Detectability and characteristics of the 2.223 MeV line emission from nearby X-ray binaries

Abstract

We present an application and an improvement of the model of Jean & Guessoum (2001) for the production of the 2.223 MeV line resulting from the capture of neutrons in the atmosphere of the secondary in an X-ray binary system. In this model, the neutrons are produced in the accretion disk around the compact primary star and radiated in all directions. The present treatment has fine-tuned and improved upon the previous model in several ways: (1) we have added CNONe nuclei in both the atmosphere of the secondary and the accretion disk around the compact object, making the production and interaction of the neutrons more realistic and accurate; (2) we have improved the physical modeling of the atmosphere of the secondary; and (3) we have considered the comptonization of the 2.223 MeV photons emitted and thus produced the spectrum of the emission both for the line and for the "continuum" below, down to some 100 kev of energy. We have performed a detailed investigation of the variation of the 2.223 MeV emissivity with respect to: the X-ray binary parameters (masses of the compact and secondary stars, separation, etc.), the rate and the model of accretion, and the metallicity of the accreted and atmospheric material. The mean fluxes, phasograms, and the spectra of the 2.223 MeV line have been calculated for several nearby X-ray binary sources, namely: A0620-00, XTE J1118+480, Cen X-4, GS 2000+25 and GRO J0422+32; and we have compared the results with present and future y-ray spectrometer sensitivities (INTEGRAL's spectrometer SPI and the Compton Telescope).