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GeV Gamma-ray Observations of Galaxy Clusters with the Fermi LAT . Keith Bechtol representing the Fermi LAT Collaboration July 14, 2009. Non-thermal view of galaxy clusters. Galaxy clusters cannot be described by mass alone! Non-thermal window Precision cosmology - PowerPoint PPT Presentation
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14 July 2009 Keith Bechtol 1
GeV Gamma-ray Observations of Galaxy Clusters with the Fermi LAT
Keith Bechtol representing the Fermi LAT Collaboration
July 14, 2009
14 July 2009 Keith Bechtol – Fermi LAT 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
Fermi will constrain hadronic component through p-p interactions
Multiwavelength observations of the Coma cluster
A. Reimer et al. 2004
14 July 2009 Keith Bechtol – Fermi LAT 3
Status of high energy observations• Existence of diffuse radio halos/relics
-> Cosmic-ray electron population
• Where are the cosmic-ray protons?
• EGRET F100 upper limits 3-5 e-8 ph cm-2 s-1 [O. Reimer et al. 2003]
• Stacking analysis of 50 clusters -> average F100 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 2009 Keith Bechtol – Fermi LAT 4
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/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
14 July 2009 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
14 July 2009 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
14 July 2009 Keith Bechtol – Fermi LAT 7
Cluster candidates
Norma
Observational challenges3C129, 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 positionsExpect steady sources to accumulate significance ~
sqrt( time ) Detailed analysis with 9-month dataset
Galactic diffuse
Radio galaxy NGC1275
PerseusAWM7
Ophiuchus
Triangulum
NGC5846
Fornax
Centaurus
HydraA075
4
NGC4636A1367M49
Coma
3C129
1-month counts map
Norma
Galactic diffuse
Radio galaxy NGC1275
14 July 2009 Keith Bechtol – Fermi LAT 8
Non-detection -> Upper limitsCluster EGRE
TFermi 9-month
Ophiuchus 5.0 1.4Fornax 1.0Coma 3.8 0.6A3627 1.2
Perseus 3.7 19.9A3526 5.3 2.5A1060 14.9 2.3M49 0.5
AWM7 3.5 0.93C129 5.3 3.2
NGC4636 0.2A1367 2.7 0.7A0754 8.2 0.8
Triangulum 8.1 1.4NGC5846 0.6
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
14 July 2009 Keith Bechtol – Fermi LAT 9
Fermi upper limits in contextCompare Fermi upper limits to EGRET and theoretical predictions
Improved sensitivity over EGRET for each clusterLimits are comparable to theoretical predictions of brightest clusters
14 July 2009 Keith Bechtol – Fermi LAT 10
Spectral energy distributionsUpper limits generally reflect LAT sensitivity
as a function of energy
Ophiuchus Fornax Coma
14 July 2009 Keith Bechtol – Fermi LAT 11
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=2Analysis cut E > 100 MeV
LAT effective area versus energy
14 July 2009 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
14 July 2009 Keith Bechtol – Fermi LAT 13
• 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
Case study: Coma
Coma ClusterXMM-Newton
Briel et al. 2000
14 July 2009 Keith Bechtol – Fermi LAT 14
Case study: Coma
αp = 2.1 αp = 2.4 αp = 2.7β = 0 (Isobaric) 0.07, 0.04 0.05, 0.04 0.09, 0.07
β = -0.5 (Clumpy) 0.03, 0.02 0.02, 0.02 0.04, 0.03β = 1 (Non-radiative)
0.24, 0.13 0.16, 0.11 0.29, 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)
αp = Proton dN/dE power-law indexβ = Cosmic ray radial density profile index
14 July 2009 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
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