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Constraints on Antimatter in Constraints on Antimatter in the Universe from Observationsthe Universe from Observations
Jacques PaulJacques Paul
Service d’AstrophysiqueService d’AstrophysiqueCEACEA--Saclay, FranceSaclay, France
XIVth Rencontres de Blois
Château de Blois16-22 June 2002
1
MATTER-ANTIMATTER ASYMETRY
Antimatter search via e+ e- studies
How to detect antimatter in the universe ?
Presence of antimatter on a cosmological scale?
Future prospects
Plan
Annihilation emission from compact sources
2
Galactic 511 keV line emission
In collecting signs of antimatter annihilation (gamma-ray studies)
Two distinct ways :In collecting antimatter particles (cosmic-ray studies)
Detection of CR antiprotons is not a definite proof of “primary”antimatter: secondary production in the course of CR propagation
How to detect antimatter in the universe ?3
Many processes can produce gamma rays in the energy domain where signs of matter-antimatter annihilation is expected
Genuine signatures to be searched for:
BUT:
Anti-nuclei in cosmic rays (see forthcoming presentations) Indisputable spectral features in gamma-ray spectra
Obvious target: cosmic diffuse background4
radio microwave IR
visi
ble
UV X-ray gamma-ray
1 keV 1 MeV 1 GeV
? (µm)
?F?
(W m
-2sr
-1)
10-6
10-8
10-10
10-12
10-14
10-16
105 1 10-5 10-10
cosmic gamma-ray background
Multi- ? spectrum of the cosmic diffuse background radiation
Two categories of possible origin: superposition of unresolved sources, truly diffuse mechanisms
The first cosmic signal detected by gamma-ray astronomers (Metzger et al. 1964, Nature 204, 766)
Of particular interest for cosmology: the universe is transparent to gamma rays back to z ~ 100
Cosmic gamma-ray background5
No more evidence for the MeV-bump which must have been due to instrumental background not accounted for in past analyses
No significant extragalactic diffuse gamma-ray radiation detected above 100 GeV
Current findings: the cosmic gamma-ray background results from superposed emission of classes of discrete extragalactic sources
No significant anisotropy: consistent with the assumption of an extragalactic origin
Spectrum of the cosmic ?-ray background6
Energy (keV)
E2
dJ/d
E (
keV
2cm
-2 s
-1 s
r-1 k
eV-1 ASCA
HEAO-1
SMM
COMPTELSAS-2
EGRET
Quasars
Seyferts
SN-Ia Blazars
Emission from the halo could mimic extragalactic truly diffuse emission to a level where two components cannot be discerned
Subtraction of the diffuse Galactic emission is a major concern for measurements of extragalactic diffuse emission
In case an extended Galactic halo exists, the diffuse Galactic emission may extend to the highest Galactic latitude
Matter-antimatter annihilation
Room for extragalactic truly diffuse sources ?
Decay or annihilation of some sort of not yet discovered relic elementary particle from the early universe
7
Emission of discrete extragalactic sources + emission from a Galactic halo leave very little room for extragalactic diffuse sources such as:
Above about 200 MeV, an exponential falling finally cutting-offat ~ 1 GeV
Annihilations occurring on the boundaries of colliding domains of matter and antimatter up to z ~ 100 should have engraved significant features in the cosmic gamma-ray background spectrum such as:
A flat peak near 1 MeV due to proton-antiproton annihilationat the highest redshifts
Above 30 MeV: the spectrum is flatter than predicted and extend up to ~ 100 GeV
Antimatter on a cosmological scale ?
No excess intensity around 1 MeV
8
Recent measurements of the cosmic gamma-ray background spectrum show no evidence for these two features:
No evidence for matter-antimatter annihilations on a cosmological scale from observations of the cosmic gamma-ray background
ß+ decay of radioactive nuclides (as e.g. 26Al, 44Ti, 56Co) synthesized during late stages of massive star evolution (WR, SN)
Study of positron annihilation is one of the major subjects of high-energy astronomy insofar as positrons are copiously produced in many cosmic sites such as:
Decay of p+ induced by p-p interactions of cosmic rays with interstellar matter
Cosmic positrons cannot annihilate at the energies at which they are produced. They need first to get thermalized down to the thermal energy of the electrons of the medium throughout they propagate
Antimatter search via e+ e- annihilation studies 9
Pair plasmas whose temperature allows equilibrium relations of the kind ? + ? ⇔ e- + e+ between energetic ?-ray photons and e- e+ pairs
where s T is the Thomson cross section and ? = Ee /me c2
Direct annihilation cross section of an ultra relativistic positron of total energy Ee is:
Where ne is the medium electronic density in e- cm-3
Mean free path of ultra-relativistic positrons10
=
??
ss1 - )(2ln
83
TA
Mpc4.3
1 - )(2ln10010
320011
2e
A
−−
−
=
???
n
The mean free path is very large in the interstellar medium: ? ~ 5500 Mpc for a 100 MeV positron
This cross section implies a mean free path:
Positrons slow down before annihilating ⇒ weak Doppler broadening of the 511 keV annihilation line
where s T is the Thomson cross section and ? = Ee /me c2
Direct annihilation cross section of a sub relativistic positron of total energy Ee is:
Mean free path of sub-relativistic positrons11
The mean free path is much smaller than the size of the confinement region
In dense plasmas confined by collapsed stars, ne reach1020 e- cm-3 and the mean positron energy is 50-100 keV, implying a mean free path:
Positrons annihilate in a very hot plasma ⇒ wide Doppler broadening of the 511 keV annihilation line
1 -83 T
A2?
