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Magnetic rigidity reveals the PeVatron acceleration region in SS 433

By analyzing multi-frequency VLBA observations to reveal a magnetic field profile that increases magnetic rigidity along the inner jet, this study identifies the baryonic ejecta of microquasar SS 433 as a hidden PeVatron capable of accelerating protons to energies exceeding 1 PeV well upstream of its extended TeV-emitting lobes.

Original authors: Lang Cui, Jie Liao, Wancheng Xu, Pak Hin Thomas Tam, Xi Yan, Linuo Yang, Ruo-Yu Liu, Liang Chen, Ning Chang, Yong-Feng Huang, Long Ji, Ashutosh Tripathi, Felix Aharonian

Published 2026-08-24

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Original authors: Lang Cui, Jie Liao, Wancheng Xu, Pak Hin Thomas Tam, Xi Yan, Linuo Yang, Ruo-Yu Liu, Liang Chen, Ning Chang, Yong-Feng Huang, Long Ji, Ashutosh Tripathi, Felix Aharonian

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the cosmos, there exists a class of cosmic accelerators so powerful they can fling subatomic particles to energies a million times greater than anything we can create on Earth. These cosmic machines are called PeVatrons. For decades, astronomers have searched for them, knowing that somewhere in our own galaxy, nature must be building particles to these staggering heights. While exploding stars have long been the prime suspects, a new candidate has emerged from a peculiar, long-studied object known as SS 433. This system is a binary pair where a black hole or neutron star is voraciously consuming a companion star, shooting out two massive jets of superheated gas in opposite directions. Recent observations have shown that this system emits high-energy gamma rays, suggesting it is indeed a PeVatron, but a critical mystery remained: exactly where inside those jets does the acceleration happen? Is it near the central engine, or far out where the jets crash into surrounding gas?

A team of astronomers has now solved this puzzle by looking at the magnetic environment inside the jets of SS 433 with unprecedented detail. Using a network of radio telescopes spread across the Earth, they mapped the strength of the magnetic field along the inner section of the jet, a region only tens of astronomical units wide. They discovered that the magnetic field does not weaken as quickly as expected as you move away from the center. Instead, it holds its strength surprisingly well over distance. This specific behavior is the key to the mystery. In physics, the ability of a magnetic field to trap and accelerate a particle depends on a combination of the field's strength and the size of the region it occupies. Because the field in SS 433 stays strong enough over such a large area, the "magnetic rigidity"—the capacity to hold onto fast-moving particles—actually grows as you travel further out along the jet.

This finding reveals that the inner part of the jet, specifically a zone a few hundred astronomical units from the central binary, is the hidden engine room where protons are being accelerated to peta-electronvolt energies. The researchers calculated that in this specific region, the magnetic conditions are strong enough to push protons to energies of about 2.6 peta-electronvolts. This matches perfectly with the maximum energy levels inferred from the gamma rays detected by the LHAASO observatory. The study effectively rules out the idea that these highest-energy particles are created by electrons in the same spot, as the intense magnetic field would cool electrons down far too quickly for them to reach such energies. Instead, the data points to protons being the primary accelerators.

The picture that emerges is one of a two-stage process. The compact, inner jets of SS 433 act as the injection site, where protons are boosted to extreme speeds by the magnetic field. Once these super-fast protons escape this inner zone, they travel outward into the surrounding nebula, a vast cloud of gas called W50. There, they collide with neutral gas atoms, creating the burst of high-energy gamma rays that telescopes on Earth can see. This separation explains why the gamma-ray emission looks different at various energy levels: the highest energy signals come from the interaction zone in the gas cloud, while the acceleration itself happens much closer to the source. By measuring the magnetic field profile, the researchers have not only located the accelerator but also confirmed that the microquasar SS 433 is a genuine PeVatron, capable of driving particles to the very limits of what is possible in our galaxy.

Technical Summary: Magnetic Rigidity Reveals the PeVatron Acceleration Region in SS 433

Problem Statement
Identifying Galactic PeVatrons—accelerators capable of driving cosmic-ray particles to peta-electronvolt (PeV) energies—remains a central challenge in high-energy astrophysics. While supernova remnants are established sites for Galactic cosmic-ray production, gamma-ray observations above tens of tera-electronvolts (TeV) indicate that additional sources are required to explain the spectrum up to the "knee" ( $10^{15}$ – $10^{16}$ eV). Microquasar jets, such as those from SS 433, have emerged as compelling candidates due to their combination of large kinetic power and magnetized, mildly relativistic outflows. Although the Large High Altitude Air Shower Observatory (LHAASO) has detected ultra-high-energy ( $>100$ TeV) gamma-ray emission from the SS 433/W50 region, implying parent proton energies on the order of PeV, the specific location within the system where these particles are first accelerated and injected remains unresolved. The $>100$ TeV emission is spatially coincident with an atomic cloud and exhibits a quasi-isotropic morphology, contrasting with the bipolar TeV emission from downstream jet-cloud interactions. This morphological discrepancy suggests the acceleration site differs from the downstream radiation zones, yet current gamma-ray instruments cannot resolve the inner radio jet (on AU scales) where such acceleration might initiate.

