Diffuse Gamma ray emission P. Sreekumar ISRO Satellite Centre Bangalore
The Gamma Ray Universe - as seen by EGRE 3C 279 Vela Pulsar Geminga Crab Galactic Center Galactic plane
The gamma-ray sky - FERMI Galactic plane
Diffuse γ -rays Production processes : 1) Neutral pion decay from cosmic ray nucleons interacting with nucleons in the interstellar gas 2) bremsstrahlung by cosmic ray electrons, 3) Inverse Compton interaction of cosmic ray electrons with ambient low energy interstellar photons. 02/18/10
Neutral pion Decay Interaction between nuclei produces pions of all charge. Neutral pions decay to gamma-ray: p+p π π 2γ 5 03/23/09
π 0 decay γ distribution from π decay Primary nucleon spectrum 6 o 03/23/09
Electron Bremmstrahlung Use cross sections of Koch & Motz (1959) Assume ISM=mostly H; 10% He; 1% heavy nuclei qem(e) = 4.7E 25 K(r) Ee α / ( α α = index of electron spectrum K(r) = normalisation for electron spectrum 1)
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> electron E photon E A 100 MeV γ ray arises from inv. Compton interaction between an electron of 1 300 GeV and a low energy photon.
Galactic diffuse emission CR + matter CR + starlight Address distribution of matter in the Galaxy Address distribution of star light Model CR distribution Consistency check with gamma ray 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 gamma ray energies
Tracing atomic hydrogen 21 cm line emission hyperfine transition (1.4 GHz) not blocked by dust!! ground state 100 3000 K gas ~ 3 billion solar mass of H in Galaxy excited state
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 1 intensity 8.5 kpc sun 2 GC velocities are positive 1,3 4 3 2 4 50 0 + 50 + 100 Doppler shift (km/s) velocities are negative
Galactic rotation curve Sun, v= 220 km/s Distance = 8.5 kpc observed rotation curve Keplerian rotation curve V = 1/ r
H1 survey : Leiden Dwingeloo 25 m radio survey in 21 cm
Giant Molecular Clouds ~105 solar mass; cold ; mostly confined to the Galactic plane
Radial profile of Atomic and Molecular Hydrogen (from Dame et al (CfA preprint 3952)
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 D(p) avg. gain in momentum SNR ush u sh dp = p dt 3 Emax ~ 1014 Z ev in the Bohm limit shock 1st order Fermi acceleration E/E α v/c SNR as sites for CR origin and acceleration Shock acceleration (SNR / ISM shocks) Maxwell Boltzmann distribution
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
Components of galactic diffuse emission Models from FERMI team π 0 decay electron bremsstrahlung Inv. Compton 03/23/09
OSSE + COMPTEL + EGRET diffuse γ ray spectrum (Strong et al 1999) Conventional CR spectrum Hard proton spectrum Pion decay bump is visible
Radial dependence of γ ray emissivity Derived CR density distribution SNR distribution: Case & Bhattacharya (1998)
Diffuse emission beyond the milky way Nearby galaxies LMC SMC Starburst galaxies (enhanced cosmic ray densities) NGC251 M82
Large Magellanic Cloud ( 50 kpc away) LMC γ ray model (Fichtel et al 1992) 30 Doradus 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
Extragalactic gamma-ray background
The gamma-ray sky - FERMI
What is meant by Extragalactic gamma-ray background? Extragalactic γ -ray Background = Observed high latitude emission { Instrumental + resolved point sources + Galactic diffuse emission} 37 03/23/09
Extragalactic Diffuse Emission Truly Diffuse Processes Unresolved Point Sources 3. AGNs 2. Normal Galaxies 3. Starburst Galaxies 4. Cluster of Galaxies Large scale structure formation Black Hole evaporation Exotic particle annihilation UHE CR interactions.. 38 03/23/09
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) 39 03/23/09
Truly Diffuse emission diffuse emission from poor angular resolution still better angular resolution highest angular resolution Point sources better angular resolution poor angular resolution Diffuse emission Images with increasing poor angular resolutions - increasing size of PSF 40 03/23/09
So how does one estimate the EGRB component of diffuse emission? I Obs Approach 1 I Obs = I EGRB + B * (NH) Approach 2 I EGRB NH Directly from pixel-by-pixel ML fit of FERMI all sky data Sreekumar et al. 1998 03/23/09
