2. Get Fermi LAT data

2.1. Spacecraft file

The spacecraft file doesn’t depend on the sky region or energy range you are interested in ... it is valid for the whole sky and all energies.

The only thing you have to watch out for is that your spacecraft file covers the time range you want to analyse. To obtain a spacecraft data file that covers the whole length of the Fermi mission (updated daily with new data) use this command

wget -O spacecraft.fits ftp://legacy.gsfc.nasa.gov/FTP/fermi/data/lat/mission/spacecraft/lat_spacecraft_merged.fits

2.2. Photon files

Usually you will do this via the FSSC data query web interface as described in the Extract LAT Data analysis thread.

TODO: give screenshots and short description.

Tip

If you are a Fermi LAT power user (e.g. run analyses on several regions or update your analyses every once in a while) or if there are several Fermi LAT data analysts at your institute you should consider downloading the weekly photon and spacecraft files as described here:

wget -m -P . -nH --cut-dirs=4 -np -e robots=off \
ftp://legacy.gsfc.nasa.gov/fermi/data/lat/weekly/photon/
wget -m -P . -nH --cut-dirs=4 -np -e robots=off \
ftp://legacy.gsfc.nasa.gov/fermi/data/lat/weekly/spacecraft/

Note that the weekly photon files currently (2013, covering five years of Fermi observations) are about 30 GB (giga-bytes) in size, so make sure you have the disk space and internet connection bandwidth.

2.3. Example: Getting data from the April 2011 Crab Nebula Flare

To carry out the aperture lightcurve excercise you will have to download the photon data from the FSSC data query web interface. We are interested in photons from a region of 1 degree radius around the Crab Nebula during the period of 16 days between April 8th and April 24th 2011. To convert between calendar dates, MJD and Fermi MET (Mission Elapsed Time, or seconds since January 1st, 2001), you can use the NASA HEASARC xTime tool.

Since we have already provided you with the whole-mission spacecraft.fits file, there is no need to download the spacecraft file for this period, so you uncheck the Spacecraft data box.

The parameters for the photon query should therefore be:

LAT query parameters
Parameter Value
Object Name Crab Nebula
Equatorial coordinates (degrees) (83.6331,22.0145)
Time range (MET) (323913600,325296000)
Time range (Gregorian) (2011-04-08 00:00:00,2011-04-24 00:00:00)
Energy range (MeV) (100,300000)
Search radius (degrees) 1

After a brief wait, download the resulting photon file to the $FERMI_HERO/excercises/lightcurve directory and, optionally, rename it to photon.fits to make it easier to remember.