3.55-keV Anomaly as the First X-ray Laser in Space

Abstract

The 3.55-keV Ly-\(\alpha\) line of the hydrogen-like ¹⁴Si can be produced in a 6x10¹² G magnetic field in close binary star systems consisting of a red giant and a neutron star. This recombination radiation to the ground Landau level is emitted by ¹⁴Si ions in the magnetic column of neutron stars in binary systems. The Ly- \(\alpha\) transition energy for hydrogen-like ¹⁴Si is calculated in magnetic fields 4x10¹² – 10¹³ G. If the 3.55-keV line is a laser line, then it can be observed up to red shifts z \(\leq\) 100. This line is probably an X-ray candle in the universe produced by a great amount of silicon in these close binary systems. The 3.55-keV line is unique due to a complex geometry in such binary systems. Laser radiation in the (1–20)-keV energy range can be also emitted by other hydrogen-like ions. A laser line is inevitably narrowed. The existence of thermal X-ray sources in space emitting lines in the keV region is also pointed out.