How To

This page contains short “how to” or “frequently asked question” entries for Gammapy. Each entry is for a very specific task, with a short answer, and links to examples and documentation.

If you’re new to Gammapy, please read the Overview and have a look at the list of Tutorials. The information below is in addition to those pages, it’s not a complete list of how to do everything in Gammapy.

Please give feedback and suggest additions to this page!

Access IACT data

To access IACT data use the DataStore. You can see how to create one with the high level interface Analysis here. You can also create it directly, see here.

Check IRFs

Gammapy offers a number of methods to explore the content of the various IRFs contained in an observation. This is usually done thanks to their peek() methods. See example for CTA here and for H.E.S.S. here.

Use gammapy for modeling 2D images

Gammapy treats 2D maps as 3D cubes with one bin in energy. To see an example of the relevant data reduction, see 2-dim sky image analysis

Sometimes, you might want to use previously obtained images lacking an energy axis (eg: reduced using traditional IACT tools) for modeling and fitting inside gammapy. In this case, it is necessary to attach an energy axis on

Extract 1D spectra

The Analysis class can perform spectral extraction. The AnalysisConfig must be defined to produce ‘1d’ datasets. Alternatively, you can follow the spectrum extraction notebook.

Extract a lightcurve

The Light curve estimation tutorial shows how to extract a run-wise lightcurve.

To perform an analysis in a time range smaller than that of an observation, it is necessary to filter the latter with its select_time method. This produces an new observation containing events in the specified time range. With the new Observations it is then possible to perform the usual data reduction which will produce datasets in the correct time range. The light curve extraction can then be performed as usual with the LightCurveEstimator. This is demonstrated in the Light curve - Flare tutorial.

Compute source significance

Estimate the significance of a source, or more generally of an additional model component (such as e.g. a spectral line on top of a power-law spectrum), is done via a hypothesis test. You fit two models, with and without the extra source or component, then use the test statistic values from both fits to compute the significance or p-value. To obtain the test statistic, call stat_sum for the model corresponding to your two hypotheses (or take this value from the print output when running the fit), and take the difference. Note that in Gammapy, the fit statistic is defined as S = - 2 * log(L) for likelihood L, such that TS = S_1 - S_0. See Datasets for an overview of fit statistics used.

Compute cumulative significance

A classical plot in gamma-ray astronomy is the cumulative significance of a source as a function of observing time. In Gammapy, you can produce it with 1D (spectral) analysis. Once datasets are produced for a given ON region, you can access the total statistics with the info_table(cumulative=True) method of Datasets. See example here.

Detect sources in a map

Gammapy provides methods to perform source detection in a 2D map. First step is to produce a significance map, i.e. a map giving the probability that the flux measured at each position is a background fluctuation. For a MapDataset, the class TSMapEstimator can be used. A simple correlated Li & Ma significance can be used, in particular for ON-OFF datasets. The second step consists in applying a peak finer algorithm, such as find_peaks. This is demonstrated in the Source detection tutorial.

Astrophysical source modeling

It is possible to combine Gammapy with astrophysical modeling codes, if they provide a Python interface. Usually this requires some glue code to be written, e.g. NaimaSpectralModel is an example of a Gammapy wrapper class around the Naima spectral model and radiation classes, which then allows modeling and fitting of Naima models within Gammapy (e.g. using CTA, H.E.S.S. or Fermi-LAT data).

Implement a custom model

Gammapy allows the flexibility of using user-defined models for analysis. For an example, see ` Implementing a Custom Model <tutorials/models.html#Implementing-a-Custom-Model>`__.

Energy Dependent Spatial Models

While Gammapy does not ship energy dependent spatial models, it is possible to define such models within the modeling framework. For an example, see ` here <tutorials/models.html#Models-with-Energy-dependent-morphologyl>`__.