Evidence for low kick velocities among high-mass X-ray binaries in the Small Magellanic Cloud from the spatial correlation function

TL;DR

This study analyzes the spatial correlation between high-mass X-ray binaries and their birthplaces in the Small Magellanic Cloud, revealing low average kick velocities and suggesting environmental influences on stellar evolution.

Contribution

It provides the first measurement of HMXB kick velocities in the SMC using spatial correlation functions, highlighting environmental effects on stellar evolution.

Findings

Significant correlation between HMXBs and OB Associations in the SMC.

Estimated average kick velocity of 2-34 km/s for HMXBs.

Lower kick velocities compared to the Milky Way, indicating environmental influence.

Abstract

We present the two-point cross-correlation function between high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud (SMC) and their likely birthplaces (OB Associations: OBAs). This function compares the spatial correlation between the observed HMXB and OBA populations against mock catalogs in which the members are distributed randomly across the sky. A significant correlation (15 sigma) is found for the HMXB and OBA populations when compared with a randomized catalog in which the OBAs are distributed uniformly over the SMC. A less significant correlation (4 sigma) is found for a randomized catalog of OBAs built with a bootstrap method. However, no significant correlation is detected when the randomized catalogs assume the form of a Gaussian ellipsoid or a distribution that reflects the star-formation history from 40 Myr ago. Based on their observed distributions and assuming a range of migration timescales, we infer that the average value of the kick velocity inherited by an HMXB during the formation of its compact object is 2-34 km/s. This is considerably less than the value obtained for their counterparts in the Milky Way hinting that the galactic environment affecting stellar evolution plays a role in setting the average kick velocity of HMXBs.