Jets in high-mass microquasars

The morphologies of detected jets in X-ray binaries are almost as diverse as their number. This is due to different jet properties and ambient media that these jets encounter. It is important to understand the physics of these objects and to obtain information about possible sites suitable for particle acceleration in order to explain the observations at very high energies. Here I present the results obtained from the first relativistic hydrodynamical simulations of jets in high-mass microquasars. Our results allow us to make estimates for the emission originated in different sites of the whole structure generated by the jets. These works represent a first step in trying to obtain a deeper understanding of the physics and emission processes related with jets in high-mass microquasars.

💡 Research Summary

The paper presents the first relativistic hydrodynamical (RHD) simulations of jets in high‑mass microquasars (HMMQs) and uses the results to assess where particle acceleration and non‑thermal emission can arise along the jet’s propagation. The authors employ the high‑resolution shock‑capturing finite‑difference code “Ratpenat” to solve the conservation form of the relativistic fluid equations in three dimensions. Two jet powers are explored, L ≈ 10³⁶ erg s⁻¹ (weak) and L ≈ 10³⁷ erg s⁻¹ (strong), injected supersonically into a realistic environment that changes with distance from the compact object.

Binary region (z < 3 × 10¹² cm).
Inside the binary, the jet encounters the dense, fast wind of an OB companion (mass‑loss rate ∼10⁻⁶ M⊙ yr⁻¹, velocity ∼2 × 10⁸ cm s⁻¹). The wind creates an asymmetric pressure distribution on the bow‑shock that surrounds the jet head. For the strong jet, the over‑pressure of the flow keeps the bow‑shock roughly symmetric; the jet remains collimated and only later suffers a strong reconfinement shock that triggers Kelvin‑Helmholtz (KH) modes along the whole beam. For the weak jet, the reconfinement shock occurs closer to the injection point, the flow becomes under‑pressured, and KH instabilities grow rapidly, leading to substantial mass entrainment, deceleration, and eventual disruption within the binary. Simulations that include a clumpy wind and radiative cooling show that dense clumps can strongly deflect the jet, increase entrainment, and accelerate the deceleration process.

Beyond the binary (z ≈ 1 pc).
At larger scales the ambient medium depends on the age of the HMXB. In a young system (t ≈ 3 × 10⁴ yr) the jet first meets the shocked stellar wind and the shell driven by the supernova remnant (SNR). A weak jet (L = 3 × 10³⁶ erg s⁻¹) is heavily slowed by the shocked interstellar medium (ISM) and would need several thousand years to cross this region. In a middle‑aged system (t ≈ 10⁵ yr) the jet penetrates a thin shocked‑ISM layer and, after a few thousand years, drills through it to reach the unshocked ISM. In an old system (t ≈ 10⁶ yr) the proper motion of the binary through the ISM creates a forward bow‑shock that acts as a wall; the jet is deflected and cannot break out of the shocked wind/ISM region.

Propagation in a uniform ISM.
Once the jet escapes all previous interaction zones, it propagates in a roughly homogeneous ISM. The classic over‑pressured jet–cocoon system forms: a forward bow‑shock, a reverse shock at the jet head, and a series of internal reconfinement shocks caused by pressure mismatches between the jet and its cocoon. These structures are natural sites for efficient particle acceleration.

Particle acceleration and radiation.
The authors identify several key acceleration sites: (a) the bow‑shock, reverse shock, and reconfinement shocks inside the binary; (b) the strong interaction region where the jet meets the shocked SNR/wind; and (c) the forward and reverse shocks in the ISM. Using typical acceleration efficiencies and magnetic field estimates, they calculate synchrotron, inverse‑Compton (IC) and proton‑proton (pp) emission. They argue that synchrotron and IC from electrons accelerated at the binary‑scale reconfinement shock (∼10¹² cm) can explain the observed high‑energy X‑ray and γ‑ray emission of several HMXBs. At larger scales, the shocked wind/SNR region can produce radio to X‑ray synchrotron and IC emission, while the ISM stage can generate a broadband spectrum from radio up to TeV γ‑rays, provided the jet power and ambient density are sufficiently high.

Conclusions.
The study demonstrates that jet power, ambient density, and system age jointly determine the morphology, stability, and radiative output of HMMQ jets. Strong jets (L ≥ 10³⁷ erg s⁻¹) survive the asymmetric wind, remain collimated, and can generate detectable non‑thermal emission across the spectrum. Weak jets are prone to disruption by wind‑induced asymmetries and clump entrainment, leading to reduced radiative efficiency. The work provides a unified, simulation‑based framework that links jet dynamics to multi‑wavelength observations, offering predictive tools for interpreting current data and guiding future high‑energy missions targeting microquasar jets.