# Glossary and references¶

## Glossary¶

- 1D Analysis
1D analysis or spectral analysis where data are reduced to a simple 1D geometry along the reconstructed energy axis. In Cherenkov astronomy, this is classically performed with a OFF background measurement.

- 3D Analysis
3D analysis or cube analysis, where data are reduced to a 3D cube with spatial coordinates and energy axes. In gammapy, these cube are represented by

`Map`

objects (see maps - Sky maps) and contained in a`MapDataset`

object.- Aeff
Is short for effective area, the IRF representing the detector collection area. See Effective area.

- Cash
The cash statistic is a Poisson fit statistic usually used when signal and background can be modeled. It is defined as \(2 \times log(L)\) See Cash : Poisson data with background model in fit statistics.

- Dataset
In Gammapy a dataset bundles the data, IRFs, model and a likelihood function. Based on the model and IRFs the predicted number of counts are computed and compared to the measured counts using the likelihood.

- EDisp
EDisp is short for energy dispersion, the IRF that represents the probability of measuring a given reconstructed energy as a function of the true photon energy. See Energy Dispersion

- GTI
Short for Good Time Interval: an continuous time interval of data acquisition. In CTA, they also represent time intervals where the IRFs are supposed to be constant.

- IRF
Short for Instrument Response Function. They are used to model the probability to detect a photon with a number of measured characteristics. See IRF Theory and irf - Instrument response functions.

- Joint Analysis
In a joint analysis individual observations are reduced to datasets and are modeled all together to extract physical informations through model fitting. This is opposed to stacked analysis.

- MET
Short for Mission Elapsed Time; see also Mission elapsed times (MET) in Time handling in Gammapy.

- Reco Energy
The reconstructed (or measured) energy (often written

`e_reco`

) is the energy of the measured photon by contrast with its actual true energy. Measured quantities such as counts are represented along a reco energy axis.- Reflected Background
Background estimation method typically used for spectral analysis.

- Ring Background
Background estimation method typically used for image analysis.

- Stacked Analysis
In a stacked analysis individual observations are reduced to datasets which are then stacked to produce a single reduced dataset. The latter is then used to obtain physical information through model fitting. Some approximations must be made to perform dataset stacking (e.g. loss of individual background normalization, averaging of instrument responses, loss of information outside region of interest etc), but this can reduce very significantly the computing and memory cost.

- True Energy
The true energy (often written

`e_true`

) is the energy of the incident photon by contrast with the energy reconstructed by the instrument. Instrument response functions are represented along a true energy axis.- WStat
The WStat is a Poisson fit statistic usually used for ON-OFF analysis. It is based on the profile likelihood method where the unknown background parameters are marginalized. See WStat : Poisson data with background measurement in fit statistics.

## References¶

This is the bibliography containing the literature references for the implemented methods referenced from the Gammapy docs.

- Albert2007
Albert et al. (2007), “Unfolding of differential energy spectra in the MAGIC experiment”,

- Berge2007
Berge et al. (2007), “Background modelling in very-high-energy gamma-ray astronomy”

- Cash1979
Cash (1979), “Parameter estimation in astronomy through application of the likelihood ratio”

- Cousins2007
Cousins et al. (2007), “Evaluation of three methods for calculating statistical significance when incorporating a systematic uncertainty into a test of the background-only hypothesis for a Poisson process”

- Feldman1998
Feldman & Cousins (1998), “Unified approach to the classical statistical analysis of small signals”

- Lafferty1994
Lafferty & Wyatt (1994), “Where to stick your data points: The treatment of measurements within wide bins”

- LiMa1983
Li & Ma (1983), “Analysis methods for results in gamma-ray astronomy”

- Meyer2010
Meyer et al. (2010), “The Crab Nebula as a standard candle in very high-energy astrophysics”

- Naurois2012
de Naurois (2012), “Very High Energy astronomy from H.E.S.S. to CTA. Opening of a new astronomical window on the non-thermal Universe”,

- Piron2001
Piron et al. (2001), “Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000”,

- Rolke2005
Rolke et al. (2005), “Limits and confidence intervals in the presence of nuisance parameters”,

- Stewart2009
Stewart (2009), “Maximum-likelihood detection of sources among Poissonian noise”

- Abdalla2018
H.E.S.S. Collaboration (2018), “The H.E.S.S. Galactic plane survey”

- Mohrmann2019
Mohrmann et al. (2019), “Validation of open-source science tools and background model construction in γ-ray astronomy”

Software references:

- Raue2012
Raue (2012), “PyFACT: Python and FITS analysis for Cherenkov telescopes”

- Robitaille2013
Robitaille et al. (2013) “Astropy: A community Python package for astronomy”

- Knoedlseder2016
Knödlseder et at. (2016) “GammaLib and ctools. A software framework for the analysis of astronomical gamma-ray data”

- FSSC2013
Fermi LAT Collaboration (2013) “Science Tools: LAT Data Analysis Tools”

- Mayer2015
Michael Mayer (2015) “Pulsar wind nebulae at high energies”

## Other gamma-ray packages¶

Here are some other software packages for gamma-ray astronomy:

Gammalib /ctools is a C++ package with Python wrapper, similar to the Fermi-LAT ScienceTools, that to a large degree uses the same input data formats as Gammapy.

3ML is a Python package that uses existing packages (e.g. the Fermi-LAT ScienceTools or the HAWC software) to deal with the data and IRFs and compute the likelihood for a given model.

Sherpa — X-ray modeling and fitting package by the Chandra X-ray Center

ctapipe — CTA Python pipeline experimental version

FermiPy — Fermi-LAT science tools high level Python interface by Matthew Wood

gammatools — Python tools for Fermi-LAT gamma-ray data analysis by Matthew Wood

pointlike – Fermi-LAT science tools alternative by Toby Burnett

naima — an SED modeling and fitting package by Victor Zabalza

Gamera — a C++ gamma-ray source modeling package (SED, SNR model, Galactic population model) with a Python wrapper called Gappa by Joachim Hahn

FLaapLUC — Fermi/LAT automatic aperture photometry light-curve pipeline by Jean-Philippe Lenain

http://voparis-cta-client.obspm.fr/ — prototype web app for CTA data access / analysis, not open source.

act-analysis — Python scripts and Makefiles for some common gamma-ray data analysis tasks by Karl Kosack

VHEObserverTools — tools to predict detectability at VHE by Jeremy Perkins

photon_simulator — Python code to simulate X-ray observations

pycrflux — Python module to plot cosmic-ray flux

Andy strong has C++ codes (GALPROP and Galplot) for Galactic cosmic rays and emission and source population synthesis at http://www.mpe.mpg.de/~aws/propagate.html .

## Other useful packages¶

In addition to the packages mentioned in the last section and at Dependencies, here’s a few other Python packages you might find useful / interesting:

See the list here: http://www.astropy.org/affiliated/

Pulsar timing package PINT