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Broadband emission of microquasar remnants

This paper models the broadband non-thermal and thermal emission of microquasar remnants, demonstrating that while super-Eddington systems can produce detectable extended radio and soft X-ray structures, their direct identification is primarily limited by low surface brightness rather than total luminosity, suggesting they constitute a hidden population of Galactic non-thermal sources.

Original authors: Leandro Abaroa, Gustavo E. Romero, Valentí Bosch-Ramon

Published 2026-08-19

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CC BY 4.0

Original authors: Leandro Abaroa, Gustavo E. Romero, Valentí Bosch-Ramon

Original paper licensed under CC BY 4.0 (http://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

The universe is filled with invisible storms. For decades, astronomers have searched for the sources of cosmic rays, which are subatomic particles traveling through space at nearly the speed of light. While some of these particles are known to come from exploding stars, the most energetic ones, capable of reaching energies a million times higher than anything we can create on Earth, have remained a mystery. These extreme particles are thought to be produced by "PeVatrons," cosmic accelerators that can boost matter to petaelectronvolt energies. Recently, powerful telescopes have spotted the footprints of these accelerators in the form of ultra-high-energy gamma rays, but the engines themselves are often hidden or have long since shut down. One promising candidate for these silent engines is the microquasar, a system where a black hole or neutron star devours matter from a companion star and shoots out twin jets of material at incredible speeds. When such a system eventually runs out of fuel and goes quiet, it leaves behind a vast, expanding bubble of trapped particles and magnetic fields, known as a microquasar remnant.

A team of researchers has now taken a closer look at these ghostly remnants to see if they can be found directly. In a new study, they modeled the behavior of these bubbles after the central engine has died, focusing on how the trapped particles glow across the entire spectrum of light, from radio waves to X-rays and gamma rays. They simulated two types of systems: those that were extremely powerful while active, and those that were more modest. The researchers found that while these remnants continue to hold onto vast amounts of energy for tens of thousands of years after their jets stop, they are incredibly difficult to spot. The problem is not that they lack energy, but that they spread it out over such a huge area that they appear very dim. It is like trying to see a single candle flame that has been stretched out to cover the size of a football field; the total light is there, but the brightness at any single point is too faint for our eyes to catch.

The scientists built a detailed computer model to track the journey of particles inside these bubbles. They assumed that when the jets were active, they injected high-speed electrons and protons into the bubble. Once the jets stopped, these particles were left trapped inside, swirling in a turbulent environment filled with magnetic fields. The researchers included a process where the turbulence inside the bubble could give these particles a second wind, boosting their energy slightly before they eventually cooled down or escaped. They calculated how these particles would interact with magnetic fields to produce radio waves, and how they would collide with light particles to produce high-energy gamma rays. They also looked at the shell of hot gas surrounding the bubble, which glows in soft X-rays as it crashes into the surrounding interstellar medium. By running these simulations for both the powerful and the modest systems, they could predict what these objects would look like to modern telescopes.

The results paint a picture of a hidden population of cosmic structures. For the most powerful systems, the model suggests that the remnants could be visible as large, faint patches of radio light and soft X-rays, provided the telescopes are sensitive enough to detect such diffuse glow. These objects would appear as elongated, smooth shapes, distinct from the sharp, bright shells of supernova explosions. However, for the more common, less powerful systems, the glow is so faint that it likely escapes detection entirely with current instruments. The study highlights that the key to finding these objects is not just looking for total brightness, but for surface brightness, which is how bright an object appears per unit of area. Because these remnants are so large, their light is diluted, making them easy to miss in surveys that are designed to find compact, bright sources.

The researchers also explored how the environment around the remnant affects what we see. If the remnant is expanding into a dense cloud of gas, the shell becomes brighter and easier to spot in X-rays. If it is in a sparse region, the shell fades, and the faint radio glow from the bubble becomes the only signature. They found that the magnetic fields inside the bubble, while strong enough to keep the particles trapped, are not strong enough to make the radio glow intense. Instead, the radio emission is smooth and extended, spreading out over degrees of the sky. This means that even if a powerful remnant is nearby, it might be mistaken for background noise or missed entirely because it does not look like a typical star or a compact nebula. The study suggests that these objects are likely scattered throughout the Milky Way, waiting to be identified by telescopes that can see large, faint structures.

