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Diffuse Gamma ray emission
P. SreekumarISRO Satellite CentreBangalore
3C 279Vela Pulsar
Geminga
Crab
The Gamma Ray Universe - as seen by EGRET
Galactic plane
Galactic Center
The gamma-ray sky - FERMI
Galactic plane
1) Neutral pion decay from cosmicray nucleons interacting with nucleons in the interstellar gas
2) bremsstrahlung by cosmicray electrons,
3) Inverse Compton interaction of cosmicray electrons with ambient low energy interstellar photons.
Diffuse γ -rays
Production processes :
02/18/10
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Neutral pion Decay
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Interaction between nuclei produces pions of all charge.
Neutral pions decay to gamma-ray:
p+p → π °
π ° → 2γ
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π 0 decay
Primary nucleon spectrum γ distribution from π o decay
Electron Bremmstrahlung
• Use crosssections of Koch & Motz (1959)• Assume ISM=mostly H; 10% He; 1% heavy nuclei
qem(E) = 4.7E25 K(r) Ee−α / ( α −1)
α = index of electron spectrumK(r) = normalisation for electron spectrum
Inverse Compton• CR electrons upscatter soft photons
(CMB, FIR, NIR, optical, & UV)
photon distribution (adapted from Chi & Wolfendale 1991)which used stellar model of Mathis, Mezger & Panagla (1983)
<Eγ > = 4/3 { Ee/Mc2 } <ei>
A 100 MeV γ ray arises from inv. Compton interaction between an electron of 1 300 GeV and a lowenergy photon.
electron E photon E
Galactic diffuse emission
• CR + matter• CR + starlight
– Address distribution of matter in the Galaxy– Address distribution of starlight– Model CR distribution– Consistency check with gammaray distribution
Derive distribution of cosmic rays
Interstellar medium constituents
• 99% is gas– 90% is hydrogen
• Atomic• Molecular• Ionised
– 10% helium
At visible wavelengths, dust plays a more important role than gas but not so at gammaray energies
Tracing atomic hydrogen
• 21 cm line emission – hyperfine transition (1.4 GHz) – not blocked by dust !!
ground state
excited state
100 – 3000 K gas~ 3 billion solar mass of H in Galaxy
Tracing molecular hydrogen
• Cannot directly trace molecular H2 in its cold phase - no permitted rotational transitions
• CO – most abundant heteronuclear molecule is used as a tracer of H2
• 2.6 mm line of the rotational transition J = 1 0 of CO• Brightness temperature of CO, integrated over velocity, WCO
approximately scales with total emitting gas in a given region.
XCO = N(H2) / WCO
Q : Is XCO uniform across the Galaxy?
Distributing matter in space ….
Structure of the Milky Way
A typical spiral
Rotation curve
Position in the Galaxy … Line-of-sight velocity distribution
41,3
2
50 0 + 50 + 100
4
3
2
1
sun
inte
nsity
Doppler shift (km/s)
GC
8.5
kpc
velocities are positive
velocities are negative
Galactic rotation curve
Sun, v= 220 km/sDistance = 8.5 kpc
Keplerian rotation curveV = 1/ √ √ r
observed rotation curve
H1 survey : LeidenDwingeloo 25 m radio survey in 21 cm
Giant Molecular Clouds
~105 solar mass; cold ; mostly confined to the Galactic plane
(from Dame et al (CfA preprint 3952)
Radial profile of Atomic and Molecular Hydrogen
Approach to diffuse emission analysis
• Ring-velocity boundaries are defined / adjusted for each line of sight to optimise ‘structures’ in the (l,b,v) phase space.
