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Spectrum simulation



To simulate a specific observation, it is not always necessary to simulate the full photon list. For many uses cases, simulating directly a reduced binned dataset is enough: the IRFs reduced in the correct geometry are combined with a source model to predict an actual number of counts per bin. The latter is then used to simulate a reduced dataset using Poisson probability distribution.

This can be done to check the feasibility of a measurement, to test whether fitted parameters really provide a good fit to the data etc.

Here we will see how to perform a 1D spectral simulation of a CTA observation, in particular, we will generate OFF observations following the template background stored in the CTA IRFs.

Objective: simulate a number of spectral ON-OFF observations of a source with a power-law spectral model with CTA using the CTA 1DC response, fit them with the assumed spectral model and check that the distribution of fitted parameters is consistent with the input values.


%matplotlib inline
import matplotlib.pyplot as plt
import numpy as np
import astropy.units as u
from astropy.coordinates import SkyCoord, Angle
from regions import CircleSkyRegion
from gammapy.spectrum import (
from gammapy.modeling import Fit, Parameter
from gammapy.modeling.models import (
from gammapy.irf import load_cta_irfs
from import Observation
from gammapy.maps import MapAxis

Simulation of a single spectrum

To do a simulation, we need to define the observational parameters like the livetime, the offset, the assumed integration radius, the energy range to perform the simulation for and the choice of spectral model. We then use an in-memory observation which is convolved with the IRFs to get the predicted number of counts. This is Poission fluctuated using the fake() to get the simulated counts for each observation.

# Define simulation parameters parameters
livetime = 1 * u.h
pointing = SkyCoord(0, 0, unit="deg", frame="galactic")
offset = 0.5 * u.deg
# Reconstructed and true energy axis
energy_axis = MapAxis.from_edges(
    np.logspace(-0.5, 1.0, 10), unit="TeV", name="energy", interp="log"
energy_axis_true = MapAxis.from_edges(
    np.logspace(-1.2, 2.0, 31), unit="TeV", name="energy", interp="log"

on_region_radius = Angle("0.11 deg")
on_region = CircleSkyRegion(center=pointing, radius=on_region_radius)
# Define spectral model - a simple Power Law in this case
model_simu = PowerLawSpectralModel(
    amplitude=2.5e-12 * u.Unit("cm-2 s-1 TeV-1"),
    reference=1 * u.TeV,
# we set the sky model used in the dataset
model = SkyModel(spectral_model=model_simu)

   name     value   error      unit      min max frozen
--------- --------- ----- -------------- --- --- ------
    index 3.000e+00   nan                nan nan  False
amplitude 2.500e-12   nan cm-2 s-1 TeV-1 nan nan  False
reference 1.000e+00   nan            TeV nan nan   True
# Load the IRFs
# In this simulation, we use the CTA-1DC irfs shipped with gammapy.
irfs = load_cta_irfs(
obs = Observation.create(pointing=pointing, livetime=livetime, irfs=irfs)

        obs id            : 0
        tstart            : 51544.00
        tstop             : 51544.04
        duration          : 3600.00 s
        pointing (icrs)   : 266.4 deg, -28.9 deg

        deadtime fraction : 0.0%

WARNING: AstropyDeprecationWarning: The truth value of a Quantity is ambiguous. In the future this will raise a ValueError. [astropy.units.quantity]
# Make the SpectrumDataset
dataset_empty = SpectrumDataset.create(
    e_reco=energy_axis.edges, e_true=energy_axis_true.edges, region=on_region
maker = SpectrumDatasetMaker(selection=["aeff", "edisp", "background"])
dataset =, obs)
# Set the model on the dataset, and fake
dataset.model = model

  Name                            : Y03hV6Th

  Total counts                    : 16
  Total predicted counts          : nan
  Total background counts         : 22.35

  Effective area min              : 8.16e+04 m2
  Effective area max              : 5.08e+06 m2

  Livetime                        : 3.60e+03 s

  Number of total bins            : 9
  Number of fit bins              : 9

  Fit statistic type              : cash
  Fit statistic value (-2 log(L)) : nan

  Number of parameters            : 0
  Number of free parameters       : 0

You can see that backgound counts are now simulated

OnOff analysis

To do OnOff spectral analysis, which is the usual science case, the standard would be to use SpectrumDatasetOnOff, which uses the acceptance to fake off-counts

dataset_onoff = SpectrumDatasetOnOff(

  Name                            : G_Ggp0ls

  Total counts                    : 278
  Total predicted counts          : 298.06
  Total off counts                : 129.00

  Total background counts         : 25.80

  Effective area min              : 8.16e+04 m2
  Effective area max              : 5.08e+06 m2

  Livetime                        : 1.00e+00 h

  Acceptance mean:                : 1.0

  Number of total bins            : 9
  Number of fit bins              : 9

  Fit statistic type              : wstat
  Fit statistic value (-2 log(L)) : 6.66

  Number of parameters            : 3
  Number of free parameters       : 2

  Component 0: SkyModel

    Name                      : E_hrDUXC
    Spectral model type       : PowerLawSpectralModel
    Spatial  model type       : None
    Temporal model type       :
      index                   :   3.000
      amplitude               :   2.50e-12  1 / (cm2 s TeV)
      reference    (frozen)   :   1.000  TeV

You can see that off counts are now simulated as well. We now simulate several spectra using the same set of observation conditions.


n_obs = 100
datasets = []

for idx in range(n_obs):
CPU times: user 220 ms, sys: 4.82 ms, total: 224 ms
Wall time: 228 ms

Before moving on to the fit let’s have a look at the simulated observations.

n_on = [ for dataset in datasets]
n_off = [ for dataset in datasets]
excess = [ for dataset in datasets]

fix, axes = plt.subplots(1, 3, figsize=(12, 4))

Now, we fit each simulated spectrum individually

results = []
for dataset in datasets:
    dataset.models = model.copy()
    fit = Fit([dataset])
    result = fit.optimize()
            "index": result.parameters["index"].value,
            "amplitude": result.parameters["amplitude"].value,
CPU times: user 4.66 s, sys: 58.7 ms, total: 4.72 s
Wall time: 4.82 s

We take a look at the distribution of the fitted indices. This matches very well with the spectrum that we initially injected, index=2.1

index = np.array([_["index"] for _ in results])
plt.hist(index, bins=10, alpha=0.5)
plt.axvline(x=model_simu.parameters["index"].value, color="red")
print(f"index: {index.mean()} += {index.std()}")
index: 3.007372509037242 += 0.08556154520735129


  • Change the observation time to something longer or shorter. Does the observation and spectrum results change as you expected?

  • Change the spectral model, e.g. add a cutoff at 5 TeV, or put a steep-spectrum source with spectral index of 4.0

  • Simulate spectra with the spectral model we just defined. How much observation duration do you need to get back the injected parameters?

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