One of the most significant findings is that these remnants can act as long-term storage tanks for high-energy particles. Even after the central engine has gone silent, the particles trapped inside the bubble can continue to interact with nearby gas clouds, producing gamma rays that appear to come from a location different from the remnant itself. This could explain why some of the gamma-ray sources detected by observatories like LHAASO do not have an obvious, active engine nearby. The gamma rays are not coming from the engine, but from the fossil fuel left behind by the engine, illuminating the surrounding neighborhood long after the fire has gone out. This mechanism allows these dead systems to remain relevant to the high-energy universe for hundreds of thousands of years.

The study also compared these remnants to other known cosmic objects to see how they might be distinguished. Unlike supernova remnants, which are roughly spherical and powered by a single, violent explosion, microquasar remnants are expected to be more elongated, reflecting the twin-jet nature of their origin. Unlike pulsar wind nebulae, which are powered by a spinning neutron star that is still active, these remnants have no central engine to light them up. The combination of an extended, faint radio and X-ray structure with no central star, and perhaps gamma rays coming from a nearby cloud, creates a unique fingerprint. The researchers suggest that finding these objects will require a multi-wavelength approach, combining radio maps that can see large, faint structures with X-ray and gamma-ray data to piece together the full story.

In the end, the paper concludes that while these microquasar remnants are likely a common feature of our galaxy, they are currently hiding in plain sight. They are too dim and too spread out to be easily recognized by standard surveys. The powerful ones might be within reach of current radio telescopes if the observations are tuned to capture large-scale, faint emission, but the weaker ones will remain invisible until we develop more sensitive ways to look at the diffuse sky. The work provides a roadmap for how to find them, emphasizing that the key is to look for the shape and the faintness of the glow rather than just the total amount of light. By understanding how these bubbles evolve and fade, astronomers can better interpret the gamma-ray sky and perhaps finally identify the silent engines that have shaped the high-energy environment of our galaxy.

Technical Summary: Broadband emission of microquasar remnants

Problem Statement
The origin of cosmic rays (CRs) near the "knee" of the spectrum and the identification of Galactic PeVatrons remain central challenges in high-energy astrophysics. While supernova remnants are traditional candidates, recent observations of ultra-high-energy (UHE) gamma rays by LHAASO suggest they may struggle to account for the most extreme energies, motivating the search for alternative accelerators. Microquasars (MQs), particularly super-Eddington systems with powerful jets, have been proposed as capable of accelerating particles to PeV energies.

A recent proposal by Abaroa et al. (2026) suggested that the long-lived remnants of extinct MQs—microquasar remnants (MQRs)—could act as "hidden" PeVatrons. In this scenario, relativistic particles injected during the active jet phase remain confined within an overpressured cocoon long after the central engine ceases activity. These particles can interact with nearby dense clouds to produce UHE gamma rays via hadronic interactions. However, a critical gap remains: the direct detectability of the MQR itself. Because the central binary becomes radiatively quiet and the emission is expected to be extended with low surface brightness, identifying the remnant via its intrinsic emission is challenging. This paper addresses whether MQRs can be directly identified through their broadband intrinsic emission, distinct from the cloud-illumination scenarios.

Methodology
The authors extend the MQR framework of Abaroa et al. (2026) by modeling the time-dependent evolution of the cocoon and the transport of relativistic particles within it. The study considers two accretion regimes: sub-Eddington ( $L_j = 10^{37}$ erg s $^{-1}$ ) and super-Eddington ( $L_j = 10^{40}$ erg s $^{-1}$ ) systems, assuming a black hole mass of $10 M_\odot$ and an active phase duration of $10^4$ years.

Key methodological components include:

Hydrodynamical Evolution: The cocoon is modeled as an adiabatic bubble expanding into the interstellar medium (ISM) after jet shutdown. The magnetic field and density evolution are derived from pressure balance and mass conservation, assuming a fixed fraction of magnetic pressure relative to thermal pressure.
Particle Transport: The authors solve a time-dependent transport equation for relativistic electrons and protons. Unlike previous work that assumed Bohm diffusion, this study adopts a Kolmogorov diffusion regime ( $\delta = 1/3$ ) to represent the turbulent cascade within the cocoon.
Stochastic Re-acceleration: A novel inclusion for this study is the modeling of stochastic re-acceleration driven by internal turbulence, treated as a second-order Fermi process. This mechanism is applied to the particle population after jet shutdown, with a phenomenological attenuation factor to account for the decay of turbulence over time.
Radiative Processes: The model computes non-thermal emission (synchrotron, inverse Compton, synchrotron self-Compton, and hadronic $pp$ interactions) from both the cocoon and the shocked shell. Thermal soft X-ray emission from the shocked shell is also calculated via thermal Bremsstrahlung.
Synthetic Observations: To assess detectability, the authors generate synthetic radio maps (at 1.3 GHz, smoothed to a 5' beam to simulate MeerKAT-like observations) and soft X-ray maps (0.5–2 keV, simulating eROSITA-like exposure). Surface brightness is prioritized over integrated luminosity as the primary metric for detectability.

