Overview: LAT Data Analysis Tools

Through a collaborative effort between the Fermi Science Support Center and the LAT instrument team a suite of instrument specific science analysis tools has been developed for public release. This software will be distributed and maintained by the FSSC. It has been designed within the framework of the HEADAS FTOOLS methodology, to ensure cross-mission compatibilities wherever possible and to minimize the learning curve for users of other high-energy astrophysics mission data sets. The general analysis flow is illustrated in this graphic:

STflow

The analysis threads are part of a hierarchical documentation set offered in support of Fermi data analysis. The threads are basically "cookbook" analysis examples which could for example be followed verbatim and then repeated but substituting data sets and observational scenarios of your own choosing. The other components of the documentation set are a broad overview guide, the "Cicerone", and individual reference guides - based on the familiar "fhelp" model - for each science tool.

The following set of Fermi-specific tools are used to analyze LAT data; some tools, such as gtbin, can also process GBM data products. Tool groups include: Utilities, Likelihood, Gamma-ray Burst Analysis, Pulsar Analysis, and Observation Simulation.

Utilities:

gtbin	Computes the counts maps for use in a binned likelihood analysis; bins are 'cubes' in latitude, longitude, and log(energy).
gtexposure	Given a counts light curve prepared using gtbin for a specific location on the sky, this tool computes the exposure (cm2-s) associated with each time bin.
gtirfs	List all available response functions.
gtltsum	Adds together livetime cubes produced by gtltcube.
gtmktime	Create Good Time Intervals (GTIs) based on selections made on the spacecraft data file variables.
gtpsf	Calculates the effective point spread function as a function of energy at a given source location, averaged over an observation.
gtselect	Creates a new filtered FITS file based on cuts applied to the values in each row of an input event file. Enablies more detailed selections to be made on data obtained from the data server.
gtsrcid	Creates a counterpart candidate catalog by correlating the objects from a list of detected sources with the objects of an existing source catalog, such as the 3EG catalog.
gtvcut	Prints a summary of the Data Sub-Space keywords.

Likelihood:

gtbin	Computes the counts maps for use in a binned likelihood analysis; bins are 'cubes' in latitude, longitude, and log(energy).
gtexpmap	Computes the exposure maps for use in an unbinned likelihood analysis.
gtexpmap2	Computes exposure maps as a function of energy. The geometry of the map may be specified by the user as pfile parameters or they may be read from a counts map cube created by gtbin. These maps are used for binned likelihoood analysis.
gtdiffrsp	Computes the event-specific response for each diffuse source in the input source model. A column containing these data is added to the event file for each source.
gtfindsrc	Optimizes a point source location using the likelihood test-statistic.
gtlike	Enables LAT data to be analyzed using an unbinned or binned formulation of the log-likelihood.
gtltcube	Integrates the livetime as a function of sky position and off-axis angle, using the spacecraft data file and the time range and GTI selections in the event file. The sky is represented using a nested HEALPix array.
gtmodel	Creates a model counts map by summing up convolved source maps that have been scaled by the spectral parameters given in the source model XML file.
gtsrcmaps	Convolves components of the specified source model with the instrument response for a given observation. The geometry in sky and energy coordinates of the output maps match that of the input counts map.
gttsmap	Computes a test statistic map for source localization and detection in an unbinned likelihood analysis.

Gamma-ray Burst Analysis:

gtbkg	Creates a background spectrum file for use in XSPEC analysis.
gtbin	Bins an event list in time, energy, or space, and results in spectra, lightcurves or a count map.
gtbindef	Utility used to read in an ASCII file with the time or energy bin definitions, and produce the FITS file, which can be used by gtbin.
gtrspgen	Creates a standard FITS file (rmf) required to analyze a binned spectrum.
gtburstfit	Analyzes burst light curves by applying a Bayesian algorithm to determine the optimum set of blocks to follow the burst profile shape; blocks may also be used by gtburstfit to construct a pulse model, which gtburstfit can fit to the data.

Pulsar Analysis:

gtpsearch	Searches for pulsation frequencies near a known, or estimated reference frequency.
gtpspec	Searches for pulsations in a wider range of frequencies, using the Discrete Fast Fourier Transfer (FFT) technique to compute power spectrum density.
gtptest	Applies all of the statistical tests available with the gtpsearch application to a series of pulse phase values stored in given event files.
gtpphase	Operates on an event file to compute the spin (pulse) phase for the time of each event, and writes this phase to the PULSE_PHASE column of the event file.
gtophase	Operates on an event file to compute the orbital phase for the time of each event, and writes this phase to the ORBITAL_PHASE column of the event file.
gtephem	Presents the user with the best ephemeris available in a pulsar database for a given pulsar and instant of time.
gtpulsardb	Creates, filters, and/or combines pulsar ephemerides database files.
gtbary	Performs a barycentering time correction to FITS files using GLAST orbit files.

Observation Simulation:

gtorbsim	Generates spacecraft orbit and attitude data for a variety of pointing strategies.
gtobssim	Generates photon events from astrophysical sources and processes those photons according to the specified instrument response functions.

Last updated by: Elizabeth Ferrara 05/25/2011