Fermi Gamma-ray Space Telescope

LAT Instrument Response Functions—Overview

The LAT performance is governed primarily by three things:

  • LAT hardware design
  • Event reconstruction algorithms
  • Background selections and event quality selections

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 LAT IRF parameters

The IRF is factored into three terms: efficiency in terms of the detector's effective area, and 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 full IRF set is further split into two complete representations describing photons that convert in either the front or back section of the LAT tracker (described in the LAT overview). Starting from the top of the instrument, the tracker consists of 12 layers of 3% radiation length tungsten converters (front or thin section), followed by 4 layers of 18% radiation length tungsten converters (back or thick section). The angular resolution for photons that convert in the front section is intrinsically better than for those that convert in the back section because the electron-positron pair is detected by the silicon detectors before significant multiple-scattering—which scales with the material thickness—has taken place. Because of this, the parameterization of both the PSF and the energy dispersion must be calculated for front and back events separately. The end result is that the IRFs applied in an analysis must match the event class selection and conversion type selection. Typically, both conversion types are selected and the combined front and back IRF is applied.

IRF selection

Because of the complexity of the IRFs, LAT data analysis must be performed carefully to avoid a mismatch between the selected events and the IRF used within the analysis. 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 has been processed using Pass 7 along with updated calibration constants up to September 2013, and is know as the "P7REP" data set. (In September 2013 the LAT team starting using the updated calibration constants for newly acquired data as well, so as to provide a single, coherent, data set.) Therefore the the corresponding P7REP IRF sets must be used to accurately represent those photons.

Up to August 2011, the FSSC was releasing data passed on the earlier "Pass 6" version of the event selection.

Between August 2011 and September 2013 the FSSC was releasing data processed with the "Pass 7" event selection, but an earlier version of the instrument calibration constants.

Pass 7 event analysis was developed post-launch using information gained from flight data. This includes updates the LAT instrument team added to their Monte Carlo simulations to include the most evident on-orbit effects, like event pile-up (i.e., signals from cosmic rays that arrive at almost the same time as the event being read out and included in the event reconstruction). In addition, they have improved their knowledge of the on-orbit effects, including second order dependencies, and singled out some major sources of systematics. The P7_V6 IRFs addressed many of these issues. Since that time, the LAT instrument team has worked to improve the calibrations in the data, and reprocessed the data to account for these improvements. The P7REP_V15 IRFs have been developed to correspond to this improved dataset. These IRFs constitute what is currently the best description of the LAT instrument the LAT team can provide while remaining in the framework of the Pass 7 event analysis. The LAT team is also working on improving the event reconstruction algorithms. These algorithm are responsible for finding the energy and direction of each event, and calculating myriad quantities with are used by the selection algorithms to separate photons from cosmic ray backgrounds. This is a much longer term project and will eventually be released as "Pass 8" of the event selection.

The main features of P7REP IRFs are:

  • Improved effective area and/or lower background contamination relative to P6. As the "Pass 7" analysis was designed using flight data, the instrument team was able to improve the photon/cosmic ray separation. In general the selections were tuned to give roughly the same background contamination rates as for "Pass 6" analysis while gaining in effective area, particularly at lower energies (below 300 MeV).
  • Inclusion of second order effects released previously: azimuthal- and livetime-dependence of effective area are also included in "Pass 7"
  • For the most part the Pass 7 event selections are based only on quantities that have been shown to be well modeled in the Monte Carlo simulations and the P7REP_V15 effective area tables do not require any flight based correction of the type that was applied in making the P6_V11_DIFFUSE IRFs. The only exception is that the effective area tables have been modified slightly (less that 5% for all energies above 300 MeV) to make the effective areas for front-converting and back-converting type events consistent. The overall (front+back) effective area was held fixed, but the relative contributions from front-converting and back-converting events were modified.
  • As in the P6_V11 IRFs the PSF derived by fitting the width of bright point sources. This was done because the PSF derived from Monte Carlo was observed to be an incorrect description of the LAT performance at high energies. In the P7REP_V15 IRFs the PSF was obtained by refitting the scaling parameters from the Monte Carlo PSF to flight data, thus retaining the angular dependence of the PSF. In contrast, in the P7_V6 IRFs the PSF was obtained by fitting directly to the data, and did include any angular dependence.

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 Pass 7 IRF sets and the photon properties as provided in the LAT photon data.

P7REP IRF name Event Class (evclass) Conversion Type Effective Area Corrections Point-spread Function Description
P7REP_ULTRACLEAN_V15 4 0+1 Livetime, phi In-flight Highest quality and lowest background selection - somewhat overconservative, this entails a significant loss of effective area. Recommneded mainly to use as a cross check that observed features are not due to cosmic-ray contamination
P7REP_CLEAN_V15 3 0+1 Livetime, phi, front/back ratio In-flight Very high quality and low background selection - recommended for analyses that integrate large regions of the sky. Reduces non-photon spectral features to very low levels.
P7REP_SOURCE_V15 2 0+1 Livetime, phi, front/back ratio In-flight High quality selection - recommended for most analysis
P7REP_TRANSIENT_V15 0 0+1 Livetime, phi Monte Carlo Lower quality selection - used for certain transient or timing analysis

Additional Information

Detailed descriptions of the LAT instrument, event analysis, and performance can be found in the following:

The IRFs shown here are based on updated simulations of the instrument that take into account effects measured in flight that were not considered in pre-launch performance estimates. Recent information regarding the results of post-launch testing which led to this formulation can be found in the following documentation:

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