Expectations for Polarized Hard X-rays from 1E1740.7-2942

Matthew G. Baring1

1 Dept. of Physics and Astronomy, MS-108, Rice University, Houston, TX 77251


E-mail contact: baring@rice.edu


The Einstein source 1E1740.7-2942 near the Galactic Center (GC) has been known to exhibit 511 keV line emission that is somewhat variable, suggesting a compact object as its source. Improved observations of hard X-rays about 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. Such polarization signatures are the subject of this paper, and have not been calculated before. Interesting degrees of polarization are computed in certain matter geometries, with implications for 1E1740.7-2942 and other accreting compact objects near the GC or elsewhere. The INTEGRAL hard X-ray/gamma-ray experiment, to be launched in late 2002, is expected to measure polarization in just this energy band sensitive to around the 10