Fermi Gamma-ray Space Telescope

Fermi Science Tools — Basic Concepts

There are certain basic concepts that you should know before you use the Fermi Science Tools. Here we do not summarize Fermi's instrument complement and their characteristics. Instrument descriptions are available in the Introduction section.

LAT Data

You will be analyzing a list of photons of astrophysical origin; the LAT instrument team will have analyzed each event that is telemetered to the ground, decided whether it is an astrophysical photon, and if so, characterized it by arrival time, best-estimated energy, and best-estimated origin on the sky. The energy and incident direction are measured quantities with a finite precision that is modeled by the Instrument Response Functions. This list of events is stored in a FITS file (FITS is described in section 3.1.2) called a 'photon' or perhaps an 'extended' file.

Fermi surveys the sky most of the time, and even when the LAT is pointed at a particular point, sources over a large fraction (~20%) of the sky will be observed simultaneously in the LAT's large field-of-view. Therefore one generally does not distinguish between distinct LAT observations of a source. The LAT data are essentially one continuous list of events from the beginning of the mission until the end with photons arriving from a large region of the sky at any given time. You, the data analyst, will select the events you want to analyze from a given time range and spatial region. Because of the LAT's relatively large point spread function and the resulting necessity of analyzing not only the source of interest but also nearby sources, you will extract events from a region around your source of interest.

To know where the LAT was pointed at any given time (and where the spacecraft was in its orbit), you need a FITS file called the 'spacecraft' file. The spacecraft file also provides the livetime; this will indicate when the LAT was not actively taking science data during SAA passages and other episodes. When you select the events from a given time range, you will usually also extract a spacecraft file covering that time range.

The processing of the LAT events by the instrument team results in user-selectable analysis classes that are tailored for different types of analysis. These classes represent trade-offs between the number of counts that are considered photons, the non-photon background, the size of the PSF, and other considerations. With the current "Pass 8" version of the event selection, several event classes are provided:

  • The TRANSIENT classes have loose selection criteria, increasing the effective area at the expense of a larger PSF and a larger fraction of counts that do not result from photons.
  • he SOURCE class is the default event class for most analyses and is appropriate for the analysis of both point sources and moderately extended sources.
  • The ULTRACLEANVETO class provides a large background reduction, but at the cost of substantial loss of effective area. This class is ideal for analysis of large regions that are more sensitive to spectral features caused by instrumental backgrounds.

Each event class has been partitioned into event types based upon specific criteria. These are

  • Front/Back: partition of the data into events that convert in the front or back of the instrument.
  • PSF: partition of the data into four divisions with increasingly better PSF.
  • EDISP: partition of the data into four division with increasingly better energy resolution.

For more details on the different events classes and types see the LAT Data Products page.

The Fermi Science Tools calculate the LAT's instrument response functions for the different components of the selected analysis class based on the spacecraft file. You will be provided tools to explore the instrument response functions to give you a better understanding of the LAT's characteristics.

Therefore, almost all your analysis of LAT data will start with a photon or extended file with the events from a region of the sky and a time range and a spacecraft file describing the LAT's pointing during the same time range.

Fermi GBM Data

Since the GBM is a burst detector, burst analysis will predominate. The basic GBM burst data will be 'Time Tagged Events' (TTE), lists of the detected events from the different GBM detectors with their arrival times and apparent energies.

The GBM detectors have a relatively high non-burst background and therefore you will need background spectra. While there will be tools to create your own background, the GBM team will provide appropriate background files for each burst for each GBM detector.

The GBM instrument response for a given detector at a given time will be represented by the Detector Response Matrix (DRM). Since the spacecraft may slew during a burst to center the burst in the LAT's field-of-view, a given detector may require a series of DRMs for a burst. Again, a tool will be provided for you to calculate DRMs, and the GBM team will provide DRMs appropriate to each burst.

Analysis Environment

The Fermi Science Tools are an extension of the FTOOLS environment. Therefore almost all the tools can be run 'ballistically' from the command line. A ballistic tool is run by invoking the tool (e.g., typing its name at the operating system prompt) and entering its parameters. Once the parameter values are input, the tool proceeds without interacting with the user. Parameters can be input in a number of ways:

  • The user can input all or some of the parameters on the command line.
  • The user will be prompted for 'query' parameters that were not input on the command line. For each query, the user is presented with the last value used; the user can accept this default by merely hitting a return. There are also 'hidden' parameters that can only be input on the command line.
  • By adding 'mode=h' on the command line, the user can accept all the defaults (the last values used) except for parameters entered on the command line, without being queried. This input method supports scripting the tools into an analysis pipeline.

In addition, some tools will send plots to your screen. Currently gtburstfit, gtlike, and gtpsearch have this feature. This option can be invoked by including 'plot=yes' on the command line. For example:

gtpsearch plot=yes

Data are input to, and output from, these tools as FITS files (FITS is short for 'Flexible Image Transport System'). FITS files consist of a series of units. Each unit has an ASCII 'header,' a list of 'keywords' and their values, followed by a data table. The data in the data table are described by keywords in the header. The first unit usually does not have a data table, and thus has only a 'primary header.' The subsequent units are called 'extensions.' FITS files can be read and modified by standard tools such as fv.

In addition to the ballistic FTOOLS, there are a number of tools that are run interactively.

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