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.

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, apparent energy, and apparent origin on the sky. We say 'apparent' because this analysis does not reconstruct the photon's precise energy and incident direction. This list of photons is stored in a FITS file (FITS is described in section 3.1.2) called an 'event file' and given the .FT1 file extension.

Fermi will probably survey the sky most of the time, and even when the LAT is pointed at a particular point, sources over a large fraction of the sky will be observed simultaneously in the LAT's large field-of-view. Therefore one cannot distinguish between distinct LAT observations of a source. The LAT data are essentially one continuous list of photons 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 photons 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 photons from a region ~20 degrees in radius.

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' and given the .FT2 file extension. The spacecraft file also provides the livetime; this will indicate when the LAT was not actively taking science data during SAA passages. When you select the photons 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, and the size of the PSF. With the current "Pass 7" version of the photon selection four event classes are provided (these are also aplicable to the newly reprocessed Pass 7 data):

  • The P7TRANSIENT class has a 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.
  • The P7SOURCE class should be considered the default, is appropriate for the analysis of point sources and was used to construct the 2FGL source catalog.
  • The P7CLEAN class reduces the particle background substantially at the expense of a smaller effective area. It is appropriate for use in studying large scale diffuse emission, and in other analyses which benefit for lower background contamination.
  • The P7ULTRACLEAN class provides even larger background reduction, but at the cost of substantial loss of effective area. This class is primarily intended to be used as a cross check with the Clean class, to ensure that observed features are not artifacts of the background contamination.

Prior to August 2011, the Fermi Science Support Center was releasing photon data which were selected with the earlier "Pass 6" version of the photon selection. For those data the basic concept of selectable events classes also applied, but the details of the selection and the names of the classes were different.

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 an event file (.FT1) with the photons from a region of the sky and a time range and a spacecraft file (.FT2) 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 photons 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.

Many tools have a gui option that can be activated by including 'gui=yes' when invoking the tool at the command line. For example:

gtselect gui=yes

In addition, some tools will send plots to your screen. Currently gtburstfit, gtlikelihood, 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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