LAT Data Selection Recommendations (adapted from the Cicerone)These recommendations apply only to an analysis of LAT data using the P6_V3 instrument response functions.
Event Selection Recommendations (P6_V3)
Time Selection Recommendations
Prior to performing any analysis on Fermi data, it is necessary to select the set of photons that are best suited for the science being performed. In most cases, the set of data selection criteria (cuts) you make will depend on the analysis being performed. Since the photon data is meant to serve for any analysis type, you will need to process the cuts before you start your analysis. There are several standard analysis types, each requiring their own sets of data cuts. These are:
There are many configurable parameters that can be used in preparing the LAT data for analysis. These parameters are inputs in various tools like gtselect. Certain cuts are strongly recommended for all data analysis processes, while others are flexible, depending on the type of analysis being performed. The tables above outline the recommended cuts to be used for the event selection (gtselect) and time selection (gtmktime) appropriate for the different analysis types. Analysis of more complex sources, or confused regions, will require optimization of some of these parameters beyond the values listed below. If a parameter is not listed, then there is no specific recommended value for its use within that analysis chain.
Preparation of data for analysisMaking cuts using the events file keywordsApplying selection cuts to the LAT data is typically performed using gtselect. Cuts are made to bound the dataset's time range, energy range, position, region of interest radius, and maximum zenith angle. The maximum zenith angle selection is designed to exclude time periods when any portion of the region of interest is too close to the Earth's limb, resulting in elevated background levels. The Earth's limb lies at a zenith angle of 113 degrees, so a suggested value of 105o provides protection against significant contamination by atmospheric gammas. Note: This cut requires careful handling when calculating livetime. (See Likelihood Livetime and Exposure.) Additionally, gtselect can be used to include/exclude events that have a lower probability of being photons (event classes 1 or 2). In general, it is recommended that only diffuse class (event class = 3) photons be used for LAT analysis. However, analysis of short duration transients such as prompt emission from gamma-ray bursts is not as sensitive to the presence of moderate background contamination, and so do not suffer from the presence of lower probability events. The instrument response functions used in the analysis tools must match the event classes selected. The reconstruction and classification of LAT events is performed by the LAT instrument team as part of the Level 1 science data processing. The version of the event reconstruction algorithms used is referred to as passN. Currently, the LAT data pipeline uses Pass 6 reconstruction algorithms. This processing is inherent in the science data, and cannot be changed. For a given reconstruction, the LAT team generates an accompanying set of parameterized instrument response functions (IRFs) that are designed for analysis of that particular dataset. These are labeled VN (where N is the version of the IRF). There may be several updates of IRFs for a given reconstruction, with one recommended for analysis (currently P6_V3). The IRFs selected later in the analysis chain must match the data reconstruction version (currently Pass 6) and the event class selected at this stage (i.e., P6_V3_DIFFUSE) for a point source analysis. (Also see Event Classes and Instrument Response Function.) The acceptance cone radius will vary with analysis type and source location. Performing a spectral analysis on a point source in the galactic plane will require a larger initial analysis region than for a source off the plane, to allow for fitting of multiple nearby sources. An acceptance cone radius of 10 deg is appropriate for spectral analysis of point sources off the Galactic plane, while 10-20 deg may be necessary for point sources located near the Galactic Plane. A timing analysis that does not use spectral modeling (for example, pulsar lightcurves) might select a smaller region to reduce background levels. These studies (timing analysis, GRB spectral analysis, extended source analysis, and others) require optimization of the acceptance cone on a case-by-case basis. Such optimization will need to consider the largest PSF of the LAT in the energy range of interest, as well as providing sufficient statistics to account for the surrounding sky structure. An all-sky analysis does not require this selection. Making cuts using the spacecraft file keywordsThe spacecraft files are used to select the good time intervals (GTIs) for data analysis by using gtmktime. Selecting times when the data quality is good (DATA_QUAL==1) will exclude time periods when some spacecraft event has affected the quality of the data. gtmktime can be used to generate and combine GTIs for any logical expression of keywords in the spacecraft file. Contamination from albedo gamma rays form the Earth is reduced by applying an additional timing selection to the maximum zenith angle cut made with gtselect. Excluding time intervals when the Earth's limb intersects the selected region of interest is one method of correcting the exposure for the zenith cut. This is accomplished by using the ROIcut=yes option in gtmktime. For other exposure correction options, see the Likelihood Livetime and Exposure section. GBM data does not typically require selection cuts prior to use. However, cuts can be applied should they be needed for a tailored analysis chain.
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