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

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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

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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

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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

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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 positionsExpect 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

NGC4636A1367M49

Coma

3C129

1-month counts map

Norma

Galactic diffuse

Radio galaxy NGC1275

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Non-detection -> Upper limits

Cluster EGRET

Fermi 9-month

Ophiuchus 5.0 1.4

Fornax 1.0

Coma 3.8 0.6

A3627 1.2

Perseus 3.7 19.9

A3526 5.3 2.5

A1060 14.9 2.3

M49 0.5

AWM7 3.5 0.9

3C129 5.3 3.2

NGC4636 0.2

A1367 2.7 0.7

A0754 8.2 0.8

Triangulum 8.1 1.4

NGC5846 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

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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

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Spectral energy distributions

Upper limits generally reflect LAT sensitivity as a function of energy

Ophiuchus Fornax Coma

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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

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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

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• 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 Cluster

XMM-Newton

Briel et al. 2000

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Case study: Coma

αp = 2.1 αp = 2.4 αp = 2.7

β = 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, 0.03

β = 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

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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