4U 1907+09: a HMXB running away from the Galactic plane

TL;DR

This paper reports the discovery of a bow shock around the high-mass X-ray binary 4U 1907+09, confirming its high velocity and runaway status, and compares it with a similar bow shock around Vela X-1.

Contribution

First detection of a bow shock around 4U 1907+09, confirming its high-velocity runaway nature through proper motion measurements and comparison with Vela X-1.

Findings

4U 1907+09 has a peculiar transverse velocity of approximately 160 km/s.

The bow shock's orientation aligns with the system's motion away from the Galactic plane.

Comparison with Vela X-1's bow shock highlights similarities in HMXB runaway phenomena.

Abstract

We report the discovery of a bow shock around the high-mass X-ray binary (HMXB) 4U 1907+09 using the Spitzer Space Telescope 24 $\mu$m data (after Vela X-1 the second example of bow shocks associated with HMXBs). The detection of the bow shock implies that 4U 1907+09 is moving through the space with a high (supersonic) peculiar velocity. To confirm the runaway nature of 4U 1907+09, we measured its proper motion, which for an adopted distance to the system of 4 kpc corresponds to a peculiar transverse velocity of $\simeq 160 \pm 115$ km/s, meaning that 4U 1907+09 is indeed a runaway system and supporting the general belief that most of HMXBs possess high space velocities. The direction of motion of 4U 1907+09 inferred from the proper motion measurement is consistent with the orientation of the symmetry axis of the bow shock, and shows that the HMXB is running away from the Galactic plane. We also present the Spitzer images of the bow shock around Vela X-1 (a system similar to 4U 1907+09) and compare it with the bow shock generated by 4U 1907+09.