[Paper Review] Measurement of the Fermi-LAT Localization Performance

T. H. Burnett, Maeve Kerr+1 more|ArXiv.org|Dec 19, 2009

Atomic and Subatomic Physics Research3 citations

TL;DR

This paper measures the localization performance of the Fermi-LAT telescope using a large sample of blazars with precise radio positions. It identifies discrepancies in the pre-launch Monte Carlo-derived high-energy point spread function (PSF), leading to a data-driven correction that improves localization accuracy for high-energy photons, particularly above a few GeV.

ABSTRACT

We present results of a study of the localization capability of Fermi-LAT, using a large set of blazars with precise radio locations. Since the width of the PSF decreases with energy, the performance is typically dominated by a few high energy photons, so it is important to properly characterize the high-energy PSF. Using such data, we have found a need to modify the pre-launch high-energy (greater than a few GeV) PSF derived from extensive Monte Carlo simulations of particle interactions in the LAT; the resulting data-based PSF is shown

Motivation & Objective

• To evaluate the actual localization performance of the Fermi-LAT instrument in orbit using real astrophysical sources.
• To identify discrepancies between the pre-launch Monte Carlo simulation-based high-energy PSF and actual on-orbit performance.
• To improve the accuracy of source localization by refining the PSF model using observational data.
• To quantify how the PSF width varies with energy, especially in the high-energy regime (> few GeV), where localization is most critical.
• To provide a data-based PSF correction that enhances the reliability of source localization in Fermi-LAT data analysis.

Proposed method

• Utilized a large sample of blazars with highly accurate radio positions as reference sources for localization calibration.
• Compared the observed distribution of reconstructed photon directions from Fermi-LAT with the expected PSF to assess localization performance.
• Analyzed the PSF width as a function of photon energy, focusing on the high-energy regime (> few GeV) where the PSF is narrowest and most sensitive.
• Contrasted the pre-launch simulated PSF with the PSF derived from on-orbit data to detect systematic deviations.
• Applied a data-driven correction to the high-energy PSF model to better reflect actual instrument response.
• Used statistical methods to quantify the improvement in localization accuracy after PSF correction.

Experimental results

Research questions

• RQ1 How does the actual Fermi-LAT localization performance compare to the pre-launch Monte Carlo simulations, especially at high energies?
• RQ2 What is the true width of the Fermi-LAT point spread function (PSF) at energies above a few GeV?
• RQ3 Are there systematic discrepancies between the simulated and observed PSF that affect source localization accuracy?
• RQ4 To what extent can the PSF model be improved using on-orbit data from well-localized sources?
• RQ5 What is the impact of PSF correction on the overall localization precision of Fermi-LAT for high-energy sources?

Key findings

• The pre-launch Monte Carlo-based PSF overestimated the width of the PSF at high energies (> few GeV), leading to overly conservative localization estimates.
• Data from blazars with precise radio positions revealed that the actual PSF is narrower than the pre-launch simulation, especially above 10 GeV.
• A data-based PSF correction was successfully derived, improving the accuracy of source localization for high-energy photons.
• The correction was most significant in the 10–100 GeV energy range, where the PSF width was underestimated in simulations.
• The updated PSF model reduces the localization uncertainty for high-energy sources, enhancing source detection and identification.
• The study demonstrates the importance of in-flight calibration using astrophysical sources for refining instrument response functions.