) in https://arxiv.org/pdf/1902.10045.pdf

## Example plot#

Here is an example plot of the model:

from astropy import units as u
import matplotlib.pyplot as plt
from gammapy.modeling.models import (
Models,
SkyModel,
SuperExpCutoffPowerLaw4FGLSpectralModel,
)

energy_range = [0.1, 100] * u.TeV
model = SuperExpCutoffPowerLaw4FGLSpectralModel(
index_1=1,
index_2=2,
amplitude="1e-12 TeV-1 cm-2 s-1",
reference="1 TeV",
expfactor=1e-2,
)
model.plot(energy_range)
plt.grid(which="both")
plt.ylim(1e-24, 1e-10)


## YAML representation#

Here is an example YAML file using the model:

model = SkyModel(spectral_model=model, name="super-exp-cutoff-power-law-4fgl-model")
models = Models([model])

print(models.to_yaml())


Out:

components:
-   name: super-exp-cutoff-power-law-4fgl-model
type: SkyModel
spectral:
type: SuperExpCutoffPowerLaw4FGLSpectralModel
parameters:
-   name: amplitude
value: 1.0e-12
unit: cm-2 s-1 TeV-1
is_norm: true
-   name: reference
value: 1.0
unit: TeV
frozen: true
is_norm: false
-   name: expfactor
value: 0.01
is_norm: false
-   name: index_1
value: 1.0
is_norm: false
-   name: index_2
value: 2.0
is_norm: false


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