Positron Annihilation throughout the Galaxy Thomas Siegert Max-Planck-Institute for extraterrestrial Physics Oct 18 th 2017, INTEGRAL Symposium, Venice, Italy
The Early Years Late 60s, early 70s: Balloon-borne experiments Collimator apertures with large FoVs First evidence of a spectral feature at 0.5 MeV with NaI scintillators in 1970 1973 Robert C. Haymes Donald D. Clayton 511 kev @ 2σ Johnson+1972 473±15 kev Haymes+1969 Johnson+1973 2
More Experiments More Confusion Sophistication with Ge detectors: Line definitely at 511 kev Positron annihilation from galactic centre direction! Leventhal+1978 Lingenfelter+198 9 Satellite missions HEAO-C ( 79-81, Ge), SMM ( 80-89, NaI), GRANAT ( 89-99, NaI) More balloon flights (GRIS 88-95, Ge) Variability in 511 kev flux from galactic centre? 3
Variability of the 511 kev Line? Transient sources? X-ray binaries with SIGMA: 1E1740.7-2942, GRS1124-68 Lingenfelter+1989: Different FoVs lead to different fluxes Diffuse and transient source(s)? OSSE solved the issue: first image! No variability! Purcell+1997 Nova Musca Lingenfelter+1989 Sunyaev+1992 GRS1124-68 Bouchet+1991 1E1740.7-2942 Great Annihilator 4
INTEGRAL/SPI 511 kev Measurements Knödlseder+2005: Image reconstruction (3 yrs) Churazov+2005, Jean+2006: Spectral analysis of GC region Weidenspointner+2008: Asymmetry in disk (5 yrs)? Bouchet+2010: No disk asymmetry, bulge shifted (7 yrs) Skinner+2014: Galactic centre source? Siegert+2016a: Component-wise spectra, faint thick disk (11 yrs) Knödlseder+2005 Weidenspointner+2008 Bouchet+2011 Skinner+2014 Jean+2006 Siegert+2016a 5
Siegert+2016a Positron Annihilation Sky Galactic positron picture after 10 years of INTEGRAL/SPI Slice through b=0 Galactic Centre Source Narrow Bulge Broad Bulge Disk 5σ 56σ 12σ Slice through l=0 6
Positron Annihilation: e + + e - = at least 2 γs Annihilation in Flight: 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 Formation of Positronium Atom (Ps): Singlet state (S=0): antiparallel spins Para-Positronium p-ps 2γ: monoenergetic γ-ray line (511 kev) Triplet state (S=1): parallel spins Ortho-Positronium o-ps 3γ: continuous spectrum 7
INTEGRAL/SPI 511 kev Spectroscopy Milky Way spectrum after 11 mission years Siegert+2017b in prep. 3σ binning! < 4%! < 18 km/s! < 47 km/s! Unprecedented accuracy! 8
Positron Annihilation Spectroscopy Annihilation spectroscopy: spatially resolved / component-wise Siegert+2016a Annihilation Continuum (Ortho-Positronium) Gaussian-shaped 511 kev Line (Para-Positronium) Galactic γ-continuum Total Significance: 56σ in 40 kev e + annihilation (=production?) rate: L e + (1.7±0.2) 10 43 e + s -1 511 kev line flux: (0.96±0.07) 10-3 ph cm -2 s -1 ; f ps 1.00 FWHM: 2.59±0.17 kev; Centroid: 511.09±0.08 kev 9
Churazov+2005, 2010; Siegert2017 What Can We Learn from Line Shapes? Guessoum+1991 Prantzos+2011 Guessoum+2005 Cold H 2 Cold H Warm neutral Warm ionised Hot plasma Line shapes trace warm and partly ionised gas: T 7000-40000 K x ion 5-20% 10
SPI 511 kev Portrayal No Disk flux asymmetry (p-ps, o-ps, ɣ-ps)! Disk scale height 1 kpc Annihilation budget: 3.1 10 43 e + s -1 Disk l<0 FWHM: 1.6 kev E 0 : 511.1 kev f Ps : 0.8-1.0 FWHM: 3.1 kev E 0 : 511.3 kev f Ps : 0.8-1.0 Disk l>0 Separate(?) annihilation site Annihilation budget: 0.1 10 43 e + s -1 GCS FWHM: 3.5 kev E 0 : 510.6 kev f Ps : 0.7-1.0 FWHM: 2.6 kev E 0 : 511.1 kev f Ps : 1.0 The Bulge Not axisymmetric Shifted to: (l/b)=(-1.25 /-0.25 ) Annihilation budget: 1.7 10 43 e + s -1 Spectral shapes similar throughout the Milky Way, but hardly represented by one! 11
Candidate Positron Sources 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 Where do they come from? DM µqs Cas A 12 44 Ti SN II Fermi Bubbles SN2014J Sgr A* 26 Al 56 Co SN Ia Pulsars
