AstroMeV511 keV emission from positron annihilation
Positrons annihilate by emitting two photons of 511 keV and a continuum in the sub-MeV to >MeV range; the latter emission depends on whether they form ortho-positronium (observations suggest that Galactic positrons do so, at the 95 % level) and on the speed of positrons at the time of their annihilation. Positron annihilation emission from the Galaxy is the first and strongest gamma-ray line detected so far, but its galactic distribution (with a bulge/disk ratio greater than unity) differs from those at any other wavelength. From the theoretical point of view it is unclear what are the main sources of positrons (conventional, like SN Ia or low-mass X-ray binaries, or more « exotic », like the Galactic black hole - which is notoriously inactive today- or even candidate dark matter particles); furthermore, positrons may propagate for several 105 yr far away from their sources before annihilating (depending on still poorly understood properties of the Galactic interstellar medium and magnetic fields), making it difficult to infer positron sources from the observed gamma-ray emission.
Required instrument performances: A large field of view is required to perform a deep Galactic Survey, including regions of low surface brightness. A sub-degree angular resolution is needed to resolve the inner bulge and detect possible point sources. A line sensitivity of 10-6 cm-2 s-1 is necessary to detect low surface brightness regions (outside the Galactic plane) and enhanced emission from inner Bulge and star forming regions (e.g. Cygnus, spiral arms) and the LMC (which will be detected for the first time).
 Spatial distribution of the 511 keV line emission from positron annihilation in the central region of the Galaxy, as observed with INTEGRAL/SPI. Credit: ESA/Bouchet et al. |
Required instrument performances: A large field of view is required to perform a deep Galactic Survey, including regions of low surface brightness. A sub-degree angular resolution is needed to resolve the inner bulge and detect possible point sources. A line sensitivity of 10-6 cm-2 s-1 is necessary to detect low surface brightness regions (outside the Galactic plane) and enhanced emission from inner Bulge and star forming regions (e.g. Cygnus, spiral arms) and the LMC (which will be detected for the first time).
| Performance parameter | Goal value | Remarks and notes |
|
Field-of-view (FWHM, deg) |
> 1 sr | Wide-field instrument to perform a deep Galactic Survey, including regions of low surface brightness |
|
Angular resolution (FWHM, deg) |
< 1 | Detailed mapping of Galactic plane, including possible enhanced emission from inner Bulge and point sources. |
|
Spectral resolution (ΔE/E @ Energy) |
0.002 @ 511 keV |
To detect Doppler effect due to Galactic rotation (intrinsic width of narrow line ~1 keV |
|
Line sensitivity (@ Energy) (cm-2 s-1, 3σ, 1 Ms) |
~ 10-6 @0.5 MeV |
To detect low surface brightness regions (outside the Galactic plane) and enhanced emission from inner Bulge and star forming regions (e.g. Cygnus, spiral arms) and the LMC |
|
Continuum sensitivity (in which energy band?) (cm-2 s-1 keV-1, ΔE=E, 3σ, 1 Ms) |
~ 10-8 @ 0.17 – 0.5 MeV ~ 2×10-9 @ 0.7 – 2 MeV |
Continuum emission from Ortho-positronium and (perhaps) in-flight annihilation in the 0.3-0.5 (and 0.5-2) MeV ranges |
| Timing performances |
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Polarimetric capability (Minimum Polarization Fraction for a Crab source in 1 Ms) |
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| Real-time data? |