14 July 2009Keith Bechtol1 GeV Gamma-ray Observations of Galaxy Clusters with the Fermi LAT Keith Bechtol representing the Fermi LAT Collaboration July 14, 2009

14 July 2009Keith Bechtol – Fermi LAT2 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 Fermi will constrain hadronic component through p-p interactions Multiwavelength observations of the Coma cluster A. Reimer et al. 2004

14 July 2009Keith Bechtol – Fermi LAT3 Status of high energy observations Existence of diffuse radio halos/relics -> Cosmic-ray electron population Where are the cosmic-ray protons? EGRET F 100 upper limits 3-5 e-8 ph cm -2 s -1 [O. Reimer et al. 2003] Stacking analysis of 50 clusters -> average F 100 upper limit 6 e-9 ph cm -2 s -1 No statistically significant correlation between Abell clusters and the 59 unidentified EGRET sources at |b|>20° EGRET All photons fluxes quoted in the range E > 100 MeV with units ph cm-2 s-1

14 July 2009Keith Bechtol – Fermi LAT4 Model predictions Fermi 1-year sensitivity Γ=2 Pfrommer 2008 Criteria based on conventional astrophysical processes [Pfrommer 2008; Ando & Nagai 2008; Blasi, Gabici, Brunetti 2007] Best dark matter candidates similar; expected flux roughly ~ M/d 2 [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

14 July 2009Keith Bechtol – Fermi LAT5 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

14 July 2009Keith Bechtol – Fermi LAT6 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

14 July 2009Keith Bechtol – Fermi LAT7 Cluster candidates Norma Observational challenges 3C129, Ophiuchus near galactic plane Radio galaxy NGC1275 in Perseus Monitor 15 clusters with highest predicted γ-ray flux [Pfrommer 2008] Monitor cumulative significance at seed positions Expect steady sources to accumulate significance ~ sqrt( time ) Detailed analysis with 9-month dataset Galactic diffuse Radio galaxy NGC1275 Perseus AWM7 Ophiuchus Triangulum NGC5846 Fornax Centaurus Hydra A0754 NGC4636 A1367 M49 Coma 3C129 1-month counts map Norma Galactic diffuse Radio galaxy NGC1275

14 July 2009Keith Bechtol – Fermi LAT8 Non-detection -> Upper limits ClusterEGRETFermi 9-month Ophiuchus Fornax1.0 Coma A Perseus A A M490.5 AWM C NGC A A Triangulum NGC % 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

14 July 2009Keith Bechtol – Fermi LAT9 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

14 July 2009Keith Bechtol – Fermi LAT10 Spectral energy distributions Upper limits generally reflect LAT sensitivity as a function of energy Ophiuchus FornaxComa

14 July 2009Keith Bechtol – Fermi LAT11 Spectral dependence of upper limits LAT effective area is a function of energy -> Inherent dependence on assumed spectral model Flux-weighted exposure Flux UL ~ Counts UL Flux upper limits E > 100 MeV relative to assuming Γ=2 Analysis cut E > 100 MeV LAT effective area versus energy

14 July 2009Keith Bechtol – Fermi LAT12 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

14 July 2009Keith Bechtol – Fermi LAT13 Rich, non-cooling flow cluster at z = Temperature and density profiles measured by x-ray observatories [Briel et al and Struble, Rood 1999, Arnaud et al. 2001] Fermi 2σ flux upper limit E > 100 MeV = 0.6e-8 ph cm-2 s-1 Case study: Coma Coma Cluster XMM-Newton Briel et al. 2000

14 July 2009Keith Bechtol – Fermi LAT14 Case study: Coma α p = 2.1α p = 2.4α p = 2.7 β = 0 (Isobaric)0.07, , , 0.07 β = -0.5 (Clumpy)0.03, , , 0.03 β = 1 (Non-radiative)0.24, , , 0.25 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 Xp = Energy density ratio (CR-proton to thermal) Yp = Pressure ratio (CR-proton to thermal) α p = Proton dN/dE power-law index β = Cosmic ray radial density profile index

14 July 2009Keith Bechtol – Fermi LAT15 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 1 st 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