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