Gamma-ray line emission from X-ray binaries

Accretion disks and jets in X-ray binary systems are sources of high-energy particles that should produce gamma-ray lines when they interact with their environment. The gamma-ray lines are emitted either by neutron capture or by annihilating positrons. These processes may happen in the atmosphere of an accreting neutron star or in the atmosphere of the companion star. Three scenarios can produce an intense gamma-ray line. (a) A narrow 511 keV line emission can be produced in a misaligned microquasar, when a jet inclined with respect to the X-ray binary orbital plane hits the atmosphere of the secondary. (b) A gravitationally redshifted 2.2 MeV line can be emitted in the atmosphere of the accreting neutron star, from radiative capture of secondary neutrons produced by He spallation. (c) The 2.2 MeV line can also be produced in the atmosphere of the companion star in close binary systems, when secondary neutrons escape the accretion disk and hit the companion.
Required instrument performances:
Good spectral performance is required to accurately measure the characteristics (centroid, width) of these gamma-ray lines. We do not expect source confusion.

Performance parameter   Goal value Remarks and notes
Field-of-view (FWHM, deg) -  
Angular resolution (FWHM, deg) - Point sources
Spectral resolution
(ΔE/E @ Energy)
0.5% @ 511 keV
1 % @ 1.5 MeV
0.5% @ 2.2 MeV
1.5 MeV line : 1% due torotational broadening of a gravitational redshifted 2.2 MeV line.
Line sensitivity (@ Energy)
(cm-2 s-1, 3s, 1 Ms)
10-6 @ 511 keV
×10-7 @ 1.5 MeV
10-6 @ 2.2 MeV
1.5 MeV line flux estimated for Sco X-1 (2.8 kpc)
2.2 MeV line : typical flux from closest candidate sources when the accretion rate is large.
Continuum sensitivity (in which energy band?)
(cm-2 s-1 keV-1, ΔE=E, 3s, 1 Ms)
Timing performances -  
Polarimetric capability
(Minimum Polarization Fraction for a Crab source in 1 Ms)
Real-time data ? -