🔭 astrophysics

Microquasar remnants as reservoirs of PeV cosmic rays

This paper proposes that microquasar remnants, which act as long-lived reservoirs of PeV cosmic rays injected during their active jet phase, could explain unidentified ultra-high-energy gamma-ray sources detected by LHAASO through delayed interactions with dense gas clouds.

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

Published 2026-03-16

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CC BY 4.0

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

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

Imagine the Milky Way as a bustling city. For years, astronomers have been hunting for the "power plants" that generate the most energetic particles in the universe—particles so powerful they can punch through almost anything. These are called PeVatrons (PeV stands for Peta-electronvolt, a unit of energy so high it's hard to comprehend).

Recently, a giant observatory called LHAASO spotted several of these power plants in our galaxy. They are glowing with ultra-high-energy gamma rays. But here's the mystery: when astronomers looked closely at these glowing spots, they couldn't find the "engine" driving them. No active black holes, no spinning neutron stars, no obvious fireworks. It was like seeing a bright light in a dark room but finding no lightbulb.

This paper proposes a clever solution: The lightbulb might be broken, but the room is still glowing because of the heat left over.

Here is the story of "Ghost Power Plants," explained simply.

1. The Active Phase: The Jet Engine

Imagine a Microquasar. This is a system where a stellar-mass black hole is eating a companion star. As it devours the star, it spits out two massive, super-fast jets of particles (like a cosmic firehose) shooting out in opposite directions.

The Analogy: Think of this like a high-powered garden hose blasting water into a dry, dusty field.
What happens: The water (the jet) hits the air and creates a massive, expanding bubble of turbulence and heat. Inside this bubble, the particles get smashed together and accelerated to incredible speeds. This is the "active phase." The black hole is the engine, and the bubble is the "cocoon" filled with high-energy cosmic rays.

2. The Engine Dies: The "Fossil" Phase

Eventually, the black hole runs out of food. The companion star stops feeding it, or the disk of gas around the black hole gets drained.

The Analogy: The water in the garden hose suddenly stops. The engine is turned off. The black hole goes silent.
The Twist: Even though the hose is off, the bubble it created doesn't vanish instantly. It's like a giant, invisible soap bubble floating in the air. Inside this bubble, the high-speed particles (cosmic rays) are still trapped. They are bouncing around, trapped in a magnetic "cage" inside the bubble.

The authors call these Microquasar Remnants (MQRs). They are the "ghosts" of dead black hole systems.

3. The Long Afterlife: The Reservoir

Here is the most important part: These bubbles are huge (about 100 light-years across) and they hold onto their energy for a very long time—hundreds of thousands of years.

The Analogy: Imagine a giant, slow-leaking water balloon. Even after you stop filling it, the water inside stays there for a long time, slowly seeping out.
The Physics: The particles inside the bubble are so energetic (PeV scale) that they don't escape quickly. They bounce around in the turbulent magnetic fields of the bubble for eons. The bubble acts as a reservoir or a battery that keeps storing cosmic rays long after the engine that made them has died.

4. The Ghostly Glow: Why We See Them Now

So, if the engine is dead, why does LHAASO see light?

The bubble is slowly expanding and drifting through the galaxy. Eventually, it bumps into a dense cloud of gas (a molecular cloud) or a clump of dust.

The Analogy: Imagine that slow-leaking balloon finally drifts into a thick fog. The water leaking out of the balloon hits the fog, creating a mist.
The Result: When the trapped cosmic rays inside the bubble hit the gas in the cloud, they crash into the atoms. These crashes create a flash of gamma rays (high-energy light).

The key insight: The gamma rays we see today aren't coming from an active black hole. They are coming from a dead black hole system, illuminating a nearby cloud with the "leftover" energy from millions of years ago.

Why This Matters

This idea solves a big puzzle in astronomy:

Missing Engines: It explains why LHAASO sees bright spots but can't find the active black holes powering them. The black holes are "ghosts"—they are there, but they are dormant.
New Population: It suggests there might be hundreds of these "fossil cocoons" in our galaxy, acting as hidden reservoirs of cosmic rays.
The Clue: If you look at these spots, you won't see a bright, compact center (like a normal black hole). Instead, you might see a faint, large, diffuse bubble of radio waves, with the gamma-ray light coming from a cloud slightly offset from the center.

Summary

Think of these Microquasar Remnants as abandoned power plants. The generators (black holes) stopped working long ago, but the massive fuel tanks (the cocoons) are still full of high-energy fuel. As the tanks slowly leak, they spill fuel onto nearby gas clouds, causing them to glow.

We are seeing the afterglow of a cosmic event that happened long ago, proving that the universe is full of "ghosts" that are still lighting up the dark.

1. Problem Statement

The Large High Altitude Air Shower Observatory (LHAASO) has identified a population of Galactic $\gamma$ -ray sources emitting beyond 100 TeV, with some approaching or exceeding 1 PeV. These sources, known as "PeVatrons," are the likely accelerators of the highest-energy cosmic rays (CRs) in the Milky Way. However, the nature of the underlying accelerators for many of these sources remains uncertain, as they often lack obvious counterparts at lower energies (e.g., radio or X-ray).

While active microquasars (MQs) with super-Eddington accretion rates are strong candidates for PeVatrons, they are transient. The authors address the gap in understanding what happens to these systems after the central engine shuts down. They propose that Microquasar Remnants (MQRs)—the fossil cocoons left behind after mass transfer ceases—act as long-lived reservoirs of PeV cosmic rays, potentially explaining the unidentified LHAASO sources.

