Positron Annihilation in the Milky Way Thomas Siegert, MPE Garching R. Diehl, G. Khachatryan, M.G.H. Krause, F. Guglielmetti, J. Greiner, A.W. Strong, X. Zhang 18th Workshop on Nuclear Astrophysics, March 15 2016, Ringberg Castle
2/24 Positron Annihilation: e + + e - = at least 2γs Expected γ-ray spectra Annihilation in Flight (AiF): Direct annihilation with E kin (e ± ) 0: E kin (e + ) = E kin (e - ) 0: narrow 511 kev line (E γ = m e c 2 ) E kin (e + ) /= E kin (e - ) > 0: Continuous spectrum (m e c 2 /2 < E γ < γm e c 2 + m e c 2 /2)
3/24 Positron Annihilation: e + + e - = at least 2γs Expected ɣ-ray spectra Spin(up) + Spin(up) = Spin 1 Spin(up) + Spin(down) = Spin 0 Formation of Positronium Atom (Ps): Triplet state (S=1): parallel spins Ortho-Positronium : o-ps Lifetime: τ=1.4 10-7 s 3γ: continuous spectrum (0 < E γ < 511 kev) Singlet state (S=0) antiparallel spins Para-Positronium : p-ps Lifetime: τ=1.3 10-10 s 2γ: monoenergetic γ-ray line (E γ = 511 kev) Para-Positronium
4/24 Measuring Gamma-rays with SPI/INTEGRAL Gamma-rays not focusable: Coded-mask telescope Van Allen Radiation Belts Earth Mission: 2002 still active! Orbit: 3 days High inclination High eccentricity Payload: 4 instruments: - SPI - IBIS - JEM-X - OMC
5/24 Measuring Gamma-rays with SPI/INTEGRAL Gamma-rays not focusable: Coded-mask telescope SPI W Mask BGO Anticoincidence shield SPI Energy range: 20 8000 kev Energy resolution: 2.2 kev @ 662 kev Spatial resolution: 2.6 Field of view: 16 x 16 19 High-purity Ge dets HPGe detectors
6/24 Measuring Gamma-rays with SPI/INTEGRAL Gamma-rays not focusable: Coded-mask telescope SPI Coded-mask Principle Camera Obscura (Pinhole camera) Principle Reconstruct (fit/deconvolve/infer) celestial emission based on varying shadow patterns during the observations
Siegert, Khachatryan, Diehl, et al. 2015 7/24 Positron Annihilation The 511 kev Sky Slice through b=0 All necessary components phenomenologically modelled as 2D-Gaussians with (l/b/σ l /σ b ) + candidate point sources Galactic Centre Source Narrow Bulge Broad Bulge Disk 5σ 56σ 12σ Crab: 31σ Cyg X-1: 11σ Slice through l=0 Disk size: 140 +25 +6-10 deg FWHM longitude; 25-4 deg FWHM latitude
Candidate Sources for Positrons Electrons are very numerously available in the Galaxy, but Where do the positrons come from? Massive stars / Novae / SNe Ia&II Radioactivity from β + -decay XRBs / Microquasars Compact γ-ray source ; Jets; Corona Sgr A* Past AGN activity; Accretion disk; Cosmic rays p-p collisions: Secondary positrons Pulsars Magnetic field interactions Dark Matter Decay; Annihilation; Excitation DM Cas A µqs 44 Ti SN II Fermi Bubbles 26 Al 56 Co SN2014J SN Ia Pulsars Sgr A* 8/24
9/24 Positron Annihilation Spectroscopy Spatially resolved spectroscopy Based on six-component sky model and BG model Intensity correlations among components due to spatial overlap Significance per energy bin Siegert et al. 2015 NB+BB = The Bulge Disk separated GCS compact or part of the Bulge Cont. sources hardly affected
