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Description
High-mass X-ray binaries (HMXBs) are systems in which a neutron star or black hole accretes material from a massive companion. They can be roughly divided into three main classes: (i) wind-fed compact objects with supergiant donors; (ii) compact objects accreting from the decretion disc of a Be star; (iii) compact objects accreting via a disc from a Roche-lobe filling companion. All HMXBs must have experienced a core-collapse supernova event during their evolution. The kick associated with this event should affect the space velocity of the system in a way that depends on the state of the binary at the time of the explosion. Here, we test whether the different evolutionary histories of HMXBs have left a detectable imprint on their peculiar velocities. Using data from Gaia Early Data Release 3 (Gaia EDR3), we first calculate the peculiar velocities ($V_p$) and associated uncertainties for 55 well-known HMXBs. The peculiar velocity distribution shows some evidence for bimodality, suggesting the existence of two distinct populations: one characterised by low velocities ($<50~\mathrm{km~s^{-1}}$), the other characterised by high velocities ($>50~\mathrm{km~s^{-1}}$). The existence of a high-velocity population is surprising for such massive systems. We use Monte Carlo simulations to set firm lower limits on $V_p$ for all of our targets, finding that at least 5 systems in our sample have $V_p>75~\mathrm{km~s^{-1}}$ at probability (p) $<2e^{-5}$.
