Fermi Gamma-ray Space Telescope

Overview of Data Exploration

The quantitative analysis that detects sources and fits spectral parameters is often preceded by qualitative analysis that suggests that a source is present, or that a spectrum has an additional feature. This qualitative analysis involves selecting and then displaying data, and will depend on the data analyst's research style.

The fundamental LAT and GBM data are simple event lists, and data selection involves making cuts on the event lists. As discussed above, data selections can be made when extracting the LAT event list from the FSSC database, and further selections are made using the gtselect tool. Thus, for example, you can extract data for a source spanning a large time range, and use gtselect to break this event list into a series of shorter time ranges. You can extract LAT events from a large spatial area that includes a gamma-ray burst, and after localizing the burst, you can select the counts from a smaller area centered on the burst.

Many of these functions are usually performed by the HEASoft tool xselect, a complex tool that has many mission-specific features. xselect may eventually operate on Fermi data, but we have decided to first add a few simple Fermi-specific tools to the Fermitools package.

Once the data are selected, they can be displayed using standard display tools such as fv and ds9. Plotting individual counts as a function of position, time or energy may be revealing, but as the number of counts increases, binning the data becomes useful. For this purpose we have developed a binning tool, gtbin, that can bin both LAT and GBM data in time, energy and space. The products are FITS files; where relevant, standard formats are used (e.g., PHA and PHA-II for spectra), which again can be examined with tools such as fv or ds9.

» Forward to Data Preparation
» Back to the beginning of Data Exploration
» Back to the beginning of the Cicerone