Particle Acceleration in Ultra-Luminous X-ray Source Wind Bubbles: A Potential Origin of Galactic PeVatrons

מושגי ליבה

Ultra-luminous X-ray sources (ULXs) are strong candidates for Galactic PeVatrons, capable of accelerating cosmic rays to PeV energies within wind bubbles formed by their powerful outflows.

תקציר

• Bibliographic Information: Peretti, E., Petropoulou, M., Vasilopoulos, G., & Gabici, S. (2024). Particle acceleration and multi-messenger radiation from Ultra-Luminous X-ray Sources -- A new class of Galactic PeVatrons. Astronomy & Astrophysics.

• Research Objective: This research paper investigates the potential of Ultra-Luminous X-ray Sources (ULXs) as Galactic PeVatrons, focusing on particle acceleration mechanisms within wind bubbles created by ULX outflows.

• Methodology: The authors develop a model of diffusive shock acceleration (DSA) occurring at the wind termination shock within ULX wind bubbles. They solve the energy-dependent and space-dependent transport equation to analyze the acceleration and propagation of high-energy protons. The model predicts multi-messenger emissions, including cosmic rays, gamma rays, radio waves, and neutrinos.

• Key Findings: The model demonstrates that ULX wind bubbles can efficiently accelerate protons to PeV energies, making them viable PeVatron candidates. The study focuses on the Galactic source SS 433, a hidden ULX, and finds that the model's predictions align with recent high-energy gamma-ray observations, particularly those from LHAASO. The authors suggest that the high-energy photons (>100 TeV) and their morphology observed from SS 433 could originate from high-energy protons accelerated within the wind bubble.

• Main Conclusions: The research concludes that ULX wind bubbles are promising sites for cosmic ray acceleration, potentially contributing significantly to the Galactic cosmic ray flux at PeV energies. The study proposes ULXs as a new class of PeVatron candidates, providing a plausible explanation for the origin of high-energy particles in the Milky Way.

• Significance: This research significantly contributes to our understanding of particle acceleration mechanisms in the Galaxy and sheds light on the long-standing mystery surrounding the origin of Galactic PeVatrons. The findings have implications for high-energy astrophysics and cosmic ray physics.

• Limitations and Future Research: The model primarily focuses on hadronic acceleration and radiation, with limited discussion on leptonic processes. Future research could explore the contribution of primary and secondary electrons to the multi-wavelength emission from ULX wind bubbles. Further investigation is warranted to constrain model parameters and refine predictions for multi-messenger observations, particularly in neutrinos and radio wavelengths.

סטטיסטיקה

The maximum energy of protons accelerated in ULX wind bubbles is estimated to reach up to 8 PeV.
The gamma-ray emission from SS 433 above 100 TeV could be explained by hadronic processes within the wind bubble.
The estimated neutrino flux from SS 433 in the TeV range is approximately 10^-12 - 10^-13 TeV cm^-2 s^-1.
The predicted radio flux from the synchrotron emission of electrons in the wind bubble is on the order of 10^-14 erg cm^-2 s^-1.

ציטוטים

תובנות מפתח מזוקקות מ:

Particle acceleration and multi-messenger radiation from Ultra-Luminous X-ray Sources -- A new class of Galactic PeVatrons

by Enrico Peret... ב- arxiv.org 11-14-2024

https://arxiv.org/pdf/2411.08762.pdf

שאלות מעמיקות

How do the characteristics and efficiency of particle acceleration in ULX wind bubbles compare to those in other proposed PeVatron candidates, such as supernova remnants or pulsar wind nebulae?

ULX wind bubbles, supernova remnants (SNRs), and pulsar wind nebulae (PWNe) are all proposed Galactic PeVatron candidates, but they exhibit distinct characteristics influencing their acceleration efficiency:
ULX Wind Bubbles:

Mechanism: Diffusive Shock Acceleration (DSA) at the wind termination shock.
Advantages:

High Wind Velocity: ULX winds can reach mildly relativistic speeds (∼ 0.1c-0.3c), leading to faster acceleration rates compared to SNR shocks.
Long Lifetime: ULX bubbles can persist for Myrs, allowing ample time for particle acceleration.

Challenges:

Geometry and Confinement: The efficiency depends on the bubble's magnetic field configuration and its ability to confine particles.
Target Density: A higher density in the shocked wind region is necessary for efficient $\pi^0$ production and subsequent gamma-ray emission.
Supernova Remnants (SNRs):

Mechanism: DSA at the expanding shock wave.
Advantages:

High Shock Velocities: Young SNRs can drive shocks at thousands of km/s, enabling PeV energies.
Efficient Injection: SNRs are known to accelerate a significant fraction of particles at the shock.