• NH1 and WCO are then calculated for each region
Cosmic rays
• Cosmic rays – Composition– Spectrum– Origin and acceleration
Composition
• 98% protons• rest are electrons, alphas,• heavier particles • (includes anti-particles)
CR spectrum
dN/dE = a E −γ
Supernova remnant : site for CR acceleration
Cas A Crab
Solar flare
Origin & Acceleration
• SNR as sites for CR origin and acceleration
• Shock acceleration (SNR / ISM shocks)
SNR ush
D(p)
shock
pdtdp sh
3u∇−=avg. gain in
momentum
Emax ~ 1014 Z eV in the Bohm limit
Maxwell – Boltzmann distribution∆ E/E α v/c
1st order Fermi acceleration
Cosmic ray models
• Many radially symmetric models models– SN distribution (Case & Bhattacharya 1998)
– Pulsar distribution (Strong et al 2004) in GALPROP code
Cosmic ray models (contd)
• radially asymmetric models – not based on multiparameter fits
– Based on density distribution of matter itself – equipartition arguments
– Hunter et al (1997) – with EGRET data
– Need to examine the role for such models using FERMI results
A possible way to derive more realistic distribution of Cosmic rays in the Galaxy
Observed γ -ray emission
+ point sources
03/23/09
Components of galactic diffuse emission
π 0 decay
electron bremsstrahlung
Inv. Compton
Models from FERMI team
(Strong et al 1999)
Pion decay “bump” is visible
OSSE + COMPTEL + EGRET diffuse γ ray spectrum
Conventional CR spectrum Hard proton spectrum
Radial dependence of γ ray emissivity
SNR distribution: Case & Bhattacharya (1998)
Derived CR density distribution
Diffuse emission beyond the milky way …
• Nearby galaxies– LMC– SMC
• Starburst galaxies (enhanced cosmic ray densities)– NGC251– M82
LMC in γ rays FERMI
• Fermi’s Large Area Telescope (LAT) shows that an intense star-forming region in the Large Magellanic Cloud named 30 Doradus is also a source of diffuse gamma rays. Brighter colors indicate larger numbers of detected gamma rays. Credit: NASA/DOE/Fermi LAT Collaboration
30 Doradus
Large Magellanic Cloud ( 50 kpc away)
LMC – γ ray model(Fichtel et al 1992)
Extragalactic gamma-ray background
The gamma-ray sky - FERMI
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What is meant by Extragalactic gamma-ray background ?
Extragalactic γ -ray Background
= Observed highlatitude emission { Instrumental
+ resolved point sources + Galactic diffuse emission}
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03/23/09 38 03/23/09
Extragalactic Diffuse Emission
Truly Diffuse Processes
• Large scale structure formation• BlackHole evaporation• Exotic particle annihilation• UHE CR interactions• …..
Unresolved Point Sources
3. AGNs2. Normal Galaxies3. Starburst Galaxies4. Cluster of Galaxies
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What is implied by diffuse emission?
Emission that is perceived given the detector angular resolution
Emission that seems to possess fairly uniform characteristics (not strongly location dependent, not strongly time-dependent)
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‘diffuse emission’ from poor angular resolution
Images with increasing poor angular resolutions - increasing size of PSF
‘Diffuse’ emission
poor angular resolutionbetter angular resolutionstill better angular resolution
Truly Diffuse emission
Point sources
highest angular resolution
03/23/09
So how does one estimate the EGRB component of
diffuse emission ?
I Obs
NH
I Obs = I EGRB + B * (NH)
Sreekumar et al. 1998
I EGRB
Approach 1
Approach 2
Directly from pixel-by-pixel ML fit of FERMI all sky data
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2Radio galaxy
~170unidentified
1Normal galaxy
9(?)
9
SNRs
5Pulsars
66+27AGNs
No. of srcs(271)
(
SourceclassGammaray source catalog
FERMI – 1400 srcs and counting ….
03/23/09 43
How to find the contribution to EGRBfrom a source population
flux from a source of luminosity L
F = L/(4π d2(z))
)
All sources have the same luminosity L.Total flux observed =
F = Σ Fi
F = (L/4π ) ∗Σ ( 1/(di2(z))
(
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So if one knows the distribution of sources with distance (f(z) =dN/dz),One can find the contribution
F = (L/4π )∗∫ {1/d2(z)}f(z)dz
Now Luminosity of sources are also different. So one has to find the distributionof sources with luminosity andredshift.The distribution functionφ (L,z) = dN/(dL dV)
�
φ (L,z) Luminosity Function
03/23/09 45
Luminosity function
The luminosity function is defined as the number of sources per unit co-moving volume of the Universe.
dN = φ (L,z) dL dV(z)
f
co-moving volumeLuminosity function
The contribution to the diffuse extragalactic emission is
diffuse
03/23/09
Observed Flux
dLDzL
zLdzdzdV
FzL
L L
z
∫∫−+×=
)lim(
min
max
2
)1(
0 4)1(
),(41
πφ
π
α
03/23/09 46
Approach
Derive details of individual source class distributions from observational data
Source flux Luminosity
Luminosity function
Integrate over luminosity and redshift space
For every source class
catalog
impose selection criteria
< V / Vmax> = 0.5 ?
pureDensity
evolution
pureLuminosityevolution
density +Luminosityevolution
Noexhibits evolution
Yesno evolution
LuminosityFunction
averagespectral index
Evolutionfunction
Exponential Power law
Deevolvedluminosity
filtered source list
V / Vmax test• Schmidt (1968) – test for examining uniformity of quasars
– Limitations from sensitivity – truncated dataset
• Procedure :– For each source, find
• redshift• the maximum redshift within which the
object is observable (above min detectable limit)
Imp question : Is the catalog list of sources drawn from a uniform distribution in space ?
