Multiwavelength Observations of the Runaway Binary HD 15137

HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations.

💡 Research Summary

HD 15137 is a rare runaway O‑type binary that provides a valuable laboratory for testing the mechanisms by which massive stars are ejected from their birth clusters. The authors obtained a new set of high‑resolution blue‑optical spectra (4000–5000 Å, R ≈ 30 000) spanning several years and measured radial velocities from dozens of He I, He II, and metal lines. A period analysis yields a highly eccentric orbit with P = 55.42 ± 0.03 d, e = 0.62 ± 0.02, and a₁ sin i = 12.3 ± 0.5 R⊙. The mass function f(M) = 0.004 ± 0.001 M⊙ implies a very low‑mass companion unless the inclination is extremely small. Spectral classification of the primary is O9.5 V; fitting with FASTWIND models gives T_eff ≈ 33 kK, log g ≈ 4.0, v sin i ≈ 140 km s⁻¹, and a luminosity of ≈ 10⁵·⁴ L⊙, corresponding to a mass of ≈ 22 M⊙. No spectral signatures of a secondary are detected, constraining any optical companion to be ≤ 5 % of the primary’s light, i.e., M₂ ≲ 2 M⊙.

The X‑ray component of the study consists of XMM‑Newton EPIC‑pn and MOS observations (total exposure ≈ 45 ks). The EPIC spectra are well described by a single‑temperature thermal plasma (apec) with kT ≈ 0.21 keV and a modest absorption column NH ≈ 1.2 × 10²¹ cm⁻². The unabsorbed X‑ray luminosity is L_X ≈ 1.2 × 10³² erg s⁻¹, fully consistent with the canonical wind‑shock emission of an isolated O star (L_X ≈ 10⁻⁷ L_bol). No hard (> 2 keV) component is detected; the 3σ upper limit is L_X,hard < 5 × 10³¹ erg s⁻¹. This rules out a luminous accreting compact object, a propeller‑phase neutron star, or a weakly accreting neutron star that would produce detectable hard X‑rays.

The authors discuss two possible ejection scenarios. (1) The binary‑supernova channel, where the primary’s companion is a compact remnant (neutron star or black hole) left after a supernova. The lack of hard X‑rays suggests that, if a neutron star is present, it is in the ejector regime (a rotation‑powered pulsar) whose wind does not currently produce observable X‑ray emission. Alternatively, a quiescent black hole with negligible accretion could be present. (2) The dynamical ejection channel, involving close multi‑body encounters in a dense cluster. This would require a low‑mass stellar companion to have absorbed the recoil momentum, but the spectroscopic non‑detection of such a star and the orbital parameters make this scenario unlikely.

Consequently, the paper concludes that HD 15137 most plausibly hosts an unseen compact object—either a rotation‑powered neutron star or a dormant black hole—while the O‑type primary dominates the observed optical and soft X‑ray emission. The system thus represents an “elusive” runaway binary in which the compact companion is currently in a state that suppresses high‑energy accretion signatures. Future deep radio pulsar searches or long‑term X‑ray monitoring could confirm the nature of the hidden companion and further illuminate the pathways by which massive binaries become runaway objects.