Winds Write the Radio Script
In the bustling dance of a neutron star paired with an OB supergiant, the radio and millimeter glow at the edges of the system carry a story that X-ray eyes miss. A team led by J. van den Eijnden of the Anton Pannekoek Institute for Astronomy at the University of Amsterdam drew a map of that story by stacking observations from ALMA, NOEMA, and the VLA across twelve neutron star high-mass X-ray binaries. The goal wasn’t to reinvent the wheel of accretion physics but to illuminate the wind that the donor star sheds and how it feeds, or sometimes starves, the neutron star’s appetite.
The authors assembled a diverse crew of targets: six systems that steadily feed the neutron star (SgXBs), four that flare in X-rays for brief moments (SFXTs), and a couple of intermediate cases. With millimeter and radio coverage spanning roughly 40 to 300 GHz, they could separate two competing sources of low-frequency light: a potential jet powered by accretion and the thermal glow of the outer stellar wind via free-free emission. The surprise is in the balance. Across nine of twelve targets, the low-frequency emission aligns with the thermal wind from the OB donor rather than a jet. In other words, the wind—the star’s constant breath—dominates the spectrum at these wavelengths, not the shadowy jet that sometimes steals the spotlight in other X-ray binaries.
What makes this especially powerful is that the wind signature is not a whisper at one color but a coherent inversion across multiple bands. The detected spectra typically show inverted shapes with spectral indices around 0.6 to 0.8, consistent with free-free emission from an expanding, radiatively driven wind that has reached, or is approaching, its terminal speed. The hydrogen atoms are not singing with the high-energy punch of the X-ray beam; they are radiating calm, thermally generated light as electrons scatter off ions in the wind’s outer reaches. The paper’s lead author and collaborators argue this wind-dominated view helps explain why these systems often look quiet in the radio but roar in X-rays: the outer wind carries the energy away in a steady, thermal fashion, while the inner regions near the neutron star host a much more chaotic, accretion-driven environment that X-ray telescopes watch closely.
Crucially, these results come with a methodological boast: the study pairs millimeter and radio data to anchor wind properties on large scales, well beyond the binary separation. That means we’re peering into the part of the wind that has had time to settle into a smoother flow, where clumping has less of a say. It’s a bit like listening to the tail of a fountain rather than the spray at the nozzle—the outer wind’s rhythm is steadier, and that steadiness is exactly what makes the wind a reliable tracer of mass loss and wind dynamics. The work’s framing and execution reflect a collaborative, cross-institutional effort, with the lead author firmly rooted in the University of Amsterdam’s astronomy group and partners across Italy, Chile, Germany, and beyond. The synthesis of facilities (ALMA, NOEMA, VLA) and the careful treatment of non-detections give the wind theory a sharper, more testable edge than ever before.
SFXTs vs SgXBs: a wind-density story
One of the central puzzles in high-mass X-ray binaries is why some systems—SgXBs—gently drink from a donor wind and glow persistently in X-rays, while others—SFXTs—shoot off flares and then quiet down for long stretches. The millimeter and radio survey adds a clear wrinkle to that story: the same wind physics that governs the outer stellar envelope also appears to shape how these systems accrete matter. The authors find that SFXTs, on average, are fainter at 100 GHz than the prototypical SgXBs. The faintness fits with the idea that SFXTs have systematically less dense winds, or winds that are faster on average, shrinking the wind material available for capture by the neutron star. In parallel, the Fe Kα fluorescence line in X-ray spectra—an X-ray echo from wind material near the neutron star—tends to be weaker in SFXTs. Put together, the millimeter luminosity and Fe Kα strength tell a coherent story: in SFXTs, the wind environment around the neutron star is, overall, sparser or more tenuous near the star, reducing both the inner accretion rate and the large-scale thermal wind glow.
