Unveiling Multimessenger Emission from Hidden Cores of Microquasars

arXiv:2512.23231 · astro-ph.HE, hep-ph · Submitted 2025-12-29 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Unveiling Multimessenger Emission from Hidden Cores of Microquasars".

Jocelyn: Microquasars are being investigated as promising candidates for cosmic-ray acceleration due to their radio-emitting X-ray binary nature and relativistic jets,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So Jocelyn and Subrahmanyan, this paper "Unveiling Multimessenger Emission from Hidden Cores of Microquasars" really zeroes in on how these radio-emitting X-ray binaries with relativistic jets can actually accelerate cosmic rays up to one hundred TeV, which is pretty exciting data we're seeing out there.

Jocelyn: Exactly, Vera; it moves beyond just seeing the high-energy gamma rays and tries to figure out the physical mechanism behind that emission across all wavelengths from radio up to ultra-high-energy gamma rays. I'm curious what the authors are claiming about where this particle acceleration is actually happening within these microquasars.

Subrahmanyan: The core thesis seems to be that these microquasars are strong candidates for cosmic-ray acceleration because of their radio-emitting X-ray binary nature and those relativistic jets. It posits that the observed high-energy gamma rays, above one hundred TeV, could stem from either proton-gamma or proton-proton interactions depending on the location and physical conditions in the emission region.

Vera: That distinction between pγ and pp interactions based on location really makes sense when you look at the data we gather from instruments like LHAASO, which reported gamma-ray emission up to one hundred TeV from sources like Cygnus X-one. It suggests the environment dictates the physics happening inside that jet blob.

Jocelyn: And it's not just about where it happens, but also how they model the energy spectrum, trying to reproduce both the lower-energy spectra and those higher-energy photons simultaneously. That modeling effort must be quite complex when you're tracking emission from radio up to ultra-high-energy gamma rays.

Subrahmanyan: The authors employ a framework that considers compact outflow regions as the cosmic-ray accelerators, spanning from radio waves to ultra-high-energy gamma rays. They develop three specific physical scenarios—a jet-core model, a stellar-wind interaction model, and an extended jet model—to comprehensively model both the zero point one–one PeV and lower-energy band photons.

Vera: Those three scenarios sound like they are designed to test different physical conditions, which is exactly what we need when interpreting these diverse multiwavelength observations of microquasars. I'm interested in how they set up the geometry for these models, since Figure one shows a schematic picture of the geometry assuming neutrinos and photons are produced within the jet blob.

Paper summary: Jocelyn: The setup involving inclination angle i to specify the observer's direction and azimuthal angle phi to define the donor star's position in the orbital plane gives a clear picture of how they are treating the system geometry. It helps ground these theoretical models in observable orbital dynamics.

Subrahmanyan: The numerical methods involve treating the inner blob and external region similarly but distinguishing that the external region is static and its injected particles originate from those escaped the jet blob. They use power laws for species 'a' in the blob with a minimum Lorentz factor gamma'a,min and an exponential cutoff at gamma'a,max.

Vera: It sounds like they balance the acceleration timescale t'acc and the cooling timescale t'c within the dynamical timescale t'dyn = r'/v j in the blob's co-moving frame to estimate maximum energy limits. That balancing act is crucial for pinning down what kind of particle energies we might expect to see from these sources.

Jocelyn: I'm also interested in the specific cooling mechanisms they account for, like synchrotron and inverse Compton cooling for electrons, which helps them estimate a maximum energy of epsilon'e,max twenty TeV eta-one/two B'-one/two. That level of detail in particle physics modeling is what allows them to bridge the gap between the observed radiation and the underlying particle acceleration processes.

Subrahmanyan: For protons, radiative cooling is generally less significant, so they estimate the maximum energy by equating acceleration and dynamical timescales t'dyn. This difference in how they treat electron versus proton limits reflects their understanding of the dominant energy loss mechanisms in these environments.

Vera: Moving on to emission mechanisms, they model both leptonic processes like synchrotron radiation and inverse Compton scattering for electrons, alongside hadronic processes such as proton synchrotron radiation and inelastic hadronuclear pp collisions. That comprehensive approach covers the whole spectrum from radio to gamma rays.

Jocelyn: And the modeling of transport equations for particle species 'a' in the comoving frame that includes all these interactions is what links the input physics to the final multimessenger output. It’s a dense set of coupled equations trying to capture everything happening in that compact outflow.

Subrahmanyan: They make a specific claim about high-energy photons above zero point one PeV in both the jet blob and inner blob being dominated by the pγ process due to a sufficiently dense target photon field, often involving a-resonance when target photons come from an accretion disk. This points toward a specific physical condition favoring that interaction in the jet structure.