10/24 Positron Annihilation Spectroscopy The Bulge Siegert et al. 2015 Annihilation Continuum (Ortho-Positronium) Gaussian-shaped 511 kev Line (Para-Positronium) Galactic γ-continuum Total Significance: 56σ in 40 kev 511 kev line flux: (0.96±0.07) 10-3 ph cm -2 s -1 FWHM: 2.59±0.17 kev; Centroid: 511.09±0.08 kev Positron production rate: L e +(Bulge) (1.7±0.2) 10 43 e + s -1 f Ps 1.00: Annihilation mainly through intermediate positronium state
11/24 Positron Annihilation Spectroscopy The Disk Siegert et al. 2015 Ortho-Positronium Continuum 511 kev Line Galactic γ-continuum Total Significance: 12σ in 40 kev 511 kev line flux: (1.66±0.35) 10-3 ph cm -2 s -1 FWHM: 2.47±0.51 kev; Centroid: 511.16±0.18 kev f Ps 0.70-1.00; Positron production rate: L e +(Disk) (3.1±1.0) 10 43 e + s -1 Fluffy, very low surface brightness disk; Scale height 1 kpc
Knödlseder+ 2005 Weidenspointner+ 2008 Bouchet+ 2011 This Work (Siegert+ 2015) 12/24 Positron Annihilation Spectroscopy Bulge vs. Disk Bulge-to-Disk flux ratio: 0.6±0.1; L(B/D) < 1 (smaller than in earlier studies) FWHM(Bulge) FWHM(Disk); E 0 (Bulge) E 0 (Disk) E lab = 511 kev f Ps (Disk) < f Ps (Bulge)? L e +(Disk) > L e +(Bulge)? L e +(Galaxy) 3.5-6.0 10 43 e + s -1 B/D smaller: Increased exposure in disk and high latitudes
13/24 Positron Annihilation Spectroscopy,,,,,,,,,,,,The Galactic Centre Source Siegert et al. 2015 o-ps Continuum? 511 kev Line γ-continuum? Total Significance: 5σ in 40 kev (l/b) (0/0) 511 kev line flux: (0.8±0.2) 10-4 ph cm -2 s -1 ; L e +(GCS) (1.0±0.5) 10 42 e + s -1 FWHM(GCS) = 3.5±0.6 kev; E 0 (GCS) = 510.6±0.3 kev < E lab = 511 kev f Ps 0.75 1.00; o-ps and γ-continuum marginally seen Broadened, red-shifted(?) and separated(?) source in the Galactic centre
14/24 The Galactic Centre Source?? GCS Position: (l/b) (0.0/0.0)±(0.2 /0.2 ) (90%) Consistent with position of Sgr A* (-0.06 /-0.05 ) Not the Great Annihilator 1E1740.7-2942 Not SNR G1.9+0.3 Size/Extent: (σ l /σ b ) (0.0/0.0)±(1.0 /1.7 ) (90%) SPI angular resolution: 3 Point-like Assuming 8.5 kpc distance to Galactic centre: Point-source encompasses region of 400 pc Reminiscent of the Central Molecular Zone (CMZ) Skinner et al. 2016, in prep. ρ(nfw) 2 ρ(nfw) Serpico & Hooper 2009 Enhanced peakedness: Dark Matter (DM)? Prantzos et al. 2011
15/24 Comparison to other Wavelengths What are the Sources? No wavelength similar to e + e - No point sources: Real diffuse emission Line shape is tracing the moderately warm (8000 K) and partly ionised interstellar gas 511 kev morphology does NOT show the sources but the annihilation sites! Consider positron budget: How many positrons can one source (type) produce? Goal: Reproduce total positron production rate in the Milky Way. Bulge: L e +(Bulge) 1.5-1.9 10 43 e + s -1 Disk: L e +(Disk) 2.1-4.1 10 43 e + s -1 GC: L e +(GC) 0.1-0.2 10 43 e + s -1 Total: L e +(MW) 3.5-6.0 10 43 e + s -1
Positron Origins 16/24 Massive Star Nucleosynthesis Milky Way in 26 Al (1.809 MeV) Positron! 1.809 MeV 26 Al produced in massive stars (WR phase) and SNe II Siegert, et al. 2016, in prep. About 2-5 M Sun of 26 Al in the Galaxy: (0.4-0.8) 10 43 e + s -1 distributed along Galactic plane? (L e +(Disk) (3.1±1.0) 10 43 e + s -1 ) Only 10-25% contribution?
