The LAT performance is governed primarily by three things:
A result of the performance analysis is the production of full Instrument Response Functions (IRFs), describing the performance as a function of photon energy, incidence angle, conversion point within the instrument, and other important parameters.
The IRF is the mapping between the incoming photon flux and the detected events. 'Detection' depends not only on the LAT hardware but also on the processing that calculates the event parameters from the observables and assigns probabilities that an event is a photon. Indeed, the different event cuts are based on tradeoffs between the non-photon background, the effective area and the spatial and energy resolution; these cuts result in analysis classes (see the section on LAT Data Products).
To evaluate the LAT response, a dedicated Monte Carlo simulation is performed. A large number of gamma-ray events are simulated in order to cover all possible photon inclination angles and energies with good statistics. This is based on the best available representation of the physics interactions, the instrument, and the on-board and ground processing to produce event classes (see Atwood et al. 2009 and Ackerman et al. 2012). The comparison between the properties of the simulated events within a given event class and the input photons gives the instrument response functions.
The IRF is factored into three terms: efficiency in terms of the detector's effective area, resolution as given by the point-spread function (PSF), and energy dispersion. The components of the IRFs are the measured representation of the corresponding figures of merit in terms of the photon true energy and incidence angle. The values associated with each parameterization are defined within the Fermi Science Tools calibration database (CALDB) which is included in the Science Tools distribution. The forms for these functions are presented in the subsequent sections.
Each event class and event type selection has its own IRFs which must be used when performing analysis of events passing that selection. Internally the IRFs for each event class contain a set of IRF parameterizations for individual event types. The event types are organized in partitions that split the events within a class according to different criteria. Whereas previous data releases provided only a single partition based on the conversion plane of the reconstructed track (FRONT versus BACK), the Pass 8 data release defines three event type partitions: FRONT/BACK (two types), PSF (four types), and EDISP (four types). The selections that define the event classes and event types are described in more detail in the LAT Photon Classification section.
The Science Tools support analysis with both individual event types as well as any superset of event types within a partition (e.g. PSF2+PSF3 or FRONT+BACK). In the case that the data selection is a superset of event types within a partition, the Science Tools will apply the response function appropriate for the sum of those event types. In the typical usage scenario in which all events within a class are selected (equivalent to evtype=3) the Science Tools will apply the response function for the sum of FRONT and BACK events.
The Pass 8 release introduces a new internal format for the IRF files whereby parameter tables for event types within each partition are stored in a single aggregated IRF file (designated FB, PSF, or EDISP in the file suffix). The event type of a parameter table within the IRF file is designated by the suffix in the key of the header/data unit. For instance the FRONT and BACK effective area tables for P8R2_SOURCE_V6 are contained in the "EFFECTIVE AREA_FRONT" and "EFFECTIVE AREA_BACK" HDUs of the IRF file "aeff_P8R2_SOURCE_V6_FB.fits".
There are multiple IRFs delivered with the Fermi Science Tools to allow the user the flexibility necessary for the different analysis types. The LAT data currently being released by the FSSC have been processed using the "Pass 8" event-level analysis. The Pass 8 analysis uses an entirely new set of event-level reconstruction algorithms that improve the instrument performance and mitigate pile-up effects. The Pass 8 data are processed with the same calibration constants used for the P7REP data release. The current Pass 8 data set is known as "P8R2".
The P8R2_V6 IRFs provide the current description of the instrument response for the P8R2 data release. Systematic uncertainties associated with these IRFs are documented in the LAT Caveats page.
The main features of the P8R2_V6 IRFs are:
Recommendations for the appropriate selection and IRF set to be used within an analysis are provided in the Data Preparation section of the Cicerone.
The tables below give the association between the P8R2 IRF sets and the photon properties as provided in the LAT photon data section. The columns "Photon File" and "Extended File" indicate whether a given class is available in the corresponding data file type.
|P8R2 IRF name||Event Class (evclass)||Class Hierarchy||Photon File||Extended File|
The following table lists the event types defined in the P8R2_V6 IRFs. The event types are organized in three partitions: FRONT/BACK, PSF, and EDISP. IRFs are provided for every permutation of event class and type.
|P8R2 Event Type Name||Event Type Partition||Event Type Value (evtype)|