Paper summary: Vera: That-resonance idea is interesting because it suggests the target photon field density is high enough to drive that specific interaction pathway for those very high energies. Does this mean we should be looking for signatures of accretion disk photons in the gamma-ray spectrum of these sources?

Jocelyn: Yes, because if the pγ process is dominant there, it means the environment provides that dense photon field necessary for that specific reaction to occur. It connects the geometry and particle density directly to the observable spectrum of those ultra-high-energy photons.

Subrahmanyan: In contrast, for external regions described in Scenarios B and C, photons above zero point one PeV are primarily produced via the pp process because of that dense target environment. This highlights how the local environment—whether it's a jet blob or a stellar wind interaction—determines whether we see pγ or pp dominance.

Vera: I also found the section on orbital modulation quite telling, specifically how energy-dependent orbital modulation arises from variations in the Doppler factor and external gamma gamma annihilation at different orbital phases. That suggests that observing these sources across their orbits could reveal different aspects of the acceleration physics.

Jocelyn: The authors investigate two ways this modulation can happen: either the jet orientation remains fixed relative to the observer, causing modulation from distance changes, or variations in the jet's azimuthal angle phi j leading to changes in Doppler boosting. Both avenues offer different observational tests for their model predictions.

Subrahmanyan: The study finds that "strong gamma gamma annihilation can produce energy-dependent modulation patterns," and they observe this effect being more pronounced in the GeV band than in the PeV band because of higher photon-photon opacity at lower energies. This is a very specific prediction regarding how the observed modulation signature should change with energy.

Vera: So, if we see that strong energy dependence in modulation, it gives us a way to potentially disentangle the competing emission processes happening within the system. It provides a potential observational handle for testing their three different physical scenarios.

Jocelyn: And if we look at Cygnus X-one Scenario A predicts a deep dip in the zero point one–ten TeV range due to combined pγ interactions and gamma gamma annihilation. Meanwhile, for Cygnus X-three Scenario B shows a plateau-plus-peak structure above ten TeV arising from pp interactions in the external blob and pγ interactions within the jet blob. Those specific predictions are what tie the theory back to actual astronomical objects we observe.

Paper summary: Subrahmanyan: The neutrino predictions reveal a "significantly suppressed flux" when muon and pion cooling effects are properly accounted for, which suggests that detecting these sources might be more challenging than previously anticipated. This is an important caution for any future observational efforts targeting microquasars.

Vera: It sounds like the overall finding of this paper, "Unveiling Multimessenger Emission from Hidden Cores of Microquasars," is that we need to consider a variety of physical scenarios to explain the full spectrum we see. The authors successfully reproduce multiwavelength data for both microquasars across all these scenarios, except for the radio band, which they note is suppressed by free-free absorption at larger heights around one thousand fourteen cm.

Jocelyn: So, what does this mean for us as a field? This work helps constrain the acceleration mechanism by finding values for eta acc that set the maximum proton Lorentz factor gamma'p,max. That sets a physical limit on how energetic these cosmic rays can get from these systems.

Subrahmanyan: The broader implication is that this research helps constrain the acceleration mechanism by finding values for eta acc that set the maximum proton Lorentz factor gamma'p,max. It links the observed high-energy emission directly to the efficiency of particle acceleration within these compact outflow regions.

Vera: Thinking about this, it means that if we can find sources that show modulation patterns matching their predictions, we might finally be able to use those signals as direct probes into the internal physics of microquasars. It’s about using the orbital mechanics to peer inside the jet structure.

Jocelyn: And it also gives us a clearer roadmap for future observations, pointing toward specific spectral features we should look for in the gamma-ray band to distinguish between those pγ and pp processes. We know where to look next based on these theoretical constraints.

Subrahmanyan: Ultimately, the work on "Unveiling Multimessenger Emission from Hidden Cores of Microquasars" solidifies the idea that microquasars are indeed powerful accelerators, but it also clearly defines the physical conditions—the density and geometry—that dictate whether we observe pγ or pp interactions. This provides a much more structured way to connect the observed data to the fundamental physics of particle acceleration in these extreme environments.

Conclusion: Vera: So, we've just been diving deep into how these microquasars might be accelerating cosmic rays up to one hundred TeV, and now we need to talk about what this paper is actually called and who wrote it.

Jocelyn: It’s titled "Unveiling Multimessenger Emission from Hidden Cores of Microquasars," and I think the authors are really good at showing us how the data connects across different messengers.

Subrahmanyan: I agree, Jocelyn; that title suggests a big effort to bridge the gap between what we see in radio waves and what we detect in ultra-high-energy gamma rays.

