M.Cassé@iap.fr 1

THE RIDDLE OF P0SITRONS IN THE GALACTIC BULGE

M.Cassé

Service d’Astrophysique,CEA, Institut d’Astrophysique de Paris

OUTLINE

SPI /INTEGRAL OBSERVATIONSPOSITRON INJECTION RATE

& MORPHOLOGY

POSITRON SOURCE(S)SNIa fall short

Hypernovae, GRB, LMXB, microquasars & NEUTRALINOSeven less convincing

Pave the way to LIGHT DARK M ATTER

(P. Fayet)

Treated byJ. Knödlseder

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OSSE/GRO map

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Knödlseder et al 2005

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Centroid: 511.06+0.17/-0.10 keV

Line width: 2.95+0.45/-0.51 keV (FWHM)

Flux:(1.05±0.06)×10-3 ph cm-2 s-1

SPI Observation of 511 keV line from the Galactic bulge

INTEGRAL/SPI

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ANNIHILATION MEDIUM

P.Jean et al 2005

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Positronium Fraction

Annihilation of e+ in the gas after thermalisation

•direct annihilation e+ e- : 2(511keV)• formation of Positronium (para)

• 1/4 of the Ps (s=0)• annihilation via 2(511keV line)

• formation of Ps (s=1)• 3/4 of the Ps (triplet)• annihilation via 3(0 - 511 keV)

fraction de positronium :fPs= 2 / (1.5 + 2.25 511 / 3)

CGRO/OSSE Kinzer et al, ApJ 559 (2001)• thin line 3 keV

• fPs= 0.93 ± 0.04

Annihilation in a warm medium (~5×104 K).

L511 =(2-1.5 fp) L e+

Positron injection rate: (1.5 ± 0.1)1043 s-1

GRO/OSSE data

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Knödlseder, 1999, Proceedings 'Astronomy with Radioactivities’

26Al map (1809 keV) CGRO/COMPTEL

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Jean et al, 2003, A&A, 407Knödlseder et al 2003, A&A, 407

Knödlseder et al 2005, astro-ph/0506026

BulgeFlux = (1.09 ± 0.04) ph cm-2 s-1

fp = 93±0.04Le+ = (2-1.5fp).L511

Annihilation rate= (1.5±0.1) 1043 e+ s-1

No evidence for a point like source

Δl (FWHM) = 8.1°± 0.9°Δb (FWHM) =7.2 ± 0.9°

No other sources (Cygnus, Vela, LMC…)

WHAT IS THE SOURCE OF POSITRONS?

FLUX & MORPHOLOGY (summary)

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GAS DENSITY(rough estimate)

Gaz density (through dE/dx) determines behaviour of positrons in the GB together with Diffusion coefficient D, both poorly known

No clear observation of HI, H2(foreground disk), except in the CMZ

Bulge = elliptical galaxy in a spiral (see models by Wyse &Silk )

Age of stars ~ 10 Gyr, Metallicity ~ solarIMF dN/dm =k Ma (Gould 2000)

(a =-2.0 for M>0.7Mo, a= -1.3 for M<0.7Mo)

1Mo<M<8Mo stars WD: 22%

Each WD ejects (PN phase) ~ 0.5 Mo Mg ~109 MoBulge Radius ~ 2 kpc

Gas Density ~ 1 cm-3 (upper limit)Accretion by stars? Sinking in CMZ new stars

Necessity dedicated calculations

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PROPAGATION OF LOW E POSITONS

Diffusion + Ionisation lossesD ~ 3 1027 cm2s-1

(Parizot et al 2005) TENTATIVE (Bykov)

Ionisation dE/dt ~ 2 10-8 (Ln gamma +6.6)nH eVs-1

tD =R2/D 108 ans

tstop = E/(dE/dt)1 0.1 0.01 cm-3

1 2.2 105 2.2 106 2.2 107

10 1.7 106 1.7 107 1.7 108

100 1.4 107 1.4 108 1.4 109

MeV Escape

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ACTIVE OBJECTS WITH A BLACK HOLE OR A NEUTRON STAR INSIDE

•Microquasars: to few, not powerful enough•LMXRB (Prantzos 2004) No 511 keV emission detected •HN/GRB (Schanne et al 2204,Parizot et al 2005):

Central SourcePb of diffusion des positons: D~3 1027cm2s-1

Rate of HN/GRB? Disk emission

NO GOOD CANDIDATE+ RADIOACTIVITY by order of importance

22Na Novae very small amounts 26Al SN et WR (1809 keV emission) not sufficient 44Ti SN: smaller contribution than 26Al 56Ni SN II MNi=0.1 M Thick envelope (10Mo)

positrons released~ 0%

SN Ia MNi= 0.6 M Thinner envelope (1 Mo) positrons released ?

