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

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)

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

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

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

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

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

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

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

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

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

All sky in gamma-rays

Ratio Map Want this to be as smooth as possible

Spectrum It’s a non-thermal spectrum!

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

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

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

Thank you! Questions?