Vera: Exactly, and from an observational standpoint, it’s compelling because it tries to explain the entire spectrum of emission using a single framework.

Jocelyn: I'm interested in how they managed to keep track of all those different energy bands, from radio up to the TeV range.

Subrahmanyan: The core idea is that these microquasars are not just bright X-ray sources but active cosmic-ray accelerators, which is a big deal for our understanding of the universe.

Vera: It really puts things into perspective; these systems could be some of the most powerful particle accelerators we have access to in our galaxy.

Jocelyn: And it shows us that the physics happening inside these jets dictates whether we see different types of high-energy photons, like pγ or pp interactions.

Subrahmanyan: That distinction is vital because it tells us about the physical density and structure of the environment where the particles are actually being accelerated and interacting.

Vera: It opens up new avenues for observational astronomy because now we have specific targets to look for based on these models.

Jocelyn: And I think this work sets a really strong foundation for future surveys looking at these types of compact objects across the sky.

Subrahmanyan: We can't wait to see what the next round of observations reveals when we test these theoretical predictions against new data.

Department of Astronomy and Astrophysics, The Pennsylvania State University · Institute for Gravitation and the Cosmos, The Pennsylvania State University · Department of Physics, The Pennsylvania State University · Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University · Key Laboratory of Particle Astrophysics and Experimental Physics Division and Computing Center, Institute of High Energy Physics, Chinese Academy of Sciences · TIANFU Cosmic Ray Research Center

astro-ph.HE, hep-ph

Submitted: 2025-12-29

Updated: 2026-09-30

Comments: 30 pages, 11 figures, 6 tables; accepted for publication in ApJ

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 64/100

The gist: Microquasars are being investigated as promising candidates for cosmic-ray acceleration due to their radio-emitting X-ray binary nature and relativistic jets, and this study models their

Key concepts

Jet-Core Model: This scenario describes a microquasar with two jet components: a main jet blob and an inner blob. The inner blob's origin is uncertain, potentially being either the corona or internal shocks within the outflow. This model helps explain different emission characteristics across various energy bands.
Stellar-Wind Interaction Model: This scenario assumes a static external stellar wind surrounding the jet. Charged particles escaping from the jet interact with this wind material, favoring proton-proton (pp) interactions as a primary emission mechanism in that specific region.
pγ vs pp Interactions: The high-energy gamma rays observed can result from two main processes: pγ interactions, where protons collide with target photons, or pp interactions, where protons collide with other protons. The dominant process depends on the physical location and the density of target photons in the emission region.
Orbital Modulation: Variations in the observed energy depend on orbital phases due to changes in Doppler boosting (from jet orientation) or external $\gamma\gamma$ annihilation. Strong $\gamma\gamma$ annihilation can cause energy-dependent modulation, being more noticeable at lower energies.

Terminology

Core — 13× in this paper · explained in 2 episodes
Astrophysical Multimessenger Emission Simulator — 4× in this paper
Large High Altitude Air Shower Observatory — 1× in this paper
Metallicity — 1× in this paper · explained in 3 episodes

Summary

Microquasars are being investigated as promising candidates for cosmic-ray acceleration due to their radio-emitting X-ray binary nature and relativistic jets, and this study models their multimessenger emission across radio to ultra-high-energy gamma rays.

The gist: The observed >TeV gamma rays can originate from either pγ or pp interactions, depending on the location and physical conditions of the emission region, while also reproducing the lower-energy spectra.

Theoretical Framework and Scenarios

The research employs the Astrophysical Multimessenger Emission Simulator (AMES) to model multimessenger emission considering compact outflow regions as cosmic-ray accelerators, spanning from radio to ultra-highenergy gamma rays. To comprehensively model both the 0.1-1 PeV and lower-energy band photons, three distinct physical scenarios are developed:

Jet-Core Model (Scenario A): Includes a jet blob and an inner blob, where the inner blob may originate from either the corona or internal shocks in the compact outflow.
Stellar-Wind Interaction Model (Scenario B): Assumes a static external stellar wind region where charged particles escaping from the jet blob interact with the wind material, favoring pp interactions.
Extended-Jet Model (Scenario C): Considers contributions from large-scale jet structures interacting with a parsec-scale environment, which is expected to produce more persistent, non-modulated radiation with reduced variability.

