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FERMILLE - Fermi LAT Low-Energy Events Catalog

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Overview

LAT Low-Energy events (LLE) are automatically produced for each GBM GRB in the GBM Trigger Catalog if the GBM GRB has a position within 90 degrees of the LAT boresight. LLE data are generated for a given position in the sky (RA, DEC) and for a given interval of time (T0, T1) corresponding to the GBM Burst.

The standard LLE selection applied to the downloaded events is the following:

(FswGamState==0 && TkrNumTracks>0 && (GltEngine==6 || GltEngine==7) && EvtEnergyCorr > 0) && (FT1ZenithTheta<90.0) && (FT1Theta<=90.0) && (((cos(FT1Dec*0.0174533)*(FT1Ra - (RA)))2 + (FT1Dec - (DEC))2) < PSF(EvtEnergyCorr, Theta)

where

 * FswGamState is the status of the Flight Sofware Gamma filter. We require
   that the event is a gamma-ray (FswGamState==0).
 * TkrNumTracks is the number of tracks in the tracker. We require that there
   is at least one track. This requires the event to have a reconstructed direction.
 * GltEngine is the status of the Global LAT Trigger.  We require that GltEngine equals 6 or 7, which
   corresponds to taking all the events that trigger in the tracker TKR but
   did not have a region of interest (ROI) associated (GltEngine 7) or all
   the events that pass the CalHI (at least 1 GeV in one crystal).
 * EvtEnergyCorr is the best estimation of the reconstructed energy, especially
   at low energy.
 * Theta is the reconstructed source direction (Theta) with respect the
   LAT boresight.
 * PSF(EvtEnergyCorr, Theta) represents the functional form of the containment
   radius of the Point Spread Function (PSF) of the LAT.

The exact cut used to select the events is saved in the keyword LLECUT in the primary header of each LLE file. If the GBM catalog position of the burst is updated (due to a refined localization from LAT or Swift or from subsequent on ground analysis), the LLE data are automatically updated and new versions of the LLE files are produced. In some cases, LLE data are manually generated (using a better localization which may or may not have been used in the GBM Trigger Catalog). For each updated position, the version of the corresponding LLE files increases by one.

There are six FITS files provided for each entry: the LLE event file, the time-binned spectrum (CSPEC) file, the CSPEC response (RSP) file, and the extracted burst spectrum (the PHA-I file) for the entire duration of the burst, an LLE event file with same time cut as the RSP and PHA-I files, and a LAT pointing and livetime history file.

There are six FITS files provided for each entry: the LLE event file (gll_lle_bnNNNNNNNNN_vMM.fit), the time-binned spectrum (CSPEC) file (gll_cspec_bnNNNNNNNNN_vMM.pha), the CSPEC response (RSP) file (gll_cspec _bnNNNNNNNNN_vMM.rsp), and the extracted burst spectrum (the PHA-I file) for the entire duration of the burst (gll_pha_bnNNNNNNNNN_vMM.fit), an LLE event file with same time cut as the RSP and PHA-I files (gll_selected_bnNNNNNNNNN_vMM.fit), and a LAT pointing and livetime history file (gll_pt_bnNNNNNNNNN_vMM.fit).

The LLE event file format is similar to the LAT photon file format with some exceptions. Because the LLE data are tightly connected to a particular object (position and time), the FITS keyword OBJECT has been added to the file. Generally, OBJECT will correspond to the entry of the GBM Trigger Catalog used to generate LLE data and corresponds to the "name" column in the FERMILLE table (and in the GBM Trigger Catalog table). For similar reasons, the position of the object used to select LLE file is written in the header of each extension of each LLE file. PROC_VER corresponds to the iteration of the analysis of LLE data. PASS_VER corresponds to the iteration for the reconstruction and the general event classification (Pass6, Pass7, etc.). VERSION corresponds to the version of the LLE product for this particular event. The update of a location of a GRB will increase the number of VERSION in the file, but will leave the PASS_VER and PROC_VER unchanged.

The CSPEC file is obtained from directly binning the TTE files. It provides a series of spectra, accumulated every second, from -1000 to 1000 seconds around the burst. Each spectrum is binned in 50 energy channels, ranging typically from 10 MeV to 100 GeV. The format of the CSPEC file is tailored to satisfy rmfit standards, and it is not directly usable in XSPEC.

The CSPEC Response file (the RSP file) is the detector response matrix calculated from Monte Carlo simulation, and it corresponds to a single response matrix for each Gamma-Ray Burst or Solar Flare.

The PHA-I file contains the count spectrum. The PHA-I file is created from the same time interval used to compute the response matrix.

The selected events file is identical to the LLE event file with an additional time selection applied to match the cut used to compute response matrix and PHA-I files.

The LAT pointing and livetime history file is identical to the standard LAT file but with entries every second (instead of every 30 seconds). It spans 4600 seconds before and 4600 after the trigger time.


Bulletin

The FERMILLE database table was last updated on 17 December 2014.

Caveats

Please see the warnings on the LAT Caveats page, http://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT_caveats.html.

References

See the Fermi Science Data Product Interface Control Document (http://fermi.gsfc.nasa.gov/ssc/library/support/Science_DP_ICD_RevA.pdf).

Provenance

The information in this table is provided by the Fermi LAT Instrument Science Operations Center (LISOC) and the Fermi Science Support Center (FSSC).

Parameters

Trigger_Name
The Fermi trigger designation that is assigned for each new trigger detected. It corresponds to the same column in the GBM Trigger Catalog. The naming scheme used is bnyymmddfff, where yymmdd is the date of the burst (yy, the year minus 2000; mm, the two-digit month; and dd, the two-digit day of the month) and fff = fraction of day.

Name
The designation of the source of the trigger. It corresponds to the same column in the GBM Trigger Catalog and the OBJECT keyword in the FITS file header.

RA
The Right Ascension of the trigger in the selected equinox. This is given in J2000 decimal degree coordinates in the original data. By default, it is the position in the GBM Trigger Catalog, but it can be a better localization which may or may not have been used in the GBM Trigger Catalog.

Dec
The Declination of the trigger in the selected equinox. This is given in J2000 decimal degree coordinates in the original data. By default, it is the position in the GBM Trigger Catalog, but it can be a better localization which may or may not have been used in the GBM Trigger Catalog.

LII
The Galactic Longitude of the trigger, derived from the trigger RA and Dec.

BII
The Galactic Latitude of the trigger, derived from the trigger RA and Dec.

Time
The start time of the observation, originally provided in Fermi Mission Elapsed Time (MET) format and converted to UTC.

End_Time
The end time of the observation, originally provided in Fermi Mission Elapsed Time (MET) format and converted to UTC.

Trigger_Time
The time at which the trigger occurred, originally provided in Fermi Mission Elapsed Time (MET) format and converted to UTC.

Trigger_Type
The classification of the trigger. The following trigger classes from the GBM Trigger Catalog are used:

  GRB     Gamma-ray burst
  SFLARE  Solar flare
  

Version
This parameter specifies the current version of the catalog entry file. New versions are provided by the LISOC as additional data are added or further analysis done.


Contact Person

Questions regarding the FERMILLE database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 16-Apr-2013