LAT GRBs - Important Table Information

Table Caveats:

This table contains preliminary results from a quick look pipeline that performed searches for gamma-ray bursts (GRBs) in Fermi-LAT data. The table is meant to facilitate follow up analysis of LAT detected GRBs and is not an official catalog. The results presented on the table have been reviewed by the Fermi-LAT burst advocates, but they could be superseded by new analysis or subsequent publications at any time without notice. Therefore, users are encouraged to consult published papers for final results. Each table entry represents a GRB for which a Fermi-LAT detection has been announced via GCN notice or published in Ackermann et al. 2013.

Table Description:

All times are in UTC. All coordinates are J2000.

GRB: The full nine character GRB name.

GCN Name: The seven character GCN name.

MET: The trigger time of the GRB in Fermi Mission Elapsed Time.

Date: The trigger date in the YYYY-MM-DD UTC format.

Time: The trigger time in the hh:mm:ss.ss UTC format.

RA, Dec, Error: The best available burst localization announced via GCN in degrees.

Source: The instrument providing the best available burst localization.

Theta: Angle in degrees between the boresight of the LAT (Z-axis) and the source position at the time of the trigger.

Zenith: Angle in degrees between the Zenith of the LAT and the source position at the time of the trigger. A Zenith angle larger than ~90 deg indicated that contamination from the Earth Limb, a bright source of gamma rays, might reduce the sensitivity of the analysis.

LLE Detection: Whether the GRB has been detected with a non-standard LAT Low Energy (LLE) analysis (see below).

Likelihood Detection: Whether the GRB has been detected with a standard Likelihood analysis (see below).

LLE Significance: Significance of the signal above the expected background in sigma pre-trial. A GRB with LLE sigma > 5 is considered detected by the LLE analysis.

TS: The value for the Test Statistic (TS) for the Likelihood Ratio Test. TS = 25 corresponds to ~5 sigma. A GRB with TS > 25 is considered detected by the likelihood analysis.

Analysis RA, Analysis Dec, Analysis Error: The position on which the Likelihood analysis was performed.

IRF: The instrument response functions (IRFs) used for the likelihood analysis (see FSSC for more details)

Likelihood Analysis:

The detection, flux determination and spectral modeling of Fermi-LAT sources is accomplished by a maximum likelihood analysis. The general concept of this analysis are described in details on the FSSC website. A detailed description of the analysis applied to GRBs is described in Ackermann et al. 2013. The likelihood detection involves performing a Likelihood Ratio Test (LRT) between a model containing only background sources (such as the Galaxy) and another model containing all background sources plus a point source (the putative GRB). The GRB is considered detected if the Test Statistic for the LRT is larger than the pre-defined threshold of 25, corresponding to ~5 sigma. Here is a summary of the data cuts used in the likelihood analysis:

Region of Interest: 12°
Zenith Angle Cut: 105°
Energy Range: 100 MeV to 10 GeV

LLE Analysis:

The LAT Low-Energy class is designed to be used for the study of transient sources such as GRBs and Solar Flares. It features a larger effective area with respect to standard classes (especially between 30 and 100 MeV where standard classes are not usable), at the expenses of a worst background rejection and a very broad PSF. The detection of a GRB is made on data accumulated from -500 to 500 s around the trigger time. A background model is constructed using data from off-pulse intervals and then interpolated to the on-pulse interval, and the GRB is considered detected if the signal in the on-pulse time interval is more than 5 sigma above the expected background level. For details, see Ackermann et al. 2013. Also, see the Fermi LAT Low-Energy Events Catalog.

Credits:

Table, Burst Advocate web interface and web infrastructure: Daniel Kocevski, J.D. Myers
Detection and localization algorithms, software infrastructure: Nicola Omodei, Giacomo Vianello, Vlasios Vasileiou, Fred Piron
Results review: the Fermi-LAT collaboration's GRB group and burst advocates