When the team plots the 100 GHz wind-calibrated luminosities against the orbital period, the SFXTs cluster in a relatively narrow lane, while the SgXBs fan out over a broader range. It’s as if the daytime wind breeze in SFXTs is gentler and less dense, while SgXBs carry a wider spectrum of wind densities and velocities that can sustain a steadier, brighter wind signal at millimeter wavelengths. The link to Fe Kα EW strengthens the case: a wind dense enough to produce stronger fluorescence near the neutron star also leaves a larger imprint on the millimeter spectrum far from the star. That parallel across scales—outer wind emission at 100 GHz and inner wind density indicated by Fe Kα—cements wind density as a unifying thread between these two subclasses.
Of course, clumping—the wind’s own irregular texture—remains a wild card. The authors emphasize that clumping can skew the simple analytic relations used to translate millimeter brightness into a mass-loss to wind-velocity ratio. In some sources, clumping can exaggerate the apparent wind strength when viewed from the outside, while others may show less clumping in the outer wind. The data suggest a picture in which SgXBs, with their brighter millimeter emission, tend to host winds that are denser or slower on large scales, whereas SFXTs carry winds that are comparatively faster or less dense. Either way, the millimeter view is offering a fossil record of wind history that complements the inner-wind measurements from UV/optical studies and the bow-shock imprints etched into the surrounding medium.
Winds vs Jets: the low-frequency disco
A natural question behind any radio study of X-ray binaries is whether the signal is a wind’s thermal glow or a jet’s non-thermal roar. The second pillar of the paper argues that, for these neutron-star–OB-supergiant binaries, the wind wins at low frequencies. The inverted spectra with α around 0.6–0.8, the typical millimeter luminosities, and the lack of obvious rapid flaring all point to free-free emission from the donor wind as the dominant source of the low-frequency light. Jets, if present, are not the primary torch at 40–300 GHz unless the system happens to reveal itself at sub-GHz frequencies where a jet’s spectrum might flatten or reveal a different slope in an observationally challenging regime.
The authors provide a careful physical balance: jets in neutron-star X-ray binaries can be weak compared with those in black-hole systems, especially when the accretion flow is magnetically choked and the wind is dense. In Be/X-ray binaries—where disks feed the jet in a disk-fed regime—the jet can dominate, especially when the wind is weaker. But in wind-fed systems like the SgXBs and SFXTs studied here, the wind’s thermal emission is a natural, expected outcome. The researchers quantify this intuition with a back-of-the-envelope jet vs. wind comparison: beyond roughly 1 GHz, the wind’s thermal luminosity generally outshines a jet unless the wind-jet interactions boost the jet power dramatically. In other words, the wind’s glow at millimeter wavelengths is robust enough that jets would have to be unusually powerful or unusually wind-bright to take the lead.
That said, the authors are careful about overclaiming. If a system were observed at sub-GHz frequencies with a sufficiently detailed multi-band spectrum, a jet contribution could become visible—especially if the wind is atypically sparse or if wind interactions with the jet amplify the jet’s luminosity. The broader message is that low-frequency emission in these particular HMXBs acts as a clean tracer of the wind, not a smoky mirror for jet activity. It also highlights how multi-wavelength campaigns—here, combining ALMA, NOEMA, and VLA data—are essential to disentangle the competing processes that can light up a binary system across the spectrum.
Wind constraints and bow shocks: mapping wind from near to far
The millimeter glow is not just a pretty signal—it’s a diagnostic tool. The study translates the measured millimeter brightness into a proxy for the ratio of the wind mass-loss rate to its terminal velocity (Ṁw/v∞). That ratio anchors the outer wind properties, which can then be compared with the inner wind parameters derived from optical/UV measurements and with the geometry of bow shocks that some systems drive into the interstellar medium. The researchers show that, for many targets, the millimeter-based wind constraints and the inner-wind measurements do not line up one-for-one. The differences aren’t paradoxes; they likely reflect wind clumping, acceleration, and radial variability as the wind settles into its outer, freely expanding regime. The result is a nuanced picture: outer winds may appear denser or slower when viewed at millimeter wavelengths, even if the inner wind appears faster or less dense in UV studies.
