ALMA resolves thin molecular filaments near HESS J1023-575; preprint suggests possible microquasar jet traces

A newly posted astronomy preprint reports that the Atacama Large Millimeter/submillimeter Array, or ALMA, has resolved a set of unusually thin, aligned molecular filaments near the bright TeV gamma-ray source HESS J1023-575. The authors argue those structures may be the long-sought traces of an ancient microquasar jet — a jet from a compact object such as a black hole or neutron star feeding on a companion star.

The paper, titled “An ALMA view of the Jet-Arc CO clouds toward the TeV γ-ray source HESS J1023-575 and Westerlund 2; Evidence for the footprints of microquasar jets, the very powerful cosmic-ray accelerator in the Galactic disk,” was posted to arXiv on Aug. 15, 2026, and is listed as submitted to PASJ as a Letter. The lead author is Yasuo Fukui of Nagoya University, with nine coauthors. Because the study is a preprint, its analysis and conclusions have not yet been peer reviewed.

The new work focuses on the previously identified Jet and Arc molecular clouds near HESS J1023-575, an extended gamma-ray source close to the young massive star cluster Westerlund 2. Using ALMA ACA plus Total Power observations of carbon monoxide emission, the team says it resolved the clouds into numerous narrow filaments, typically about 0.5 parsec wide and about 10 to 20 parsecs long. The filaments, the authors write, are aligned with the Jet-Arc axis, while the larger Jet/Arc structure extends across roughly 170 parsecs. Those size estimates depend on the paper’s adopted distance of 7.5 kiloparsecs, based on earlier CO studies.

From that morphology, combined with magnetohydrodynamic modeling, the researchers argue that “the thin filamentary clouds are the footprints of the microquasar jets.” In the same paper, they go further, estimating how much energy may be stored in cosmic-ray protons if the gamma rays have a hadronic origin — meaning they are produced when high-energy protons interact with surrounding gas. Under that scenario, using the inferred gas density and gamma-ray luminosity, they estimate a proton energy of about 7 × 10^48 erg. “Wp in HESS J1023 is therefore an order of magnitude larger than that derived in the young TeV γ-ray SNRs,” the authors write, comparing their estimate with values they cite for the supernova remnants RX J1713.7-3946 and RX J0852.0-4622. They also argue that if the source remained active for 1 million to 10 million years, its lifetime-integrated cosmic-ray output could equal at least about 1,000 supernova remnants.

Those broader claims are much less secure than the underlying observational result. The energy and lifetime estimates depend on several linked assumptions: that the clouds are 7.5 kiloparsecs away, that the gas densities are correct, that the TeV gamma rays are hadronic in origin rather than produced by energetic electrons, and that the cloud morphology really does record a long-lived microquasar jet. The source itself remains debated, and the paper does not identify a confirmed compact object powering such a jet.

HESS J1023-575 has been an open question since the H.E.S.S. collaboration reported it in 2007 as an extended TeV source spatially coincident with Westerlund 2 and the RCW 49 region. Earlier CO studies by Fukui’s group and collaborators had already identified the Jet and Arc clouds and proposed unusual explanations, including a microquasar or an anisotropic supernova remnant. Other work in the region has pointed to possible contributions from the star cluster environment and from a bright gamma-ray pulsar. The new ALMA data strengthen the case that the gas has an unusual, highly ordered structure. Whether that structure marks the aftermath of a microquasar jet — and an unusually powerful Galactic cosmic-ray accelerator — is the part that now awaits scrutiny.