GeV Gamma-ray Observations of Galaxy Clusters with the Fermi LAT

1. GeV Gamma-ray Observations of Galaxy Clusters with the Fermi LAT Keith Bechtol representing the Fermi LAT Collaboration July 14, 2009 Keith Bechtol

2. Non-thermal view of galaxy clusters Galaxy clusters cannot be described by mass alone! • Non-thermal window • Precision cosmology • Large scale shock acceleration • Intergalactic magnetic fields • 3 primary non-thermal energy bands • Radio • Hard x-ray • GeV gamma-ray • Gamma-ray emission from neutral pion decay is the most direct indicator of cosmic-ray proton energy content Multiwavelength observations of the Coma cluster A. Reimer et al. 2004 Fermi will constrain hadronic component through p-p interactions Keith Bechtol – Fermi LAT

3. Status of high energy observations EGRET • Existence of diffuse radio halos/relics -> Cosmic-ray electron population • Where are the cosmic-ray protons? • EGRET F100 upper limits 3-5 e-8ph cm-2 s-1 [O. Reimer et al. 2003] • Stacking analysis of 50 clusters -> average F100 upper limit 6 e-9ph cm-2 s-1 • No statistically significant correlation between Abell clusters and the 59 unidentified EGRET sources at |b|>20° All photons fluxes quoted in the range E > 100 MeV with units ph cm-2 s-1 Keith Bechtol – Fermi LAT

4. Model predictions Pfrommer 2008 Fermi 1-year sensitivity Γ=2 Criteria based on conventional astrophysical processes [Pfrommer 2008; Ando & Nagai 2008; Blasi, Gabici, Brunetti 2007] Best dark matter candidates similar; expected flux roughly ~ M/d2 [refer to TeVPA talk by T. Jeltema] Several clusters with anticipated flux over the LAT 1-year sensitivity Ophiuchus, Fornax, Coma, Perseus, Norma, Centaurus, … …but if clusters have soft spectra, detection after 1-year would be surprising Keith Bechtol – Fermi LAT

5. Fermi Large Area Telescope • Pair conversion telescope • Energy range: 20 MeV to over 300 GeV • All-sky survey instrument • Field of view covers 1/5 of sky • Full-sky coverage every 3 hours Keith Bechtol – Fermi LAT

6. LAT sensitivity 1-year flux sensitivity [ph cm-2 s-1] Γ =2 LAT PSF as function of energy • 1-year extragalactic flux sensitivity • 4e-9 ph cm-2 s-1 for Γ = 2 • 2e-8 ph cm-2 s-1 for Γ = 3 Currently developing tools to measure source extension Challenges increase with softer index, diffuse emission Keith Bechtol – Fermi LAT

7. Cluster candidates 1-month counts map Coma M49 A1367 NGC4636 Galactic diffuse NGC5846 Hydra Centaurus Ophiuchus A0754 3C129 Norma Perseus Triangulum AWM7 Fornax Radio galaxy NGC1275 Monitor 15 clusters with highest predicted γ-ray flux [Pfrommer 2008] Galactic diffuse Norma Observational challenges 3C129, Ophiuchus near galactic plane Radio galaxy NGC1275 in Perseus Radio galaxy NGC1275 • Monitor cumulative significance at seed positions • Expect steady sources to accumulate significance ~ sqrt( time ) • Detailed analysis with 9-month dataset Keith Bechtol – Fermi LAT

8. Non-detection -> Upper limits 95% C.L. UL Flux E>100 MeV [1e-8 ph cm-2 s-1] • Flux upper limits • Event selection • E > 100 MeV • 9-month data set • Assume • Point source spatial model • Power law spectral model • dN/dE ~ E-Γ • Photon index Γ = 2 • Plan to address alternative spatial and spectral models in a 1-year publication Keith Bechtol – Fermi LAT

9. Fermi upper limits in context Compare Fermi upper limits to EGRET and theoretical predictions Improved sensitivity over EGRET for each cluster Limits are comparable to theoretical predictions of brightest clusters Keith Bechtol – Fermi LAT

10. Spectral energy distributions Upper limits generally reflect LAT sensitivity as a function of energy Ophiuchus Fornax Coma Keith Bechtol – Fermi LAT

11. Spectral dependence of upper limits Counts UL Flux UL ~ Flux-weighted exposure • LAT effective area is a function of energy -> Inherent dependence on assumed spectral model LAT effective area versus energy Flux upper limits E > 100 MeV relative to assuming Γ=2 Analysis cut E > 100 MeV Keith Bechtol – Fermi LAT

12. Cosmic-ray proton energy content • Hydrostatic cluster mass estimates assume balance between gravity and thermal pressure • Fermi upper limits constrain cosmic-ray proton contribution to energy density and pressure of intercluster medium • Follow calculation by Pfrommer, Ensslin 2004 and Ando, Nagai 2008 • Gamma-ray emissivity depends on • Intercluster gas distribution • Energy spectrum of CR-protons • Radial distribution of CR-protons Keith Bechtol – Fermi LAT

13. Case study: Coma • Rich, non-cooling flow cluster at z = 0.0231 • Temperature and density profiles measured by x-ray observatories [Briel et al. 1999 and Struble, Rood 1999, Arnaud et al. 2001] • Fermi 2σ flux upper limit E > 100 MeV = 0.6e-8 ph cm-2 s-1 Coma Cluster XMM-Newton Briel et al. 2000 Keith Bechtol – Fermi LAT

14. Case study: Coma Xp = Energy density ratio (CR-proton to thermal) Yp = Pressure ratio (CR-proton to thermal) αp = Proton dN/dE power-law index β = Cosmic ray radial density profile index Upper limits on cosmic-ray proton energy density and pressure ratios (Xp, Yp) Cosmic-ray pressure less than 15% of thermal gas pressure over much of parameter space Keith Bechtol – Fermi LAT

15. Summary • Fermi Large Area Telescope provides new opportunity to study non-thermal activity of galaxy clusters • No LAT detection of individual galaxy cluster candidates • Upper limits improve over the EGRET era with 9 months of data • Planning a 1st year LAT publication • 1-year extragalactic flux sensitivity E>100 MeV = 4 e-9 ph cm-2 s-1 • Additional spectral and spatial extension models • Interpretation • Non-detection in gamma-ray band will still provide stringent constraints on the hadronic energy content of galaxy clusters Next steps Keith Bechtol – Fermi LAT