2003ApJ...587..748B

The jet axes and the orbital planes of microquasar systems are usually assumed to be approximately perpendicular, even though this is not currently an observational requirement. On the contrary, in one of the few systems where the relative orientations are well constrained, V4641 Sgr, the jet axis is known to lie not more than ∼36° from the binary plane. Such a jet, lying close to the binary plane, and traveling at a significant fraction of the speed of light, may periodically impact the secondary star, initiating nuclear reactions on its surface. The integrated yield of such nuclear reactions over the age of the binary system (less the radiative mass loss) will detectably alter the elemental abundances of the companion star. This scenario may explain the anomalously high Li enhancements (roughly ∼20-200 times the Sun's photospheric value) seen in the companions of some black hole/X-ray binary systems. (Such enhancements are puzzling, since Li nuclei are exceedingly fragile, being easily destroyed in the interiors of stars, and Li would be expected to be depleted rather than enhanced there.) Gamma-ray line signatures of the proposed process could include the 2.22 MeV neutron-capture line as well as the 0.478 MeV ⁷Li* deexcitation line, both of which may be discernible with the INTEGRAL satellite if produced in an optically thin region during a large outburst. For very energetic jets, a relatively narrow neutral pion gamma-decay signature at 67.5 MeV could also be measurable with the GLAST satellite. We argue that about 10%-20% of all microquasar systems ought to be sufficiently misaligned to be undergoing the proposed jet-secondary impacts.