Clustering between HMXBs and OB associations in the MilkyWay

Abstract

We present the first direct measurement of the spatial cross-correlation function of high-mass X-ray binaries (HMXBs) and active OB associations in the Milky Way. This result relied on a sample containing 79 hard X-ray detected HMXBs (mostly IGRs) and 458 OB associations. Clustering between the two populations is detected with a significance above 7σ for distances ≤1 kpc. Thus, HMXBs closely trace the underlying distribution of the massive star-forming regions that are expected to produce the progenitor stars of HMXBs. The average minimum separation of 0.4±0.2 kpc between OB associations and HMXBs is consistent with the view that the latter have large runaway velocities (100±50 km s−1). This suggests that the offset is mostly due to the runaway velocity acquired by the binary during and after the supernova phase (with a few exceptions). The characteristic scale of the correlation function indicates an average kinematical age (≡ time between the supernova and X-ray phase) of 4±2 Myr for the HMXB population. Despite being derived from the “grand design” of our Galaxy, these signatures of HMXB evolution are consistent with theoretical expectations and observations of individual objects.