Two systems where bow shocks have helped ground-truth the wind parameters receive particular attention: Vela X-1 and 4U 1907+09. Bow shocks—structures carved where the stellar wind meets the surrounding interstellar medium—offer an independent way to estimate wind properties by balancing ram pressure against the ambient medium. The combined analysis yields a consistent, cross-validated view: for Vela X-1, the millimeter inference lines up with a reasonable interstellar density around the bow shock, yielding wind parameters in line with broader measurements. For 4U 1907+09, the bow-shock constraints point to a somewhat different ISM density, with the millimeter data still compatible when the model components are allowed to vary. These cross-checks reinforce a central conclusion: low-frequency wind emission is a meaningful, externally checkable window on how massive-star winds behave when they crash into a compact companion and then push into the galaxy at large.
Beyond the steps of the bow shock, the study also highlights wind variability as a potential driver of millimeter flux changes. While no rapid (minute-scale) flares were detected in the ALMA observations, the data hint at slower, day-scale or multi-day variations in some sources. That pattern matches what one might expect if global wind parameters—mass loss rate, wind speed, or clumping—change on longer timescales and propagate out to the millimeter-emitting regions. In practical terms, the wind’s voice changes slowly, and we need sustained observing campaigns to hear the full chorus. The authors sketch a plausible framework: if winds vary by tens of percent over days to weeks, the millimeter light should reflect that, offering a way to track wind evolution in real time across several binary orbits.
A new window on massive stars and their winds
Why does this matter beyond the neat taxonomy of SgXBs and SFXTs? Because massive stars sculpt galaxies, seed the interstellar medium with heavy elements, and drive complex feedback loops that regulate star formation. The wind’s density and velocity determine how much energy and momentum a star’s wind can inject into its surroundings. And when the wind interacts with a compact companion, the fate of accretion, X-ray output, and jet formation is scrambled in ways that are hard to predict from X-ray data alone. The millimeter and radio findings provide an outside view—an independent check on the outermost winds of the donor stars and, crucially, a way to connect outer wind properties to inner accretion physics. It’s a reminder that the universe often communicates across wavelengths, and that listening at long wavelengths can reveal the scaffolding that shapes more dramatic, high-energy events.
From a broader perspective, this work also tightens the bridge between stellar astrophysics and the physics of accretion. The wind carries energy and material far from the binary, influencing not just the binary’s own evolution but also the surrounding ISM and, on longer timescales, the evolution of the binary’s mass transfer history. The study’s approach—combing multi-band millimeter and radio data with X-ray context and bow-shock measurements—offers a road map for future work. It shows how to use the outer wind to test inner-wind physics, how to place constraints on clumping and acceleration, and how to frame questions about wind-driven feedback in a universe where the most interesting processes often hide in plain sight at the cold, steady edge of the spectrum.
As a final reflection, the research underscores the value of big, coordinated observing campaigns. The twelve targets are not merely a sample; they are a curated laboratory in which different donor types, orbital configurations, and accretion regimes all speak the same wind-language. The results—nine millimeter detections, inverted spectra, wind-dominated SEDs, and the SFXT/SgXB dichotomy—are not just data points. They are a narrative about how massive stars shed their mass, how that mass feeds a compact companion, and how the cosmos writes its most energetic stories in the quiet language of thermal radiation out at the edge of the spectrum.
Lead author J. van den Eijnden and collaborators published this work with the Anton Pannekoek Institute for Astronomy at the University of Amsterdam as the institutional home base, reflecting a richly collaborative project across Europe and the Americas. The study represents a milestone in using millimeter wavelengths to probe winds in a class of systems where accretion and wind physics are tightly interwoven, and it sets the stage for a new era of wind-driven astrophysics that can be tested across multiple wavelengths and with next-generation facilities.