ss =
TeA
0.6s
?n
= cm10
1091
20e3
A
−
=
n?then
In 25% of the case, Ps forms as para-positonium (spin 1S0) which annihilates in ≥ 10-10 s in two 511 keV ?-ray photons
Cosmic Ps forms by charge exchange of kind e+ + H2 ⇒ Ps + H2+ or
by radiative recombination of kind e+ + e- ⇒ Ps + photon
The case of positronium12
In 75% of the case, Ps forms as ortho-positonium (spin 3S1) which annihilates in 1.5 10-7 s in three ?-ray photons, the energy of each photon is < 511 keV and the sum of the photon energy is 1022 keV
When the temperature of the medium is not too high (T < 106 K) a positron may join with an electron to form positronium (Ps), a two particle system analog to H atom
13
Ortho-positronium annihilation
Spectrum of ?-ray photons produced bythe ortho-positronium 3-photon annihilation
Flu
x (a
rbitr
ary
units
Energy (keV)
1978
1982 Confirmation with the gamma-ray spectrometer aboard the HEAO-3 satellite
Leventhal et al. 1978, ApJ 225, L11
First precise measurement of a positron annihilation line emission from the GC region (balloon borne spectrometer)
Riegler et al. 1981, ApJ 248, L13
Historical milestones14
Milne et al. 2000, AIP Conf. Proc. 510, 21
positron fountain?
OSSE map of the 511 keV line emission15
16
OSSE spectrum of the GC region
Kinzer et al. 2001, ApJ 559, 282
ortho-positronium contribution
Recent estimates of the nova production of radioactive nuclei with ß+ decay ⇒ small contribution from novae
High-positronium fraction (94-100%) for the inner Galaxy ⇒ warm medium where positrons annihilate (T > 5 103 K)
Origin of the galactic annihilation emission17
Contribution from compact sources ?
The annihilation luminosity from the Galactic disk corresponds to the annihilation of 3-4 1043 positrons per second
ß+ decay from 26Al, 44Ti, 56Co and from old stellar population products ⇒ might be the main contribution
Accreting stellar black hole identified by SIGMA in the GC region producing sporadic bursts of annihilation radiation
0
Galactic longitude (degrees)
Gal
actic
latit
ude
(deg
rees
)
4
2
6
024 358 356 354
1E 1740.7-2942
Goldwurm et al., Nat 371, 589, 1994 Mirabel et al., Nat 358, 215, 1992
18
The great annihilator
Further recognized as a source of bipolar relativistic outflow
Contour map of the GC region derived from the SIGMA data recorded in the 330-570 keV band on 13-14 October 1990
Sunyaev et al. 1991, ApJ 383, L49
Spectrum of the source coinciding with 1E 1740.7-2942 derived from the SIGMA/GRANAT data recorded on 13-14 October 1990
Bouchet et al. 1991, ApJ 383, L45
SIGMA spectrum of 1E 1740.7-294219
1E 1740.7-2942
Gaussian linewidth 150 keV
20
The case of Nova Muscae 1991
SIGMA observations of Nova Muscae 1991 - also recognized as a genuine accreting stellar black hole - have revealed on 20 January 1991 an other sporadic source of annihilation radiation
Map of the Nova Muscae field derived from the SIGMA data recorded on 20 January 1991 in the 430-530 keV band
Goldwurm et al. 1992, ApJ 389, L79
21
SIGMA spectrum of Nova Muscae 1991
Goldwurm et al. 1993, A&AS 97, 293
Spectrum of the source coinciding with Nova Muscae 1991 derived from the SIGMA/GRANAT data recorded on on 20 January 1991
Spectral-imaging view of Nova Muscae 1991 derived from the SIGMA data recorded on 20 January 1991
Gaussian linewidth < 40 keV
22
The INTEGRAL mission
To be launched on October 17, 2002
Imaging and spectroscopy in the 15 keV to 10 MeV bandSource monitoring in the X-ray (2-30 keV) and visible bands
23
Spectrometer SPIImager IBIS
10-5
10-6
10-7
10-8
10-9
0.1 1
Energy (MeV)
10
Sen
sitiv
ity (p
hoto
n cm
-2s-
1ke
V-1
)
IBIS continuum sensitivity
NGC 4151
3s , 10 6 s, ?E = E
24
10 -6
10 -5
10 -4
10 100 1000 104
SPIIBISOSSECOMPTEL
511 keV
T=10 6 s3σ
Pho
ton
cm-2
s-1
3s , 10 6 s
511 keV
SPIIBISOSSECOMPTEL
10-4
10-5
10-6
10-2 0.1 1Energy (MeV)
10
INTEGRAL sensitivity to narrow lines25
Towards a gamma-ray lens
Plane parallel crystals which intercept a beam of very short wavelength radiation ? act as a 3-D diffraction array
When entering a crystal underan angle ?, a beam of high-energy photons can be diffracted under the same angle ? defined by the Bragg condition: 2 d sin(?) = n ?
This process (Laue diffraction) is well suited to gamma-ray line observations, and, in particular, the 511 keV e+ annihilation line
26
Projects involving a gamma-ray lens
Balloon borne prototypefirst flight in June 2000
Mission proposed in answerto the CNES AO (06/2002)
CLAIRE
27
~ 20 mlens
spacecraft
MAX
detector spacecraft