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M. G. Baring (Rice University)
Compton reflection features of 511 keV pair annihilation radiation have been known for a long time, being observed in laboratory experiments at around 170 keV. Emission features around 170 keV have been seen in connection with transient annihilation emission in Nova Muscae and the Einstein source 1E1740.7-2942 near the Galactic Center, yielding a flurry of observational and theoretical activity around a decade ago. These 170 keV ``bumps" were interpreted as part of a Compton reflection feature of the 511 keV line, presumed to arise due to the proximity of the pair annihilation source to an accretion disk or some thick blob of material. The asymmetry of such ``target" geometries automatically implies significant hard X-ray/soft gamma-ray polarization that distinctively couples to the spectral shape. This paper presents such polarization signatures for certain matter geometries and observational perspectives, thereby providing powerful probes of the disk orientations and environments for 1E1740.7-2942 and Nova Muscae, and potentially also for other accreting compact objects in the galaxy. These model developments are germane to the INTEGRAL hard X-ray/gamma-ray experiment, launched in late 2002, which is expected to measure polarization in this energy band, sensitive to around the 10 galactic and extragalactic sources.
This work was sponsored by the NASA Astrophysics Theory Program.
Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.