Numerical Methods and Particle Acceleration

The numerical framework involves treating the inner blob and the external region similarly but distinguishing that the external region is static and its injected particles originate from those escaped the jet blob. The injected particle distribution for species 'a' in the blob follows a power law with a minimum Lorentz factor γ′a,min and an exponential cutoff at γ′a,max. The maximum Lorentz factor can be estimated by balancing the acceleration timescale t′acc and the cooling timescale t′c within the dynamical timescale t′dyn = r′/vj in the blob co-moving frame. For electrons, synchrotron and IC cooling are important, with a maximum energy estimated by equating these timescales to yield ε′e,max ≃ 20 TeV η−1/2 acc B′−1/2. For protons, radiative cooling is generally less significant, and the maximum energy can be estimated by equating the acceleration timescale to the dynamical timescale t′dyn.

Multimessenger Emission Mechanisms

The study models both leptonic processes (synchrotron radiation and inverse Compton scattering from relativistic electrons) and hadronic processes (proton synchrotron radiation, inelastic hadronuclear (pp) collisions, Bethe-Heitler (BH) pair production, and photomeson (pγ) interactions). The transport equation for particle species 'a' in the comoving frame accounts for these interactions. For high-energy photons above 0.1 PeV in both the jet blob and inner blob, the production is dominated by the pγ process due to a sufficiently dense target photon field, often involving a ∆−resonance when target photons are from an accretion disk. In contrast, for external regions (Scenarios B and C), photons above 0.1 PeV are primarily produced via the pp process due to the dense target environment.

Orbital Modulation and External Absorption

The energy-dependent orbital modulation arises from variations in the Doppler factor and external γγ annihilation at different orbital phases. The authors investigate two primary mechanisms: (i) jet orientation remains fixed relative to the observer, leading to modulation from changes in distance between the donor star and the jet blob, or (ii) variations in the jet azimuthal angle ϕj lead to changes in Doppler boosting. The study finds that strong γγ annihilation can produce energy-dependent modulation patterns, with a more pronounced effect in the GeV band than in the PeV band due to higher photon-photon opacity at lower energies.

Results and Predictions for Cygnus X-1 and Cygnus X-3

The models successfully reproduce multiwavelength data for both microquasars across all scenarios, except for the radio band, which is suppressed by free-free absorption, suggesting the emission region lies at larger heights (∼ 1014 cm). For Cygnus X-1, Scenario A predicts a deep dip in the 0.1–10 TeV range due to combined pγ interactions and γγ annihilation. For Cygnus X-3, Scenario B exhibits a plateau-plus-peak structure above 10 TeV, arising from pp interactions in the external blob and pγ interactions within the jet blob. The neutrino predictions reveal a significantly suppressed flux when muon and pion cooling effects are properly accounted for, indicating that detecting these sources may be more challenging than previously anticipated. The models also constrain the acceleration mechanism by finding values for ηacc that set the maximum proton Lorentz factor γ′p,max.

Improvements for AI systems

As a fastidious researcher, I have analyzed this paper, Unveiling Multimessenger Emission from Hidden Cores of Microquasars, focusing on its methodology (AMES simulator), physical scenarios (Jet-Core, Stellar Wind Interaction, Extended Jet), and results concerning particle acceleration and emission mechanisms.

Here are the specific improvements I would make to AI systems based on this scientific paper, followed by what the improved AI system can achieve.

)

Improve AI Systems using this Scientific Paper:

The core improvement lies in building an Astrophysical Multimessenger Emission Simulator (AMES) that is not just a model-fitting tool, but an integrated, self-consistent physical solver capable of navigating complex parameter spaces to predict observable phenomena across all messenger bands.

Here are the specific improvements:

Improvements to the AI System Architecture (The AMES Solver):
Integration of Coupled Physical Transport Equations:
Enhanced Scenario-Specific Modeling Capability:
Advanced Multi-Messenger Prediction Module (Neutrino/Gamma Ray Cross-Check):
Orbital Modulation and Geometry Analysis Engine:

)

Improvements to the AI System Architecture (The AMES Solver):

The current system is a sequence of models. The improved AI must be a holistic, differentiable solver capable of handling non-linear, coupled transport equations (Eqs. 13-19) simultaneously across different physical regimes (leptonic vs. hadronic).

)

Integration of Coupled Physical Transport Equations:

The AI must implement the full set of coupled transport equations for all relevant species (photons, electrons, protons, muons, pions) in the co-moving frame. This requires a solver that can handle:

Time evolution and spatial distribution (Eq. 13).
Energy exchange terms (self-production rates C'b→a).
Crucially, the inclusion of cooling processes for both electrons/positrons and muons/pions (Eqs. 14, 17, 19), ensuring that the suppression of neutrino flux due to muon/pion cooling is self-consistently calculated across all energy bands.

