Transcript Keith Bechtol

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
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Non-thermal view of galaxy clusters
Galaxy clusters cannot be
described by mass alone!
• Non-thermal window
Multiwavelength observations of
the Coma cluster
– Precision cosmology
– Large scale shock acceleration
– Intergalactic magnetic fields
• 3 primary non-thermal energy
bands
A. Reimer et al. 2004
– 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
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Fermi will constrain hadronic
component through p-p interactions
Keith Bechtol – Fermi LAT
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Status of high energy observations
• Existence of diffuse radio halos/relics
-> Cosmic-ray electron population
EGRET
• 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°
All photons fluxes quoted in the range
E > 100 MeV with units ph cm-2 s-1
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Model predictions
Criteria based on conventional
astrophysical processes
[Pfrommer 2008; Ando & Nagai 2008;
Blasi, Gabici, Brunetti 2007]
Pfrommer
2008
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, …
Fermi 1-year
sensitivity
Γ=2
…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]
1-year extragalactic flux sensitivity
• 4e-9 ph cm-2 s-1 for Γ = 2
Γ =2
• 2e-8 ph cm-2 s-1 for Γ = 3
Currently developing tools to
measure source extension
LAT PSF as
function of
energy
Challenges increase with softer
index, diffuse emission
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Cluster candidates
Monitor 15 clusters with highest predicted γ-ray flux [Pfrommer 2008]
1-month counts map
Coma
M49
Galactic
Galacticdiffuse
diffuse NGC5846
3C129
Perseus
Ophiuchus
A1367
NGC4636
Centaurus
Hydra
A0754
Norma
Norma
Triangulum
Observational challenges
AWM7
3C129, Ophiuchus near galactic plane
Fornax
Radio
galaxy
Radio Radio
galaxy galaxy NGC1275 in Perseus
NGC1275
NGC1275
Monitor cumulative significance at seed positions
Expect steady sources to accumulate significance ~ sqrt( time )
Detailed analysis with 9-month dataset
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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
EGRET
Fermi 9-month
Ophiuchus
5.0
1.4
Fornax
Coma
• 9-month data set
1.0
3.8
A3627
Assume
• Point source spatial model
• Power law spectral model
dN/dE ~
Cluster
E-Γ
• Photon index Γ = 2
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
Plan to address alternative
spatial and spectral models in a
1-year publication
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0.6
0.2
A1367
2.7
0.7
A0754
8.2
0.8
Triangulum
8.1
1.4
NGC5846
Keith Bechtol – Fermi LAT
0.6
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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
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Spectral energy distributions
Upper limits generally reflect LAT sensitivity as
a function of energy
Ophiuchus
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Fornax
Keith Bechtol – Fermi LAT
Coma
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Spectral dependence of upper limits
• LAT effective area is a
function of energy
-> Inherent dependence on
assumed spectral model
LAT effective area
versus energy
Flux UL ~
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Flux upper limits E > 100 MeV
relative to assuming Γ=2
Counts UL
Flux-weighted exposure
Keith Bechtol – Fermi LAT
Analysis cut E > 100 MeV
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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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Case study: Coma
Coma Cluster
XMM-Newton
Briel et al. 2000
• 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
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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)
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
Cosmic-ray pressure less than 15% of thermal gas
pressure over much of parameter space
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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
Next steps
• 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
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