ISSI Workshop on Galactic Positrons
- Science -
The
511 keV line emission was first detected and identified with balloon
born experiments in the 70ies. With a flux of about 2 10-3 photons cm-2 s-1 from the Galactic center (GC) direction, it is
the most intense gamma-ray line in the sky. The detected flux, combined
to
the GC distance of 8 kpc, implies that
about 1.5 1043 positrons annihilate every second, i.e. the
luminosity in that single spectral line is several thousand times
larger than
the solar luminosity.
Several
astrophysical processes may give rise to copious production of
positrons: β+ decay of radioactive nuclei, interactions between
energetic
particles (e.g. between photons, or between an electron and a strong
magnetic
field, or between energetic protons, through the subsequent decays of
the
resulting charged pions and mesons), and (perhaps) annihilation of dark
matter
particles. Those processes may take place in various astrophysical
sites, like
stellar explosions (supernovae and novae), rotating magnetized neutron stars (pulsars), the interstellar
medium (ISM) hit by cosmic rays, or in
the jets emitted from compact objects accreting matter from a companion
star
(microquasars in X-ray binaries or XRBs). Some of those processes are
rather
well understood and certainly produce positrons (e.g. radioactivity,
which is
known to power the late lightcurves of supernovae, or cosmic ray
interactions
with the ISM), while others are only theoretical possibilities for the
moment
(e.g. jets from XRBs or dark matter annihilation). However, the amount
of
positrons released from any one of those sites (their positron yield)
is
largely unknown at present, due to uncertainties in theory and lack of
relevant
observational constraints.
This feature is quite exceptional, since the Milky Way does not display such a large bulge/disk ratio in any other wavelength; and, from the theoretical point of view, it is hard (but not necessarily impossible) to conceive of any class of stellar objects (old or recent, alive or dead, in single or multiple systems) having a distribution so strongly concentrated in the inner Galaxy. In order to interpret the SPI observations, some rather “exotic” novel ideas were suggested, like the annihilation of light dark matter particles with their antiparticles, which may lead to positron production (Boehm et al. 2004). In view of the importance of dark matter in modern astronomy and physics, this suggestion received quite some attention recently, mainly from particle physicists. It should be stressed, however, that the invoked dark matter particles must have rather unusual properties (mass, type of interaction, cross-section), which do not feature in the Standard Model of particle physics or in its main extensions.
The aforementioned observational and theoretical difficulties prompted recently a closer examination of the main assumption linking positron sources to observed gamma-ray images, namely the idea that positrons annihilate close to their sources. This idea stems from the fact that low energy positrons, like those resulting from radioactivity, are expected to suffer strong ionization losses and to be rapidly thermalized and brought to rest in the ISM, where they annihilate. This picture reflects reasonably well the situation in the local ISM, which is neutral and has a relatively high density (of about 1 atom per cm3), but it is not appropriate for the hot, ionized and dilute ISM (with density hundreds of times smaller), which dominates elsewhere and, in particular, away from the plane of the Galactic disk; in such conditions, positrons may travel for millions of years and get more than ten thousand light-years away from their sources (Jean et al. 2006). The actual situation is far more complex, since the various phases of the ISM (atomic, molecular, ionized) coexist, with volume filling actors varying across the Galaxy (Ferrière 1998). Moreover, positrons are charged particles and their motion is affected by the Galactic magnetic field, the properties of which (configuration, intensity, etc.) are very poorly known at present. It has even been suggested that, under some extremely idealized conditions, a fraction of the disk positrons may be channeled by the poloidal component of the Galactic magnetic field to the bulge, and annihilate there (Prantzos 2006); this would alleviate some of the difficulties presently encountered when trying to interpret the SPI observations within conventional schemes (e.g. with positrons resulting from the radioactivity of SNIa).
It
is obvious then, that an understanding of the origin of galactic
positrons requires a thorough investigation of several distinct
astrophysical
topics:
- Sources of
positrons (physics, positron yields, Galaxy wide distribution for:
SNIa, XRBs,
massive star explosions, pulsars, dark matter, etc.)
- Interstellar
matter and magnetic field in the Galaxy (physical properties of the
various
phases of the ISM and corresponding volume filling factors as a
function of
position in the Milky Way, configuration and intensity of the various
components of the Galactic magnetic field)
- Propagation
of positrons (slowing down and thermalization in various media and
propagation
in the Galactic magnetic field)
- Annihilation
of positrons (resulting gamma-ray signatures as a function of the
physical
conditions in the annihilation site)
- The 511
keV gamma-ray sky (spatial morphology and spectroscopy of the emission)
The
planned study aims precisely at this kind of interdisciplinary
questions, by a team of internationally known experts (both observers
and
theoreticians) in all those topics.
References
- - Boehm, C.,
Hooper, D., Silk,
J., et al., (2004), Physical Review Letters, 92,
101301
- -
Ferrière, K. (1998),
Astrophysical Journal, 503, 700
- - Jean, P., Knoedlseder, J., Gillard, W., et al., (2006), Astronomy and Astrophysics 445, 579
- - Knoedlseder J., Jean, P., Lonjou, P., (2005), Astronomy and Astrophysics 441, 513
- - Prantzos, N., (2006), Astronomy and Astrophysics 449, 859
Literature
collected from ADS: set
of references Last update: 2007-11-21 by R. Diehl
Authorized by N. Prantzos