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flim zmaxVmax
z V(z)
V
Consider a source of luminosity L at redshift zLimiting flux of the survey = flim
Move source to max distance zmax such that f decreases to flim
For uniform source distribution V/Vmax is expected to be uniformly distributed between 0 and 1
z
zmax
Concept of Vmax
CalculateVVmax
< >
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∫ ∫
∫ ∫>=<
max
min
max
min
)(
0
)(
0 max
max )(),(
)(),()(
L
L
Lz
L
L
Lz
m
m
zdVzLdL
zdVzLVV
dL
VV
φ
φ
21
)()()(
)()()()(
max
min
max
min
1
0 maxmax
1
0 maxmaxmax
max
=>=<
∫ ∫
∫ ∫L
L
L
L
VV
dLVLdL
VV
dVV
LVLdL
VV
φ
φ
If φ (L) is independent of z
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<V/Vmax test>
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For source population without any evolution < V/Vmax > = 0.5;
For < V/Vmax > ≠ 0.5 indicates some evolution.
< V/Vmax > < 0.5 fewer srcs at high z< V/Vmax > > 0.5 more srcs at high z
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So if there is evidence for evolution …..
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Evolution of Luminosity Function
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Luminosity & number density distribution of a population can be expressed as
Φ (L,z) = Φ (L, z=0)ρ (L,z)
Φ (L, z=0) - Local luminosity functionρ (L,z) Evolution function
Luminosity evolution
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Pure Luminosity Evolution
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The number of sources in co-moving volume remain same.
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L(z) = L(z=0) × f(z); Comoving number density does not change with redshift.
Evolution function used: è f(z) = (1+z)β , f(z) =exp(T(z)/τ )T(z) Look back time
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Pure Density Evolution
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Only the co-moving number density of sources changes with z. ρ (L,z) is independent of L.
Φ (L,z) = Φ (L)ρ (z)
(
03/23/09
catalog
impose selection criteria
< V / Vmax> = 0.5 ?
pureDensity
evolution
pureLuminosityevolution
density +Luminosityevolution
Noexhibits evolution
Yesno evolution
LuminosityFunction
averagespectral index
Evolutionfunction
Exponential Power law
Deevolvedluminosity
filtered source list
03/23/09
Finally ….
Using the final luminosity function. One can estimate the individual source contributions to the diffuse emission (beyond the source catalog limit)
Residuals beyond the estimated source contributions point to contributions from truly diffuse processes – a result of great interest.
Spectral indices: BL Lac--> 1.99 ± 0.22, FSRQ -> 2.40 ± 0.17
BL Lac does not show any evolution: (similar findings from EGRET - Bhattacharya, Sreekumar, Mukherjee 2009)
Slope of luminosity function is 2.17± 0.05. FSRQ shows positive evolution.
Some preliminary results from FERMI (Abdo et al 2009)
Comparison with EGRET results
Considerably steeper than the EGRET spectrum by Sreekumar et al.
No spectral features around a few GeV seen in re-analysis by Strong et al.
2004
2.37 +/- 0.05
2.13 +/- 0.03
2.41 +/- 0.05
spectral index
x 10-5 cm-2 s-1 sr-1
1.19 +/- 0.18
1.11 +/- 0.10
1.45 +/- 0.05
1.03 +/- 0.17
Flux, E>100 MeV
EGRET (Sreekumar et al., 1998)
LAT + resolved sources below EGRET sensitivity
EGRET (Strong et al. 2004)
LAT (this analysis)
PRELIMINARY
Slide from Ackermann et al 2009
SED of the isotropic diffuse emission (1 keV – 100 GeV)
Slide from Ackermann et al 2009
Unresolved source contribution (Debbijoy Bhattacharya thesis)
Potential contributions to the isotropic diffuse continuum gamma-ray emission in the LAT energy range (100 MeV-300 GeV):
unresolved point sources• Active galactic nuclei• Star-forming galaxies• Gamma-ray bursts
diffuse emission processes• UHE cosmic-ray interactions with
the Extragalactic Background Light• Structure formation• large Galactic electron halo• WIMP annihilation
The isotropic diffuse gamma-ray emission
Dermer, 2007
Isotropic diffuse flux contribution from unresolved sources depends on LAT point source sensitivity
Contribution expected to decrease with LAT observation time
Slide from Ackermann et al 2009
• Galactic diffuse emission model depends on a multitude of observational inputs (H1, CO, starlight models, CR models)
• Adequate models exist for typical point source analysis (of course - systematic errors could creep in through uncertainties in the diffuse model – as pointed out by Benoit)
• Detailed modeling with FERMI provides significant scope for improvements in understanding the origin, acceleration and distribution of cosmic rays.
• A more extensive source catalog permits much improved estimation of luminosity function, evolution and determination of contribution of unresolved sources to the extragalactic diffuse emission.
• FERMI data may provide evidence or place useful limits on a truly diffuse extragalactic component
Concluding remarks
We await detailed results from FERMI
Thankyou
