X-ray Lines in Binary Star Systems and the 3.55-keV Anomaly

Abstract

A close binary star system that consists of a red giant and a neutron star is considered. X-ray emission from H-like Si XIV emerges during its recombination in the magnetic column of a neutron star with a superstrong magnetic field. The Lα Si XIV line with an energy of 3.55 keV is formed in a magnetic field of 6 × 1022 G. The identification of this line in the X-ray spectrum is discussed. The detection of Lα emission features during the recombination of such ions as C VI, N VII, O VIII, Ne X, Mg XII, S XVI, Ca XX, and Fe XXVI is predicted at energies both below and above 3.55 keV. A table for the energies in the Lα line for these ions in magnetic fields from 4 × 1012 to 1013 G is presented. The presence of anisotropy in the line emission is emphasized. It is shown that the realization of a recombination laser is possible and, as a consequence, the detection of these X-ray features up to redshifts z ≤ 100 is probable, although this will already be the ultraviolet range. It is noted that in the presence of a laser, line narrowing is inevitable.