Challenges:

Limited Lifetime: The acceleration efficiency decreases as the SNR expands and slows down.
Magnetic Field Amplification: Reaching PeV energies requires strong magnetic field amplification beyond typical ISM values.
Pulsar Wind Nebulae (PWNe):

Mechanism: Magnetic reconnection and/or Fermi-type acceleration in the pulsar's relativistic wind.
Advantages:

Extreme Energies: Pulsars, with their rapidly rotating magnetic fields, can potentially accelerate particles to ultra-high energies, exceeding PeV.

Challenges:

Acceleration Efficiency: The exact mechanisms and efficiency of particle acceleration in PWNe are still under debate.
Geometry and Escape: The geometry of the pulsar wind and the nebula's magnetic field structure significantly impact particle confinement and escape.
Comparison:
ULXs, with their fast winds and long lifetimes, appear promising for sustained PeV particle acceleration. While SNRs are widely considered likely PeVatrons, their efficiency decreases over time. PWNe hold the potential for exceeding PeV energies, but the acceleration processes require further clarification.

Could the observed gamma-ray emission from SS 433 be fully explained by leptonic processes, such as inverse Compton scattering of low-energy photons by relativistic electrons, challenging the need for a hadronic component?

While leptonic processes like inverse Compton (IC) scattering can contribute to the gamma-ray emission from SS 433, fully explaining the observed spectrum, particularly at the highest energies (> 100 TeV), poses challenges:
Leptonic Scenario Challenges:

High-Energy Cutoff: IC scattering typically produces a softer spectrum compared to the observed hard spectrum extending beyond 100 TeV. Achieving such high energies through IC would require exceptionally high-energy electrons and/or a very intense photon field.
Morphology: The morphology of the gamma-ray emission, becoming more spherical at higher energies, aligns better with a spatially extended source like a wind bubble rather than the collimated jets expected in a purely leptonic scenario.
Multi-wavelength Constraints: Fitting the entire spectral energy distribution, from radio to gamma-rays, solely with leptonic models, while simultaneously satisfying observational constraints in other wavelengths, can be difficult.
Hadronic Component Support:

Hard Spectrum: The hard gamma-ray spectrum observed by LHAASO favors a hadronic origin, as proton-proton interactions naturally produce a harder spectrum extending to higher energies compared to typical leptonic processes.
Spatial Extent: The extended, bubble-like morphology of the highest energy gamma-rays supports a hadronic scenario where protons, with their longer diffusion lengths, can produce a more isotropic emission pattern.
Conclusion:
While a leptonic component likely contributes to the overall emission from SS 433, a purely leptonic origin faces challenges in explaining the highest energy gamma-rays and their morphology. The observations favor a significant hadronic contribution, supporting the classification of SS 433 as a potential PeVatron.

If ULXs are indeed powerful cosmic accelerators, what are the implications for the Galactic ecosystem, particularly regarding the energy balance and the impact of cosmic rays on the interstellar medium?

If ULXs are confirmed as powerful cosmic accelerators, their contribution to the Galactic ecosystem could be significant:
Energy Budget of the ISM:

Cosmic-Ray Injection: ULXs, through their long-lasting wind bubbles, could inject a substantial amount of energy into the ISM in the form of cosmic rays. This injection could contribute to the overall energy balance of the ISM, potentially influencing processes like star formation and galaxy evolution.
Heating and Ionization: Cosmic rays from ULXs can ionize and heat the ISM through collisions with interstellar gas. This heating and ionization could impact the chemical and thermal evolution of the ISM, influencing the formation of molecules and the structure of interstellar clouds.
Impact on Star Formation:

Turbulence Generation: Cosmic rays can generate turbulence in the ISM, potentially influencing the formation of molecular clouds, the birthplaces of stars.
Triggering Star Formation: In some cases, cosmic-ray pressure from nearby ULXs could compress interstellar gas, potentially triggering or quenching star formation in their vicinity.
Gamma-Ray Background:

Diffuse Gamma-Ray Emission: The cumulative emission from a population of ULXs accelerating protons could contribute to the diffuse gamma-ray background observed in our Galaxy.
Neutrino Production:

Galactic Neutrino Sources: ULXs, as potential PeVatrons, would also be sources of high-energy neutrinos. Detecting these neutrinos would provide crucial evidence for hadronic acceleration in ULXs and offer insights into the origin of Galactic cosmic rays.
Further Research:
Quantifying the precise impact of ULXs on the Galactic ecosystem requires further research, including:

Population Studies: Determining the number and distribution of ULXs in the Milky Way.
Modeling Cosmic-Ray Propagation: Simulating the propagation of cosmic rays from ULXs through the ISM to understand their impact on different Galactic environments.
Multi-messenger Observations: Combining gamma-ray, neutrino, and cosmic-ray observations to constrain the properties and efficiency of particle acceleration in ULXs.