Key Results

Electron Distributions and Re-acceleration: The inclusion of stochastic re-acceleration modifies the temporal evolution of the electron distribution, redistributing particles in energy space. However, radiative losses, adiabatic expansion, and escape continue to reduce the overall normalization of the distribution as the remnant ages. A transient "ankle-like" feature appears in early super-Eddington curves, marking the transition between the lower-energy population and the cooling high-energy tail.
Spectral Energy Distributions (SEDs):
- Super-Eddington: The peak luminosity is dominated by synchrotron radiation from the cocoon, reaching $\nu L_\nu \sim 10^{35}–10^{36}$ erg s $^{-1}$ . Hadronic $pp$ interactions become relevant at very high energies (VHE).
- Sub-Eddington: These remnants are significantly fainter, with peak luminosities around $\nu L_\nu \gtrsim 10^{32}$ erg s $^{-1}$ , dominated by thermal emission from the shell at soft X-ray energies.
- Evolution: Non-thermal emission decreases rapidly as the system evolves, while thermal emission shifts to lower energies and slightly increases in luminosity before fading.
Surface Brightness and Detectability: The study concludes that the direct detectability of MQRs is controlled primarily by surface brightness rather than integrated luminosity.
- Radio: At 1.3 GHz, the modeled surface brightness for young, powerful remnants is $\Sigma_\nu \sim 10^{-19}$ W m $^{-2}$ Hz $^{-1}$ sr $^{-1}$ . This brightness decreases rapidly as the remnant expands. While powerful remnants may be detectable as extended synchrotron radio cocoons if large angular scales are recovered, sub-Eddington remnants are orders of magnitude fainter and likely undetectable in current surveys.
- X-rays: The soft X-ray emission is dominated by the shocked shell. In super-Eddington systems, the shell may be detectable under favorable conditions, but the emission fades as the shell decelerates and cools. Sub-Eddington systems produce very low count rates, making them difficult to detect with shallow observations.
Parameter Space: Variations in ISM density and diffusion regimes (Kolmogorov vs. Kraichnan) confirm that jet power is the dominant parameter controlling SED normalization. Sub-Eddington remnants in the parameter scan reach peak values of $\nu L_\nu \sim 10^{32}–10^{34}$ erg s $^{-1}$ , whereas super-Eddington remnants can reach $\nu L_\nu \sim 10^{36}–10^{37}$ erg s $^{-1}$ . Denser environments enhance shell emission, while diffusion regimes primarily affect high-energy tails and escape efficiency.

Significance and Claims
The paper claims that MQRs constitute a potentially hidden population of extended Galactic non-thermal sources. The primary significance of the work lies in demonstrating that while MQRs can remain energetically relevant for $10^5$ years, their identification is severely limited by low surface brightness.

The authors argue that:

Morphology and Context: MQRs can be distinguished from other extended sources (like supernova remnants or pulsar wind nebulae) by their elongated or bipolar morphology, the absence of a central active engine, and potentially offset gamma-ray emission from illuminated clouds.
Multiwavelength Strategy: Identification requires a multiwavelength approach combining radio (to trace the cocoon), soft X-rays (to trace the shocked shell), and gamma rays (to trace high-energy particle interactions).
Population Estimate: Based on a quasi-steady-state assumption, the Milky Way could host tens to hundreds of MQRs, though only a fraction may be detectable given observational thresholds.

The study emphasizes that current radio and X-ray surveys may miss these sources due to their diffuse nature and low surface brightness, suggesting that future identification efforts must focus on morphology, surface brightness limits, and environmental context rather than just integrated flux. The results support the hypothesis that MQRs are viable hidden PeVatrons, but their direct detection remains a significant observational challenge.

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