Positron Annihilation in the Milky Way and beyond

Transcription

1 Positron Annihilation in the Milky Way and beyond Thomas Siegert, MPE Garching R. Diehl, A. C. Vincent, F. Guglielmetti, M. G. H. Krause, C. Boehm Research Area G Science Day, October 20 th 2016

2 Positron Annihilation: e + + e - = at least 2 γs Formation of Positronium Atom (Ps): Triplet state (S=1): parallel spins Ortho-Positronium o-ps Lifetime: τ= s 3γ: continuous spectrum Singlet state (S=0) antiparallel spins Para-Positronium p-ps Lifetime: τ= s 2γ: monoenergetic gamma-ray line (511 kev) Annihilation in Flight (AiF): Direct annihilation with E kin (e ± ) 0: E kin (e + ) = E kin (e - ) 0: 511 kev line E kin (e + ) /= E kin (e - ) > 0: continuous spectrum Occurences depend on (statistics and) ISM characteristics Positronium fraction f Ps e - free or from atoms/molecules in the ISM? Charge exchange with H, He, D, H 2, (spectral details) e + from where? Ortho-Positronium Para-Positronium 2/19

3 Measuring Gamma-rays with INTEGRAL/SPI Gamma-rays not focusable: Coded-Mask telescope SPI W Mask HPGe detectors 19 High-purity Ge dets building the camera Energy range: kev Energy resolution: kev Spatial resolution: 2.6 Field of view: 16 x 16 Degradation: Restoration by annealings (twice per year) 3/19

4 Complete sky coverage 11 year data set Total exposure 160 Ms At least 0.1 Ms throughout Energy band kev 0.5 kev binning (SPI res. 2.1 kev) pointings ( 1.2 Mio spectra) Background dominated data Elaborate BG model based on selfconsistent description of physical processes in the spacecraft INTEGRAL/SPI Data Set How to distinguish between BG and sky? Energy [kev] 4/18

5 Background in Different Detectors Separation: Continuum + Lines 203 Bi 69 Ge from detector array! from veto shield! 5/19

6 Background Patterns Are Constant in Time Sky Patterns Are Not! Simultaneously distinguish between BG and Sky by disentangling measured detector patterns over time according to modelled BG and Sky (IRF) patterns 6/19

7 Siegert+2016a Positron Annihilation Sky Model Slice through b=0 Galactic Centre Source Narrow Bulge Broad Bulge Disk 5σ 56σ 12σ 511 kev line Crab: 31σ Cyg X-1: 11σ Continuum Slice through l=0 Disk size: deg FWHM longitude; 25-4 deg FWHM latitude 7/19

8 Positron Annihilation Spectroscopy The Bulge Siegert+2016a 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: ±0.08 kev f Ps 1.00; Positron production rate: L e + (1.7±0.2) e + s -1 Fully consistent with earlier measurements Annihilation in warm ( K) and partly ionized (2-7%) gas 8/19

9 SPI 511 kev Portrayal No Disk flux asymmetry (p-ps, o-ps, γ-ps)! Disk scale height kpc Annihilation budget: e + s -1 Disk l<0 FWHM: 1.6 kev E 0 : kev f Ps : FWHM: 3.1 kev E 0 : kev f Ps : Disk l>0 Separate(?) annihilation site Annihilation budget: e + s -1 GCS FWHM: 3.5 kev E 0 : kev f Ps : FWHM: 2.6 kev E 0 : kev f Ps : 1.0 The Bulge Not axisymmetric Shifted to: (l/b)=(-1.25 /-0.25 ) Annihilation budget: e + s -1 Annihilation conditions similar throughout the MW? (It seems that one spectral shape does not fit all) 9/19

10 Churazov+2005, 2010 Large Variety of Possible Conditions Model by Churazov et al. 2005,2010: Single phase medium: pure H, dust free, temperature, ionisation state Spectral parameters from: Milky Way (total) Bulge Disk (total) Disk (l<0) Disk (l>0) GCS T(Bulge) > T(Disk)? T(Bulge) >< T(GCS)? x(bulge) x(disk)? x(bulge) < x(gcs)? Disk(l<0) Disk(l>0) Peculiar sampling of dust dominated regions? 10/19

11 Candidate Sources for Positrons Massive stars / Novae / SNe Ia&II Radioactivity from β + -decay of 13 N, 18 F, 22 Na, 26 Al, 44 Ti, 56 Co, Cas A 26 Al LMXRBs / Microquasars 44 Ti compact γ-ray source, jets, corona Sgr A* Fermi bubbles as a relict of past AGN activity; Accretion disk; Cosmic rays µqs Fermi Bubbles SN2014J p-p collisions: p+p π + +X + ; π + e + +ν e ; π + µ + +ν µ ; µ + e + +ν e +ν µ ; Sgr A* Dark Matter Decay, Annihilation, Excitation DM Caveat: Gamma-ray morphology not necessarily source morphology due to e + propagation in the ISM! 11/19

