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arXiv · 2509.05427

Gamma-ray lines, electron-positron annihilation, and possible radio emission in X-ray pulsars

Alexander A. Mushtukov·Emir Tataroglu·Alex J. Cooper·Sergey S. Tsygankov
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Abstract

Accretion onto neutron stars (NSs) in X-ray pulsars (XRPs) results in intense X-ray emission, and under specific conditions, high-energy nuclear interactions that produce gamma-ray photons at discrete energies. These interactions are enabled by the high free-fall velocities of accreting nuclei near the NS surface and give rise to characteristic gamma-ray lines, notably at 2.2 MeV, 5.5 MeV, and 67.5 MeV. We investigate the production mechanisms of these lines and estimate the resulting gamma-ray luminosities, accounting for the suppression effects of radiative deceleration in bright XRPs and the creation of electron-positron pairs in strong magnetic fields. The resulting annihilation of these pairs leads to a secondary emission line at $\sim 511$ keV. We also discuss the possibility that non-stationary pair creation in the polar cap region could drive coherent radio emission, though its detectability in accreting systems remains uncertain. Using a numerical framework incorporating general relativistic light bending and magnetic absorption, we compute the escape fraction of photons and distinguish between actual and apparent gamma-ray luminosities. Our results identify the parameter space - defined by magnetic field strength, accretion luminosity, and NS compactness - where these gamma-ray signatures may be observable by upcoming MeV gamma-ray missions. In particular, we highlight the diagnostic potential of detecting gravitationally redshifted gamma-ray lines and annihilation features for probing the mass-radius relation and magnetospheric structure of NSs.

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Alexander A. Mushtukov, Emir Tataroglu, Alex J. Cooper, Sergey S. Tsygankov. 2025-09-05. Gamma-ray lines, electron-positron annihilation, and possible radio emission in X-ray pulsars. https://arxiv.org/abs/2509.05427

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