POSITRON GENERATION IN TYPE IA SUPERNOVAE
A.Hirschmann, E. Bravo & J. Isern
Institut d´Estudis Espacials de Catalunya, (IEEC/CSIC/UPC), Barcelona, Spain
Various scenarios have been proposed to understand the emission of positrons from the galactic bulge : GRB-hypernovae (Bertone et al. 2004),
Mirror matter (Foot & Silgadaze, 2004) and Type Ia Supernovae (SN Ia) (Cassé et al. 2004). This emission has been measured by ESA’s gamma
ray satellite INTEGRAL. 1D simulations models have obtained a value for the rate of supernovae explosions rather similar to Cassé et al. results
which follows up very well to the value of the 511 keV positron-electron emission line coming from the galactic center. 3D simulations are being
conducted at the moment to achieve results on this matter.
Possible Progenitors:
⇒ GRBs/Hypernovae (Bertone et al, 2004)
⇒ Type Ia Supernovae (Cassé et al, 2004)
⇒ Mirror Matter (Foot & Silgadaze, 2004)
ANNIHILATION OF POSITRONS WITH ELECTRONS
SUPERNOVA TYPE IA
Positron
GAMMA RAY BURST
Positronium formation ~90 %
POSITRONIUM
Bound state with electron
POSITRON GENERATION BY β+ DECAY OF RADIOACTIVE ELEMENTS
Direct annihilation ~10 %
Electronic Capture:
β+ decay:
(Z+1,A)
ORTO-POSITRONIUM
75% cases
(Z,A) + υe
(Z+1,A) + e-
e+ +υe +(Z,A)
τ1/2 ≈ 111 days
56Ni
56Co
Emission Line
511 keV
Annihilation positron-electron
56Fe
β+ decay 19%
2 γ rays with
E= 511 keV
Continuum
Elec. Cap. 81%
Elec. Cap. 100%
τ =10-10s
τ =10 s
3 γ rays with
E< 511 keV
Positron
τ1/2 ≈ 8.8 days
PARA-POSITRONIUM
25 % cases
-7
56Fe
Gamma rays are emitted in every radioactive decay since these particles decay into
excited states and emit γ rays in order to reach the temporary ground state of the
daughter nuclei. Only 19% of the 56Ni generated in the explosion can produce positrons
since no positrons are produced during electronic capture (56Ni → 56Co) and only that
19% mentioned above is generated by β+ decay (56Co → 56Fe).
e+ e-
γγ
2 γ of 511 keV each
When positrons are produced, they can annihilate with the electron either by direct
annihilation( 10% of the cases) or form positronium (90%) previously. Direct annihilation
produces 2 γ rays of 511 keV each. If the positron becomes bound with the electron, it will
annihilate into 2 γ rays of 511 keV, in 25% of the cases (para-positronium) giving rise to
the emission line, or into 3 γ rays of energies below 511 keV, 75% of the cases (ortopositronium), which will form the continuum below the emission line of 511 keV.
OBSERVATIONS OF POSITRON-ELECTRON 511 KEV LINE
1D MODEL SIMULATIONS RESULTS
MODELS
MODELS
DDTa
Emission of the positron-electron emission line at 511 keV from the galactic bulge has been
observed by INTEGRAL. The galactic bulge is emitting particles at this energy at a rate of:
DDT(a-e): Delayed Detonation model
PDD(a-e): Pulsating Delayed Detonation model
DET: Detonation model
SUB: Sub-Chandrasekhar model
DEF(a-f): Deflagration model
ACCUMULATED e+
1.897x1054
ΓSN/cent
Γe+ ~ 1.3 x 1043 e+s-1
The rate of supernovae explosions is calculated by means of the following expression:
0.023
DDTe
8.488x1053
0.048
PDDa
2.522x1054
0.016
PDDe
1.358x1054
0.031
DET
4.708x1054
0.009
SUB
1.507x1054
0.027
Γ SN =
N (e + / s)
N e+ / event
The rate of supernova explosions occurring in the galactic bulge has been obtained by
Cassé et al. 200, which is:
The figure displays the accumulated positrons as a function of time. At early epochs, the
expanding ejecta is very dense and thus, a small percentage of positrons can escape. As
the ejected material becomes transparent, a larger amount of positrons will be able to
escape and thus will increase drastically in the amount of escaped particles. By very late
epochs, there are no more considerable radioactive decays that will generate positrons and
thus the values reach a constant slope.
CONCLUSIONS
Positrons that escape the expanding ejecta are
accumulated over the entire integration time (fesc
is the fraction of escaped positrons as a function
of time)
∞
Np =
∫N
p
( t ) f esc dt
0
According to the values obtained for supernova explosion rates in the left table, we can
reach several conclusions:
1) Since we do not know which theoretical model is the correct one, we can not discard
that Type Ia Supernovae are considered possible progenitors of such emission.
3D MODELS SIMULATIONS
The same procedure is being conducted for 3D simulations of Type Ia Supernovae models
since these carry indeed more information related to possible asymmetries in the system,
they can provide a more realistic scenario of the situation.
2) Type Ia Supernova can be possible progenitors but since the values obtained indicate a
higher emission,for certain models, than that observed by INTEGRAL, it is necessary to
analyze this scenario of positron emission in 3D.
3) The rate of supernova explosions obtained from the galactic bulge may be
underestimated and would need to be re-calculated.
References:
Bertone et al. 2004, submitted (astro-ph/04050005)
Cassé M., Cordier B., Paul J. & Schanne S., 2004, ApJ, 602, L17
Cassé et al. 2004, submitted (astro-ph/0404492)
Foot R. & Sildagaze Z.K., 2004 submitted (astro-ph/0404515)