AAS 205th Meeting, 9-13 January 2005
Session 76 X-Ray Binaries, and Pulsars
Oral, Tuesday, January 11, 2005, 10:00-11:30am, Royal Palm 1-3

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[76.02] The Polarization of Reprocessed Annihilation Line Radiation in Compact Objects

M. G. Baring (Rice University)

Compact objects in the Milky Way provide excellent candidates for probing source geometry using X-ray and soft gamma-ray polarimetry, since they are luminous and possess highly anisotropic environs. The Einstein source 1E1740.7-2942 near the Galactic Center (GC) is an interesting case: it has been known to exhibit 511 keV line emission that is somewhat variable. Improved observations of hard X-rays just over a decade ago saw evidence of a bump at around 170 keV that was not connected to positronium decay emission. This bump was interpreted as part of a Compton reflection feature of the 511 keV line that manifests itself at 170-511 keV. The reflection is presumed to take place off an accretion disk or some blob of material. The asymmetry of such "target" geometries automatically implies significant hard X-ray/soft gamma-ray polarization that couples to the spectral shape. This paper computes such polarization signatures for certain matter geometries and observational perspectives, thereby providing environmental diagnostics for 1E1740.7-2942 and other accreting compact objects such as Nova Muscae. These model developments are germane to the INTEGRAL hard X-ray/gamma-ray experiment, launched in late 2002, which can potentially measure polarization in this energy band, sensitive to around the 10% level in 1E1740.7-2942 and other galactic and extragalactic sources.

This work was sponsored by NASA as part of the INTEGRAL Guest Investigator Program.

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