Adam Zok Science Undergraduate Laboratory Internship Program August 14, 2008

GLAST: Key Concepts High energy: 30 MeV – 300 GeV Limited spatial resolution: 0.15° - 3.5° Resolution worsens at low photon energies Coulomb scattering from heavy nuclei Targets of study: typically < 1°

Identifying Sources Many potential gamma-ray emitters may lie within GLAST’s spatial uncertainty Some emitters are not point sources, but are spatially extended (they have a measurable angular size) Spatially extended sources are much less common than point sources, so identifying one can narrow down the list of candidate objects significantly.

Software Tools gtobssim Creates virtual gamma-ray emitters, outputs.fits file that represents how GLAST may view the source sourcefit Works backwards: subtracts background radiation, reconstructs source parameters and calculates confidence limits Optimizes likelihood (probability that a given set of data came from a particular distribution) Python modules: PyFITS, ROOT

Gtobssim Simulation

Testing Sourcefit Options Sourcefit allows the user to specify certain fitting options, or simply use the defaults In particular, I wanted to see how the energy binning and energy range used affected fit quality To determine how to most effectively use the program, I ran fits on the same sources using several different combinations of settings

Energy Ranges Red: default range Brown: 100 MeV – 100 GeV Green: 500 MeV – 100 GeV Blue: 1 GeV – 100 GeV

Energy Binning Red: default binning (irregular) Brown: 2 bins per decade Green: 3 bins per decade Blue: 4 bins per decade Pink: 6 bins per decade

Determining Sourcefit’s Limits Needed to find out which kinds of sources could be accurately modeled by sourcefit Used two different fitting algorithms: Minuit and Simplex Generated 4 arrays of simulated sources obscured by background radiation Different flux for each array, varied size and spectrum within the arrays Investigated accuracy of fits in terms of size and position, as well as the calculated confidence limits

Array Fit Results Minuit and Simplex performed comparably Both algorithms did a poor job of calculating reasonable confidence limits Sources with a high flux and low spectral index (lots of energetic photons) were most successfully parameterized for both size and position

Simplex Position Fitting Results Flux = 3 x s -1 m -2

Simplex Position Fitting Results Flux = s -1 m -2

Simplex Position Fitting Results Flux = 3 x s -1 m -2

Simplex Position Fitting Results Flux = s -1 m -2

TS Values, Flux = s -1 m -2

Minuit Point Source Fitting Red = unacceptable fit ( > 0.01° ) Blue = good fit ( < 0.01° ) Green = very good fit ( < o.oo1° )

Final Thoughts Default energy range usually works best, but low flux, soft spectrum sources may be better fit with a wider energy range (including more low energy photons) TS value correlates most strongly with source size and spectral index More background (incorrectly) detected for large, soft- spectrum sources

Future Work Problems with error matrix calculated by sourcefit need to be fixed Array plots that quantify error, instead of “yes” or “no” classification Analyze sources with less regular spectra Introduce background radiation from galactic sources Additional simulations to rule out statistical irregularities