Secondary Positrons from Cosmic-Rays p + p π + X p + p K + X K ± µ ± + ν µ K ± e ± + ν e K ± π 0 + π ± π 0 γ + γ π 0 γ + e - + e + π + µ + + ν µ π + e + + ν e µ + തν μ + ν e + e + Sizun+2006 Collision products have large masses: K 0 (498), K ± (494), π 0 (135), π ± (140), µ(106) e + produced have large kinetic energies: MeV to GeV But no annihilation in flight excess in spectrum? e + originated at energies not greater than few MeV (well...) Cosmic-ray positrons cannot explain 511 kev flux (0% contribution?) 13
β + -Unstable Nuclei Astrophysically important isotopes that produce non-negligible amounts of positrons: p n + e + + ν e Isotope Decay chain Type %e + T 1/2 Sources 26 Al 26 Al 26 Mg β + 82 7.1 10 5 yr Massive Stars, CCSNe 44 Ti 44 Ti 44 Sc 44 Sc 44 Ca 56 Ni 56 Ni 56 Co 56 Co 56 Fe EC β + 94 EC β + 19 60 yr 3.9 h 6.1 d 77.2 d SNe SNe Ia 22 Na 22 Na 22 Ne β + 90 2.6 yr Novae 14
e + Origins Massive Star Nucleosynthesis Milky Way in 26 Al (1.809 MeV) 13 yr SPI Spectrum Full sky Siegert2017 Plüschke+2001 Positron! T 1/2 0.7 Myr 1.809 MeV 26 Al produced in massive stars Various stellar associations (foregrounds) 2.8 M Sun of 26 Al in the Galaxy 0.3 10 43 e + s -1 along Galactic plane (L e +(Disk) 3.1 10 43 e + s -1 ) 10% contribution 15
e + Origins Supernova Nucleosynthesis Core-collapse supernovae: Young remnants and 44 Ti Positron! SN1987A in LMC Seitenzahl+2014 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 But: SN1991bg-like events (SN Ia) may explain B/D and total positron content (Crocker+2016)? 16
e + Origins Type Ia Nucleosynthesis Positron! White dwarf merger Binary mass transfer 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 17
Crocker+2016 e + Origins Type Ia Nucleosynthesis? Credits: Fiona H. Panther; http://auntiematter.space/ 18
Life of a Positron in the Galaxy MeV/ GeV Source distribution ev Sink distribution Positron propagation required! Energy 19
The Picture doesn t Fit? 430 MHz IR Visible CO X-ray Free-Free 1.809 MeV 511 kev Dust >100 MeV Hα 656 nm 21 cm > 1GeV 20
A Bulge Component: Dark Matter? Decay, Annihilation, De-excitation? F 511 ρ DM n? Siegert+2016a GCS Skinner+2014: Dark matter profile with n=2 can replace all three bulge components Test the dark matter scenario with SPI on dwarf satellite galaxies? offset peak Galactic centre DM main halo Dark matter simulation Kuhlen+2008 Dwarf galaxies DM sub-haloes ρ(nfw) 2 ρ(nfw) 21 Serpico & Hooper 2009
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+2016c e + e - (511 kev) from DM? Probably not. log M dyn /L 511 (0. 25 ± 0. 11)M V 23
A Bulge Component: Microquasars Microquasars identified as galactic positron sources! γ + γ e + + e - e + annihilated: 10 42 s -1 Duty cycle: 10-3 Escape fraction: 20% Siegert+2016b V404 Cygni 10 3 10 8 µqs expected in the Milky Way: contribution tens of %! 24
One step further Kinematics Longitude-velocity diagram for 511 kev in the inner radian Siegert+2017b in prep. Orientation of Galactic rotation! Faster than rotation velocity Faster than 26 Al Sliding window method Another possibility? Velocity measurements 25
Open Questions e + Trace the WNM/WIM? Guessoum+2005 Cold H 2 Cold H Warm neutral Warm ionised Hot plasma HI (21 cm) and/or Hα (656 nm) should fit the 511 kev data well Line shapes agree with warm and partly ionised gas: T 7000-40000 K x ion 5-20% Annihilation only in WNM/WIM? HI 21 cm 511 kev but they don t! Hα 656 nm 26
Log(T) χ 2 Improvement Is it Really the ISM? Why should positrons choose this phase (T 7000-40000 K, x ion 5-20%)? Star maps (IR) provide a decent fit Is it stellar atmospheres? Is it stellar flares (Bisnovati-Kogan+2016)? 511 kev IR (1.25 4.9 µm) Siegert2017 Height above surface (km) Tracer Photon Energy (ev) 27
Summary Undoubtedly, the positrons giving rise to the observed feature come from a variety of processes. Marvin Leventhal, 1978 Plurality must never be posited without necessity. William of Ockham, 14 th century If two things do not suffice to verify a proposition, one must posit a third. Walter Chatton, 14 th century 28