Vol. 674
1. Letters to the Editor

Vacuum polarization alters the spectra of accreting X-ray pulsars

by E. Sokolova-Lapa, J. Stierhof, T. Dauser, and J. Wilms 2023, A&A, 674, L2

The X-ray spectrum of high mass X-ray binaries (HMXBs) sometimes shows a feature around an energy of 10 keV. Absorption features in the HMXB spectra are often due to cyclotron absorption in the high neutron star magnetic field. However, in addition to the "10 keV" feature, in many of these sources we do already see the cyclotron feature at higher energies, ruling out this explanation. Other authors suggested that this 10 keV feature might be caused by calibration issues in the X-ray detectors. It is known that the high neutron star magnetic field alters the propagation of X-ray photons, making the vacuum birefringent. At variance with common belief, in this Letter, Sokolova-Lapa and collaborators show that the effects of vacuum polarization can play a role in the propagation of X-rays within the hot plasma accreting onto the neutron star, suggesting an intriguing new explanation for the 10 keV feature.