2. Methodology

The authors developed a time-dependent physical model to simulate the evolution of a microquasar system from its active phase to its fossil remnant stage.

System Parameters:
- Central Engine: A stellar-mass black hole ( $M_{BH} = 10 M_\odot$ ) undergoing super-Eddington accretion for a duration $t_0 = 5 \times 10^4$ years.
- Jets: Two transrelativistic jets with kinetic luminosity $L_j = 5 \times 10^{39}$ erg s $^{-1}$ and bulk Lorentz factor $\gamma_j = 3$ .
- Environment: A homogeneous interstellar medium (ISM) with density $\rho_{ISM} = 1.7 \times 10^{-25}$ g cm $^{-3}$ .
Evolutionary Stages:
1. Active Phase ( $t < t_0$ ): Jets inflate a hot, overpressured cocoon. A reverse shock (RS) at the jet termination point accelerates particles.
2. Fossil Phase ( $t > t_0$ ): Mass transfer stops, jets vanish, and the RS disappears. The cocoon continues to expand adiabatically as a "wind-blown bubble," retaining the injected energy and particles.
Particle Physics & Transport:
- Acceleration: Assumed to occur via first-order Fermi acceleration at the RS. A hadron-dominated scenario is adopted ( $L_p = 100 L_e$ ), with 10% of jet power converted to CRs.
- Injection: Protons follow a power-law distribution ( $Q \propto E^{-2}$ ) with a maximum energy $E_{max,p} \approx 10^{16}$ eV at $t_0$ .
- Transport: The authors solve time-dependent transport equations for protons in both the RS region and the cocoon. This includes:
  - Advection: Particles moving from the RS into the cocoon via backflow.
  - Diffusion: Spatial diffusion within the turbulent cocoon (modeled under a Kraichnan regime, $\delta=0.5$ ).
  - Losses: Radiative and adiabatic losses are accounted for.
Emission Modeling: The model calculates the interaction of relic CRs with dense gas (molecular cloud clumps) inside or outside the cocoon. The resulting $\gamma$ -ray emission is derived from inelastic $pp$ collisions and subsequent neutral pion decay.

3. Key Contributions

The "Fossil" Hypothesis: The paper introduces the concept that MQRs are not dead systems but "ghost" PeVatrons. Even after the central engine dies, the cocoon retains a massive population of PeV protons for $\sim 10^5$ years.
Delayed Illumination: The authors demonstrate that these relic CRs can diffuse out of the cocoon and interact with nearby dense molecular clouds long after the jet activity has ceased, producing delayed $\gamma$ -ray emission.
Morphological Distinction: The paper distinguishes MQRs from Supernova Remnants (SNRs). Unlike SNRs, which have shell-like morphologies and active shocks, MQRs are expected to appear as diffuse, centrally underluminous bubbles with $\gamma$ -ray emission offset from the geometric center (tracing the dense gas rather than the shock).
Population Estimate: The authors estimate that the Galactic population of MQRs could range from tens to a few hundred objects, suggesting they constitute a significant fraction of the unidentified LHAASO sources.

4. Results

CR Retention: Simulations show that while the jet termination region empties quickly after the jets switch off, the cocoon retains the bulk of the CR population. Particles with energies $\gtrsim 1$ PeV persist in the cocoon for hundreds of thousands of years due to slow diffusion times in the turbulent interior.
Cocoon Expansion: The cocoon expands to sizes of $\sim 100$ pc over timescales of $t \sim t_0$ and continues to expand slowly thereafter, remaining overpressured relative to the ISM.
Spectral Energy Distribution (SED):
- The $\gamma$ -ray spectrum from an irradiated cloud is time-dependent and space-dependent.
- Higher-energy particles arrive at distant clouds first, leading to spectral steepening with increasing distance from the reservoir.
- For a cloud located 50 pc from the MQR (with density $10^3$ cm $^{-3}$ ), the $\gamma$ -ray emission remains detectable by current and future facilities (Fermi, SWGO, LHAASO) for a source at 3 kpc, even $10^5$ years after the MQ shuts down.
Source Clustering: A single MQR can illuminate multiple clumps or external clouds, potentially creating a small cluster of $\gamma$ -ray sources with varying fluxes and spectral shapes, rather than a single point source.

5. Significance

This work provides a compelling alternative explanation for the "missing" PeVatrons detected by LHAASO. It suggests that the search for PeV sources should not be limited to currently active accelerators but must include the fossil remains of past high-energy events.

Observational Strategy: The paper proposes that the key to identifying MQRs lies in searching for faint, large-scale radio structures (the cocoons) surrounding clusters of unidentified LHAASO sources.
Morphological Signatures: The predicted spatial offset between the $\gamma$ -ray source (tracing dense gas) and the geometric center of the remnant, combined with energy-dependent spectral steepening, offers a unique signature to distinguish MQRs from young SNRs.
Cosmic Ray Origin: By validating MQRs as long-lived reservoirs, the study reinforces the role of microquasars (both active and fossil) as major contributors to the Galactic cosmic ray budget, particularly at the PeV scale.

In conclusion, the authors argue that the "ghost" cocoons of extinct super-Eddington microquasars are a previously overlooked, non-negligible population of Galactic PeVatrons capable of explaining the nature of several unidentified ultra-high-energy $\gamma$ -ray sources.