Enhanced Scenario-Specific Modeling Capability:

The AI must be explicitly trained on the three distinct physical scenarios (A: Jet-Core, B: Stellar Wind Interaction, C: Extended Jet). Instead of just running these as separate parameter sets, the AI should use a learned scenario discriminator to select the appropriate dominant interaction pathways (pγ vs. pp) and external targets based on input observational constraints (e.g., spectral shape or variability timescale).

If input data shows strong variability, prioritize Scenario A (short dynamical timescales).
If input data suggests dense ambient environments, prioritize Scenarios B and C.

Advanced Multi-Messenger Prediction Module (Neutrino/Gamma Ray Cross-Check):

The AI must incorporate the calculated energy-dependent optical depths for both internal and external absorption (Eqs. 28, 29) directly into the final flux prediction pipeline (Fig. 3). This allows for a true messenger cross-check:

Predict the primary high-energy photon spectrum (TeV/PeV) based on pγ/pp interactions.
Simultaneously calculate the suppression factor due to internal γγ absorption.
Predict the neutrino flux, accounting for muon/pion cooling effects derived from Eqs. 14 and 17.

This module must output not just a single spectrum, but a set of predicted fluxes for all channels (Photon + Neutrino) as functions of detector type (IceCube vs. IceCube-Gen2).

Orbital Modulation and Geometry Analysis Engine:

The AI must be equipped with an engine to analyze the complex phase dependence shown in Figure 10. This involves:

Inputting orbital parameters (inclination, jet angle, donor star position) and calculating the resulting Doppler factor variations (Eq. 2).
Modeling the energy-dependent modulation caused by varying Doppler boosting and external γγ absorption across different orbital phases.
The AI should be able to quantify the discrepancy between predicted modulation patterns (e.g., 1 GeV vs. PeV) and observational data, flagging specific geometric parameters (like the required jet polar angle or phase offset) that resolve these tensions, as discussed in Section 4 and Appendix A.

)

Improved AI System Capabilities:

The improved AI system can move beyond simple parameter fitting to become a powerful astrophysical discovery engine capable of:

Predicting Multimessenger Signatures: The system can take observational data from radio, X-ray, GeV, and PeV bands and simultaneously predict the expected flux in all messenger channels (photons and neutrinos), providing a clear smoking gun signature for specific physical scenarios (e.g., identifying whether a source is dominated by pγ interactions or pp interactions).
Constraining Particle Acceleration Mechanisms: By comparing the predicted maximum particle energies (derived from balancing acceleration timescales, Eq. 4-8) against observed high-energy cutoffs, the AI can definitively constrain whether protons or electrons are being accelerated to PeV energies in microquasars.
Distinguishing Physical Environments: The system can robustly distinguish between the three primary emission geometries (Jet-Core vs. Stellar Wind vs. Extended Shell) by analyzing their unique spectral signatures (e.g., the presence of a plateau in Scenario B/C versus a deep dip in Scenario A).
Optimizing Future Observations: By simulating the expected orbital modulation patterns across different energy bands, the AI can provide specific, testable predictions for future high-energy variability observations (CTA), guiding observational strategies to resolve ambiguities between competing geometric models.
Identifying System Constraints: The system can identify which physical constraints are most critical; for instance, determining whether a lack of a PeV feature in Cygnus X-1 is due to insufficient target photon density or insufficient acceleration power, allowing researchers to prioritize the next set of necessary observations.

Abstract

Microquasars are radio-emitting X-ray binaries with relativistic jets and are established sources of about 100 TeV gamma rays, making them promising candidates for cosmic-ray acceleration. Motivated by recent detections of about 100 TeV photons from Cygnus X-1 and multi-PeV photons from Cygnus X-3, we employ the Astrophysical Multimessenger Emission Simulator (AMES) to model their multimessenger emission from radio to ultrahigh-energy gamma rays. Our modeling suggests that particle acceleration is extremely efficient in the jet region. The observed 0.1 PeV gamma rays can originate from either pγ or pp interactions, depending on the location and physical conditions of the emission region, while also reproducing the lower-energy spectra. These configurations yield observationally testable predictions. In the poorly constrained 0.1 - 10 TeV band, the models predict either a deep valley, a mild suppression, or a power-law spectrum. Additionally, models involving compact emission regions comparable to the orbital separation predict strong variability, while those invoking more extended and static external zones show more stable behavior. We also provide a possible qualitative explanation for the energy-dependent modulation patterns, relying primarily on changes in the Doppler factor and external γγ absorption. In particular, explaining the PeV emission from Cygnus X-3 favors a compact emission region, for which a magnetic field strength of order 10 2 G is required. Finally, after accounting for pion and muon cooling, the predicted neutrino flux is suppressed, implying that detection is more challenging than previously thought.