Methodology
To locate the acceleration region, the authors utilized archival, multi-frequency Very Long Baseline Array (VLBA) observations of SS 433 from June 16, 1998, at 5, 8.4, and 15 GHz. The study focused on the eastern (approaching) jet to avoid free-free absorption affecting the western jet.

Data Processing: The multi-frequency data were restored to a common circular beam ( $\Theta_{maj} = 3.537$ mas) and registered to a common frame using frequency-dependent core positions to account for core-shift effects.
Slicing and Spectral Analysis: Ten transverse slices of the jet, spanning deprojected distances of 24–75 AU (3.6–11.5 mas), were analyzed. For each slice, the local flux density, beam-deconvolved transverse width, and two-point spectral indices were measured. The spectra confirmed optically thin synchrotron emission in the mid-jet region.
Magnetic Field Estimation: Under the standard minimum-energy (equipartition) assumption, the magnetic field strength ( $B_{eq}$ ) was calculated for each slice. The analysis accounted for uncertainties in flux density, width, and spectral indices via Monte Carlo simulations and tested sensitivity to the ratio of non-radiating to radiating particle energy ( $k$ ).
Hillas Limit and Acceleration Timescales: The authors constructed a spatially resolved profile of the magnetic field $B(H)$ as a function of distance $H$ from the central compact object. They evaluated the speed-weighted Hillas limit ( $E_{H,\beta} \propto \beta_j Z e B R_{acc}$ ) to determine if the magnetic rigidity is sufficient to confine PeV protons. They further compared the acceleration timescale ( $t_{acc}$ ) against the dynamical flow time ( $t_{dyn}$ ) and particle escape times to verify if acceleration can occur before particles are advected away.
Gamma-Ray Modeling: The LHAASO $>100$ TeV spectrum was fitted using a lepto-hadronic model (incorporating inverse-Compton and pion-decay components) to infer the maximum proton energy ( $E_{p,max}$ ) required to produce the observed emission.

Key Results

Magnetic Field Profile: The study derived a magnetic field profile along the inner jet declining as $B(H) \propto H^{-0.50 \pm 0.12}$ . This decline is significantly shallower than the $H^{-1}$ scaling expected for a purely toroidal field in a conical jet with constant speed, or the $H^{-2}$ scaling for a poloidal field.
Increasing Magnetic Rigidity: Because the magnetic field declines more slowly than $H^{-1}$ , the magnetic rigidity ( $B \times R_{acc}$ ) for a conical jet ( $R_{acc} \propto H$ ) increases with distance downstream. Specifically, $E_{H,\beta} \propto H^{0.50 \pm 0.12}$ .
PeV Acceleration Capability: At a distance of $H \approx 100$ AU, the estimated speed-weighted Hillas energy reaches $\sim 2$ PeV. At $H \approx 140$ AU, the limit reaches the few-PeV range, consistent with the proton cutoff energy ( $E_{p,cut} \approx 2.63$ PeV) inferred from the LHAASO gamma-ray spectrum.
Acceleration Efficiency: The acceleration timescale for near-Bohm first-order Fermi acceleration is shorter than the dynamical flow time ( $t_{acc} < t_{dyn}$ ) at these distances. Furthermore, the ratio $t_{acc}/t_{dyn}$ decreases downstream, making the acceleration condition progressively less restrictive with distance.
Electron vs. Proton Origin: The strong magnetic fields in the compact ejecta would cause PeV electrons to cool via synchrotron radiation on timescales ( $\sim 110$ s) far shorter than the flow time, ruling out the compact ejecta as the site for PeV electron inverse-Compton emission. This supports a hadronic origin for the highest-energy gamma rays.
Energy Budget: The inferred sub-gauss magnetic fields (0.20–0.35 G for $k=1$ , potentially higher for larger $k$ ) are energetically plausible, requiring only a small fraction ( $\sim 0.7\%$ – $1.3\%$ ) of the total jet kinetic power.

Significance and Claims
The paper claims to reveal a "hidden PeVatron" within the baryonic ejecta of the microquasar SS 433, located well upstream of the extended TeV-emitting lobes. The primary significance lies in identifying the compact radio-resolved ejecta (tens to hundreds of AU) as the specific region where PeV protons are accelerated and injected.

The authors propose a multi-zone acceleration picture:

Acceleration Zone: The inner jet (compact ejecta) acts as the accelerator, utilizing the increasing magnetic rigidity to boost protons to PeV energies.
Injection and Radiation: These accelerated protons escape or are advected into the surrounding W50 nebula, where they interact with neutral hydrogen gas (H I) via $pp$ collisions to produce the observed extended $>100$ TeV gamma-ray emission.
Downstream Zones: The parsec-scale jet-cloud interaction regions (lobes) remain the site for lower-energy TeV electron acceleration and reprocessing, consistent with H.E.S.S. observations.

This work provides the first direct evidence linking the magnetic rigidity profile of a microquasar jet to the acceleration of PeV protons, resolving the spatial discrepancy between the acceleration site and the observed high-energy gamma-ray morphology. It demonstrates that microquasar jets can serve as efficient Galactic PeVatrons, with the acceleration occurring deep within the relativistic outflow rather than solely at terminal shocks.

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