Grefenstette, et al. 2014 Positron Origins 17/24 Core Collapse Supernova Nucleosynthesis Fast moving (outer) knots CCSN remnant Cassiopeia A Siegert, et al. 2015 44 Sc* 78 kev line Fe Siegert, et al. 2015 44 Ti 44 Ca* 1157 kev line Positron! Seitenzahl, et al. 2014 SN1987A About 10-5 - 10-4 M Sun of 44 Ti per (young) CCSNR 0.3 10 43 e + s -1 along Galactic plane? (L e +(Disk) (3.1±1.0) 10 43 e + s -1 ) Only 10% contribution?
18/24 Positron Origins Type Ia Supernova Nucleosynthesis M82 Galaxy 847 kev line 1238 kev line 158 kev line SN2014J 812 kev line Positron! White dwarf merger Binary mass transfer Diehl, Siegert, Hillebrandt, et al. 2014 & 2015 About 0.5 M Sun of 56 Ni per SN Ia 10 55 e + per SN Ia event! Supernova rate and positron escape very uncertain: 0.25 per century @ few % escape Potentially 2 10 43 e + s -1 in the whole Galaxy (L e +(Galaxy) 3.5-6.0 10 43 e + s -1 ) 40% contribution?
19/24 Positron Origins Dark Matter Positron? ρ(nfw) 2 GCS ρ(nfw) Enhanced peakedness towards Galactic Centre Siegert, et al. 2015 Serpico & Hooper 2009 Hypothesis: If 511 kev emission in the Milky Way originates in DM- annihilation, then DM-dominated regions show a similar signal Flux ~ M 2 D -2 R -5 & log(m Dyn /L 511 ) ~ +M V Test: Satellite galaxies are believed to be DM dominated log(m Dyn /L V ) ~ +M V In addition to diffuse emission, test for all MW satellites No contribution? log(m Dyn /L V ) ~ +M V OK! Siegert, et al. 2016, in prep. 3σ detections : 2 of 39 log(m Dyn /L 511 ) ~ M V?
20/24 Positron Origins Microquasars? Microquasar Done, et al. 2007 GRO J1655-40 spectra Black hole binary systems: Stellar mass black hole accreting matter from a companion star State transitions: Changes in X-ray emission caused by accretion-disk instabilities (Magnetic Rotation Instability / Hydrogen Ionisation Instability / Radiation Pressure Instability / ) accretion flow changes Positrons!
21/24 Positron Origins Microquasars? Pair-plasma spectra Pair outflow + Compton equilibrium: Narrow line (kt 10s of kev) Annihilation inside / close to the source: Broad line (kt 100 kev) Thermal pair annihilation
22/24 Positron Origins Microquasars! Microquasar Siegert, Diehl, Greiner, et al. 2016 V404 Cygni Positron! Pair creation through photon-photon interactions in optically thick region around compact gamma-ray source: γ + γ e + + e - Electron-positron-,,,,,pair plasma At least 10 42 e + s -1 for microquasar V404 Cygni (duty cycle 10-3 ) About 20 sources known in the Milky Way (few 1000 expected) Potentially 2 10 43 e + s -1 in the whole Galaxy from microquasars,,,,,(l e +(Galaxy) 3.5-6.0 10 43 e + s -1 ) 40% contribution?
23/24 Positron Creation Positron Annihilation? Positron Propagation! Han 2004 Once positrons are created, most do not annihilate immediately (E Kin (e + ) 1 MeV) Gamma-ray morphology most probably not source morphology because of propagation of the positrons! Positron annihilation predominantly in the bulge (low surface brightness disk): Positrons created anywhere in the Galaxy or in the disk? Spiraling from disk to bulge along Galactic magnetic field? Trapped in bulge? Spectral shape diagnostics: Positronium fraction 1.0 and broadened annihilation line Slowing down from MeV to few ev (ortho-positronium binding energy 6.8 ev) Positrons lose energy due to ionisation, H excitation, Coulomb collisions, Slowing down time scale between 10 5 10 7 yr (mean free paths up to kpc scales) Line shapes Similar annihilation conditions (one line shape doesn t fit all spectra)
24/24 Summary / Conclusion 511 kev emission in the Milky Way traces the gas in which positrons annihilate Main positron contributors probably: µqs (40%) + SNe Ia (40%) + Massive Stars (20%) Positron propagation through the ISM inevitable 26 Al CCSN SN Ia SPI 511 kev Portrayal µq