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Point Source Sensitivity and IRFs for GLAST: Preliminary Findings and Test Data Generation

This document presents preliminary results on point source sensitivity and Instrument Response Functions (IRFs) for the GLAST project, authored by Toby Burnett from the University of Washington. The study focuses on generating test data sets across a range of energies from 18 MeV to 356 GeV, analyzing uniform angular distributions within the upper GLAST hemisphere. Key findings detail the effective area, field of view (FOV), and point spread function (PSF) characteristics, laying groundwork for further sensitivity calculations and future work in validating the performance through beam tests and refining the impact of background sources.

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Point Source Sensitivity and IRFs for GLAST: Preliminary Findings and Test Data Generation

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  1. PreliminaryPost-DC2/HandoffInstrument Response Functions (IRFs) and Point Source Sensitivity Toby Burnett University of Washington T. Burnett

  2. Generation of test data set (“all-gamma”) • Energy: uniformly in log(energy) from 18 MeV to 178 (or 356 GeV) • Angles: Uniform in upper GLAST hemisphere • Position: uniform over 6 m2 disk (1.38 m radius) perpendicular to incoming, centered at GLAST coordinate origin. • About 2/3 miss • Current Data set: 20 M generated • Bill used 2M. • Start with basic cuts z    y x T. Burnett

  3. Effective Area, FOV Loose cuts Count number in each bin, divide by total generated in that bin, multiply by 6 m^2 T. Burnett

  4. Angular detail (1-10 GeV) 66 cut 66 cut T. Burnett

  5. Acceptance: integrate over angles, >66 Relevant for scanning mode sensitivity T. Burnett

  6. PSF • Front (thin) vs. back (thick) – factor of 2 in width • Separate, measure independently • Energy dependence: E-0.8 • Factor of 100 from 100 MeV to 10 GeV! • Bin in energy, fit scaled deviation • CTBCORE • Introduced by Atwood • Function of other variables,trained to predict good PSF From Atwood T. Burnett

  7. PSF representation • Two traditional measures • Scale: 68% containment radius • Shape: ratio of 95% to 68% radius • (assume azimuthally symmetric) • Simple function has two parameters:  for scale,  for shape (power-law). T. Burnett

  8. PSF fits • Categories: front/back, 8 bins in log energy, 8 bins in cos() • Also fit combined angles, 0-66 Front,CTBCORE>0.5 Note: =2 corresponds to 95%/68% ratio =3. This is of course Monte Carlo: the beam test must validate it! T. Burnett

  9. Check PSF in CTBCORE intervals 3 Combined 0-66 degrees Data sets 4 2 1GeV 1 10GeV 100 GeV T. Burnett

  10. Point-source sensitivity • Figure of merit combining FOV, effective area, PSF • What is the weakest E-2 power law source, in the presence of the nominal EGRET extra galactic diffuse background, that is detectable with a likelihoodTS >25 for a one year livetime, in a scanning mode? • Current estimate: 4E-9 cm-2 s-1, E>100 MeV From GLAST LAT performance T. Burnett

  11. The ingredients: for each event class T. Burnett

  12. Sensitivity Calculation T. Burnett

  13. Point source sensitivity: Check TS for 4E-9 cm-2 s-1, one year livetime • E-2 power law • Standard EGRET extragalactic background • Combined 0-66 deg in one bin, scanning mode • Ignore effects of dispersion ‘5” requirement: (TS) > 25 T. Burnett

  14. Status/Plans • No post-DC2 surprises: parameterizations used then still seem applicable • Preliminary set of IRFs ready for Science Tools • Need to reconcile with beam test • Generate ~100M All-gamma data set T. Burnett

  15. Dispersion • R. Rando’s formula, compared with Gaussian with same variance T. Burnett

  16. Fit results (typical) 3.9% 3.7% Tail contents may be a problem: let’s cut on that good energy measurement guy! T. Burnett

  17. Cut on CTBBestEnergyProb>0.5 Reduced acceptance 2.4% ~0% Consequence for the sensitivity:TS = 15.5 +7.6 This tail was in the DC2 data T. Burnett

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