12 A Bulge Component with its Own Source Dark matter: Decay, Annihlation, De-excitation Process Light DM (m DM MeV) CDM (m DM =GeV-TeV) n Decay Annihilation e.g. axion decay (1-300 MeV, Hooper+2004) e.g. scalar WIMP (1-100 MeV, Boehm+2004) Not possible (Fermi limits) 1 Not possible (Fermi limits) 2 De-excitation? e.g. WIMP de-ex. by e + e - Decay Ann. De-ex. Skinner+2014? F 511 ρ DM n emission (Finkbeiner+2007) Skinner+2014: NFW profile with α=1, β=3, γ=1, R=20 kpc, and n=2 can replace all three bulge components 511 kev morphology offset to negative longitudes Kuhlen+2013: We show that the position of the central DM density peak may be expected to differ from the dynamical center of the Galaxy by several hundred parsecs. 2? GCS offset peak ρ(nfw) 2 ρ(nfw) Serpico & Hooper /19

13 Testing the DM Scenario with SPI Dwarf galaxies are believed to be DM-dominated: log M dyn /L V +(0. 22 ± 0. 02)M V Siegert+2016b Expect M/L = const. across magnitude range But intrinsically fainter objects seem to have an additional unseen mass Interpreted as Dark Matter 13/19

14 Testing the DM Scenario with SPI Dwarf galaxies are believed to be DM-dominated? Siegert+2016b Include model of all 39 satellite galaxies of the MW in maximum likelihood fit in addition to diffuse large scale emission model (bulge+disk). Extract individual spectra for all additional sources and determine 511 kev flux. Expectation if 511 kev emission is due to DM annihilation: Remember: log M dyn +0.2M L V & log L V 0.4M V log M dyn 0.2M V V 2 L 511 M dyn R 5 2 F 511 M dyn R 5 D 2 log M dyn L 511 log M dyn +0.2M V 14/19

15 511 kev Emission from MW Satellites Siegert+2016b #tested 39 #expected > 1σ > 2σ 6 2 > 3σ 1 0 Empirical 2σ detection 511 kev: ph cm 2 s 1 T 6 /T 6 exp In general more sources with a non-zero 511 kev flux than expected. Hard to judge which sources are real and which are not. One hint of a source with 3.1σ from the direction of Reticulum II. 15/19

16 Testing the DM Scenario with SPI Dwarf galaxies are believed to be DM-dominated log M dyn /L V + (0. 22 ± 0. 02)M V DM in dwarf galaxies? Maybe yes. Siegert+2016b e + e - (511 kev) from DM? Probably not. log M dyn /L 511 (0. 25 ± 0. 11)M V 16/19

17 More Evidence Against DM Ret II (l/b) (266.3 /-49.7 ) Distance 30 kpc M dyn M Sol F 511 (1.7±0.5) 10-4 ph cm -2 s -1 C 0 (5.4±2.2) 10-6 ph cm -2 s -1 kev -1 E 0 FWHM 510.8±0.4 kev 1.2±0.8 kev L 511 (Ret II) ( ) e + s -1 L 511 (MW) ( ) e + s -1 The case of Reticulum II SPI spectrum of Ret II If 511 kev in Ret II was purely from DM, then Milky Way bulge should be 100 times brighter than what is seen with SPI! 17/19

18 Positrons in Reticulum II? Massive stars / Novae / SNe Ia&II Radioactivity from β + -decay of 13 N, 18 F, 22 Na, 26 Al, 44 Ti, 56 Co, Cas A 26 Al LMXRBs / Microquasars compact γ-ray source, jets, corona Sgr A* Fermi bubbles as a relict of past AGN activity; Accretion disk; Cosmic rays p-p collisions: p+p π + +X + ; π + e + +ν e ; π + µ + +ν µ ; µ + e + +ν e +ν µ ; Dark Matter Decay, Annihilation, Excitation DM µqs 44 Ti Fermi Bubbles SNe Ia SMBH No young population. Little to no gas. Old population? Microquasar? (Siegert+2016c) Peculiar supernovae? Delay time? (Crocker+2016) 18/19

19 Conclusions 511 kev is a unique observing window: Cosmic-ray propagation in diluted ISM Ann. morphology Source morphology Observationally confirmed positron producers: 26 Al / 44 Ti / 56 Ni from massive stars, CCSNe, SNe Ia Pair plasma from microquasars DM ruled out (>99%) as positron source: GCS not related to DM GCS separate ann. site: CND / Fermi bubbles / Sgr A* Remaining sources (contributions) to be clarified: Pulsars, Cosmic-rays, SMBH, MW Ret II connection? 19/19