Gamma ray source catalog Source class AGNs No. of srcs (271) 66+27 Pulsars 5 SNRs 9(?) Normal galaxy 1 Radio galaxy 2 unidentified ~170 FERMI 1400 srcs and counting. 42 03/23/09
How to find the contribution to EGRB from 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)) 43 03/23/09
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 distribution of sources with luminosity and redshift. The distribution function φ (L,z) = dn/(dl dv) φ (L,z) Luminosity Function 44 03/23/09
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) co-moving volume Luminosity function The contribution to the diffuse extragalactic emission is 1 F= 4π diffuse z max 0 dv dz dz Llim( z ) Lmin L(1 + z ) (1 α ) φ ( L, z ) dl 2 4πDL Observed Flux 45 03/23/09
Approach Derive details of individual source class distributions from observational data Source flux For every source class Luminosity Luminosity function Integrate over luminosity and redshift space 46 03/23/09
catalog impose selection criteria average spectral index filtered source list No < V / Vmax> = 0.5? exhibits evolution pure Density evolution pure Luminosity evolution density + Luminosity evolution De evolved luminosity Evolution function Exponential Power law Yes no evolution Luminosity Function
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?
Concept of Vmax Consider a source of luminosity L at redshift z Limiting flux of the survey = flim Move source to max distance zmax such that f decreases to flim z z V(z) flim zmax zmax Vmax Calculate < V Vmax > For uniform source distribution V/Vmax is expected to be uniformly distributed between 0 and 1 49 03/23/09
Lmax V L < >= min Vmax z m( L) dl 0 Lmax V ( )φ( L, z ) dv ( z ) Vmax zm( L) dl Lmin φ( L, z )dv ( z ) 0 If φ (L) is independent of z Lmax V < >= Vmax 1 V V dlφ( L)Vmax ( L) ( )d ( ) Vmax Vmax Lmin 0 Lmax 1 V dlφ( L)Vmax ( L) d ( ) Vmax Lmin 0 50 1 = 2 03/23/09
<V/Vmax test> For source population without any evolution < V/Vmax > = 0.5; For < V/Vmax > evolution. 0.5 indicates some < V/Vmax > < 0.5 fewer srcs at high z < V/Vmax > > 0.5 more srcs at high z 51 03/23/09
So if there is evidence for evolution.. 03/23/09
Evolution of Luminosity Function 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 function ρ (L,z) Evolution function 53 03/23/09
Luminosity evolution
Pure Luminosity Evolution The number of sources in co-moving volume remain same. 55 03/23/09
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 56 03/23/09
Pure Density Evolution 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) 57 03/23/09
catalog impose selection criteria average spectral index filtered source list No < V / Vmax> = 0.5? exhibits evolution pure Density evolution pure Luminosity evolution Luminosity Function density + Luminosity evolution De evolved luminosity Evolution function Exponential Yes no evolution Power law 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. 03/23/09
Some preliminary results from FERMI (Abdo et al 2009) 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.
Comparison with EGRET results PRELIMINARY 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 Slide from Ackermann et al 2009 Flux, E>100 MeV spectral index 1.03 +/- 0.17 2.41 +/- 0.05 EGRET (Sreekumar et al., 1998) 1.45 +/- 0.05 2.13 +/- 0.03 EGRET (Strong et al. 2004) 1.11 +/- 0.10 LAT + resolved sources below EGRET sensitivity 1.19 +/- 0.18 LAT (this analysis) x 10-5 cm-2 s-1 sr-1 2.37 +/- 0.05
SED of the isotropic diffuse emission (1 kev 100 GeV) Slide from Ackermann et al 2009
Unresolved source contribution (Debbijoy Bhattacharya thesis)
The isotropic diffuse gamma-ray emission Potential contributions to the isotropic diffuse continuum gamma-ray emission in the LAT energy range (100 MeV-300 GeV): Dermer, 2007 Isotropic diffuse flux contribution from unresolved sources depends on LAT point source sensitivity Contribution expected to decrease with LAT observation time 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 Slide from Ackermann et al 2009
Concluding remarks 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
We await detailed results from FERMI Thankyou