1. The Search for Gamma-Rays From Galaxy Clusters
Lisa Lee
SULI 2011

2. What is Fermi? Fermi Gamma-Ray Space Telescope
Main instrument: Large Area Telescope (LAT)
Detects energies from <20 MeV to >300 GeV
Started survey Aug 2008
(3 yrs of data to date)

3. What are Galaxy Clusters?
Largest gravitationally bound structures in the Universe
Multi-wavelength emission hints at relativistic particles in ICM
Galaxy Cluster Composition
1% Galaxy
visible
9% Intracluster Medium (ICM)
High temperature gas between galaxies.
90% Dark Matter
Not yet detected

4. Synchrotron Radiation
Particle encounters magnetic field
Spirals around
Emits radiation
Frequency of emission related to speed of electron
In order for emission to have any astronomical value, electrons must be relativistic

5. As electron travels around the magnetic field, it gives up energy as it emits photons.
The longer it is in the magnetic field, the more energy it loses, emits EM radiation at a longer wavelength.
Hence the non-thermal spectrum

6. Non-thermal spectrum Not a curve (that’s thermal distribution)
Rather, increasing
emission at longer
wavelengths
Power law

7. Galaxy Clusters should also emit Gamma-Rays
Reasons to believe this:
Cosmic-ray hadrons
Radio-emitting leptons
Decay/annihilation of dark
matter particles (hm…)
Collisions, decays, scatterings

8. Galaxy Clusters and Gamma-Rays
…but yet to be detected
What I did
Energies below 1 GeV not searched before. went down to 63 MeV

9. General Process Get photon file, spacecraft file
Do some processing, make a model and compare it to the counts file
Significant bright point in residual map?  free it (allow to vary)
Determine the test-statistic value. If greater than 25, it is real
Calculate Upper-Limit Flux
Check for a non-thermal spectrum

10. Counts Map
Map of the photons in a certain region of the sky

11. Model Map, generated by computer program
Want to be as accurate to our counts map as possible

12. All sky in gamma-rays

13. Ratio Map
Want this to be as smooth as possible

14. Spectrum
It’s a non-thermal spectrum!

16. Results 3 out of 32 galaxy clusters potentially interesting
Further analysis required

17. Conclusions
We have calculated gamma-ray flux upper limits that set lower limits on the magnetic field
Detection of gamma-rays from a couple galaxy clusters takes us one step closer to determining the activities and mechanisms happening in the clusters

18. Acknowledgements Rolf Buehler and Marco Ajello Department of Energy
SULI Program
SLAC
Brent Harris

19. Thank you!
Questions?