paper

A Two-Stage Kick Scenario for the Peculiar LMXB GX 1+4

arXiv:2509.21177 · doi:10.3847/2041-8213/ae060a

Abstract

The low-mass X-ray binary (LMXB) GX 1+4 stands out with its unique properties. Despite being an old system, it hosts a strongly magnetized neutron star (NS), a trait usually linked to younger systems. Its exceptionally long orbital period (1160 days) and low eccentricity (0.101) imply that the NS formed with minimal mass loss and a weak natal kick. These features collectively point towards the NS having formed through the accretion induced collapse (AIC) of a white dwarf (WD). However, GX 1+4's unusually high peculiar velocity (~ 189.36 km/s) defies standard AIC explanations. To address this discrepancy, we propose a two-stage kick scenario within the AIC framework: an initial natal kick followed by a delayed electromagnetic "rocket effect" kick. Our Monte Carlo simulations indicate that while the natal kick (< 100 km/s) can generate a wide range of orbital eccentricities, the subsequent rocket kick (~ 240-480 km/s) explains both the high systemic velocity and low eccentricity. This two-stage kick mechanism naturally reproduces the observed characteristics of GX 1+4, provided that the NS's initially buried magnetic field re-emerges after the acceleration process ends. Our study represents the first attempt to quantitatively constrain the kick velocities in GX 1+4 and underscores the importance of possible rocket kicks in forming such peculiar LMXB systems.

9 pages, 5 figures; Published in the Astrophysical Journal Letters

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