Potential sources of e+

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TYPICAL PARAMETERS OF SNIa: M=1.34 Mo ejected mass E=1.17×1051 erg Kinetic energy (E=1/2 M v2) MNi = 0.6 Mo mass of 56Ni synthesized

ENERGY SOURCE :

56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + e+

•Simple Model of explosion (spherical symmetry ) - free homologous expansion : R = v t - radioactive nuclei (56Ni) deeply buried- production of positrons: 56Co 56Fe + e+ (19%)

- absorption: R = = mean free path ~ 0.6 g cm-2

-Escape time: M E-1/2 • t~390 d consistent with inflexion of L.C..• 3.3% of 56Co remaining

Number of positrons released Ne+ = 8×1052

Production of positrons by SNIa (Milne et al, Schanne et al )

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RATE OF SNIa IN THE BULGE

Calibration with M (10Calibration with M (101010Mo) and B-K (+4.8) ;Mo) and B-K (+4.8) ;Schanne et al 2004, 2005Schanne et al 2004, 2005)) :  : 0.030.03±±0.02/century0.02/century,, in agreement with theoretical models in agreement with theoretical models (Matteucci, Nomoto…)(Matteucci, Nomoto…)

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HYPERNOVA SN2003dh

SNIA: PUREMELY RADIOACTIVE OBJECTS

L.C. fed by gammas & positrons

56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + e+

E=847 keV, <Ee+>= 640 keV (19%), Emax=1.4 MeV

Envelope dense & opaque to & e+ ,then progressive leak out

Light curve of SN2003dh declines faster than typical SNIa faster ejection (earlier transparency) faster escape of and e+

larger number of released positrons

25 times more than in SNIa

~ 0.02/century required

BUT: Rate? Diffusion of positrons? Disk/Bulge ratio?

Ma

gn

itud

e d

iffe

ren

ce (

V-b

an

d)

Time (days since max)

56Ni 56Co (t1/2=6.1 j) 56Fe (t1/2=77j) + e+ (Br=19%)

-3,50

-3,00

-2,50

-2,00

-1,50

-1,00

-0,50

0,00

0,50

-20,00 -10,00 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00

SN03dh-modelSN03dh-pointsSN92alSN96x

Schanne et al 2004

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hypothesis :  ANNIHILATION OF DARK MATTER

NATURE ? NEUTRALINOS (favorite candidate of particle physicists)

551keV EMISSION DOUBLE ANNIHILATION 

e+e- or

= a + b Z + c Ho,1 + d Ho,2 photino higgsino

  MAJORANA PARTICULE (particle = antiparticle)

  Positrons produced by different channels

WW,ZZ,tt,+-

 (qq-) (bb-)

° 2+ + e+

- - e-

 final product, (e+,e-), (, -), (p,p-)

~~ ~~

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Energy ~ 1043 Ee+ ergs-1 if Ee+ ~ 1 GeV 

Lbol > 1040 erg s-1 >> Lbol(non thermal)

 

Radio-synchrotron + Bremmstrahlung Not observed

 

Branching (e+ e-)/ ~ 1 huge flux of HE Not observed

NEUTRALINOm = 500GeV – 10TeV

e+ injected ar high Energy

GB Positrons do not emanate from the annihilation of neutralinosE>30 MeV excluded by bremmstrahlung (EGRET 30-100 MeV)

Hooper et al., 2004

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BREMMSTRAHLUNG (EGRET)

Ionisation

Bremsstrahlung

No hint of a bulge

Lb(bulge) < 2 1036 erg/sThick target

Lb =τb/τi Li

Li = 5 1039 erg/sLb (100-0 MeV) ~1039 erg/sLb (100-30 MeV) ~2 1038 erg/s

Mx < 100 MeV

Hunter et al 1997

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CONCLUSION

OTHER SOURCE(S) OF POSITRONS NEEDED

LIGHT DARK MATTER(TO AVOID PRODUCTION OF π°

and HIGH ENERGY GAMMA RAYS)

Pierre Fayet