🔭 astrophysics

TeV-PeV Gamma-ray and Neutrino Emission in the Galactic Plane

This paper models LHAASO's observation of diffuse TeV–PeV gamma rays in the Galactic plane as a combination of leptonic and hadronic emissions, demonstrating that uncertainties in the inner Galaxy's infrared radiation field have only modest effects on the overall fit and neutrino constraints, while highlighting the potential for future multi-messenger observations to probe the inner-Galaxy cosmic-ray population and interstellar radiation field distribution.

Original authors: Saikat Das, Nayantara Gupta, Siyao Xu

Published 2026-06-23

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CC BY 4.0

Original authors: Saikat Das, Nayantara Gupta, Siyao Xu

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 our Milky Way galaxy as a giant, bustling city. For decades, astronomers have been trying to figure out where the "noise" in this city comes from. Specifically, they are looking at high-energy light (gamma rays) and ghostly particles (neutrinos) that rain down on Earth from the galactic plane.

This paper is like a detective trying to solve a mystery: Is this noise coming from a few loud, individual sources (like specific factories), or is it a general hum from the whole city?

Here is the story of what the authors did, explained simply:

1. The Two Types of Noise

The authors propose that the high-energy light we see comes from two main "machines":

The Leptonic Machine (The Pulsar Wind Nebulae): Think of these as thousands of tiny, unresolved lighthouses (pulsars) scattered throughout the galaxy. They spin fast and shoot out particles that create light. The authors modeled these as a "fog" of light because we can't see every single lighthouse individually.
The Hadronic Machine (Supernova Remnants): This is the "smoke" from old explosions (supernovas). When these explosions happen, they shoot out protons (atomic nuclei) that crash into gas clouds. These crashes create a different kind of light and also produce neutrinos.

2. The Foggy Window Problem (The ISRF)

To see clearly through the galaxy, you have to look through a "window" filled with dust and light. In astronomy, this is called the Interstellar Radiation Field (ISRF).

The Problem: The authors weren't sure how thick the "fog" (dust and infrared light) is in the very center of the galaxy.
The Experiment: They built a model of the galaxy and then tried looking through different versions of this "fog." Some versions had a little more dust in the center; others had way more dust (like a thick smog).
The Result: Surprisingly, it didn't matter much which version of the fog they used. Because the telescope (LHAASO) was looking at specific areas and ignoring the very center of the galaxy (where the fog is thickest), the "fog" didn't change the overall picture of the diffuse light much. The main signal remained the same.

3. The Ghost Particles (Neutrinos)

When the "Hadronic Machine" (the crashing protons) works, it doesn't just make light; it also makes neutrinos. These are ghostly particles that pass through everything.

The authors calculated how many of these ghosts should be coming from the "Galactic Ridge" (the busy center of the city).
The Check: They compared their prediction to what other detectors (IceCube, ANTARES, KM3NeT) have seen so far.
The Verdict: Their prediction fits perfectly with what we already know. The amount of "ghosts" they calculated is safe; it doesn't break any current rules or exceed what other telescopes have seen.

4. The Special Case: The Very Center

While the "fog" didn't change the big picture of the whole galaxy, the authors realized it would matter if we were looking at a single, specific light source right in the center of the galaxy (near the Central Molecular Zone).

The Analogy: Imagine a single bright streetlamp in a thick fog. If the fog gets thicker, the light gets dimmer and changes color.
The Finding: If there is a lot of dust in the center, it would absorb high-energy light from specific sources and change their appearance. This means that if we look at individual sources near the center in the future, we might see them look different depending on how thick the dust is.

5. The Missing Piece

The authors noticed that their model worked well for the inner part of the galaxy but slightly underestimated the light coming from the outer part of the galaxy at the highest energies.

The Clue: This suggests there might be something else in the outer galaxy contributing to the noise that their current model isn't capturing yet. Maybe the "factories" (supernovas) are distributed differently than they thought, or there are other sources we haven't identified.

The Bottom Line

The authors built a model to explain the high-energy light and ghost particles coming from our galaxy. They tested how much "dust" in the center of the galaxy changes the view.

For the big picture: The amount of dust doesn't change the main conclusion much.
For the details: If we look at specific sources near the center, the dust matters a lot.
The Future: New telescopes (like KM3NeT) will help us separate the "ghost" particles from the light, helping us understand exactly how the "factories" in the center of our galaxy are working and how thick the cosmic fog really is.

Technical Summary: TeV–PeV Gamma-ray and Neutrino Emission in the Galactic Plane

Problem Statement
The origin of the diffuse TeV–PeV gamma-ray emission from the Galactic plane remains an open question in high-energy astrophysics. While the emission is widely interpreted as a calorimeter of Galactic cosmic rays (CRs) interacting with interstellar gas and radiation fields, the relative contributions of hadronic processes (proton-proton collisions) and leptonic processes (inverse-Compton scattering from unresolved pulsar wind nebulae, or PWNe) are not fully constrained. A critical systematic uncertainty lies in the radial distribution of the interstellar radiation field (ISRF), particularly its infrared (IR) component in the inner Galaxy. Variations in the ISRF density affect the optical depth for gamma-gamma ( $\gamma\gamma$ ) pair production, which attenuates high-energy gamma rays, and influence the inverse-Compton (IC) cooling rates of electrons. This paper investigates how uncertainties in the inner-Galaxy IR photon density profiles impact the interpretation of LHAASO diffuse gamma-ray data and the associated neutrino flux predictions.

Methodology
The authors model the LHAASO diffuse gamma-ray spectrum as the superposition of two components:

Leptonic Emission: Unresolved emission from PWNe and pulsar halos. The authors generate a synthetic population of parent pulsars based on the ATNF catalog, applying age-dependent beaming fractions and empirical relations between spin-down power ( $\dot{E}$ ) and gamma-ray flux. The resulting flux is distributed over the sky, masked to exclude known bright sources, and summed to estimate the unresolved contribution.
Hadronic Emission: Diffuse emission from supernova-injected CR protons propagating through the Galaxy. The authors solve the steady-state transport equation for CR protons in cylindrical coordinates, incorporating spatially dependent diffusion coefficients and $pp$ interaction losses. The source distribution follows the radial profile of supernova remnants (SNRs) from Case & Bhattacharya (1996).

To address the ISRF uncertainty, the authors adopt the baseline dust distribution of Misiriotis et al. (2006) and introduce four alternative radial profiles (M1–M4) that enhance the dust density (and thus IR photon density) toward the Galactic Center by factors ranging from a few to nearly two orders of magnitude. These profiles are normalized to match the baseline model at the solar neighborhood. The authors calculate the $\gamma\gamma$ optical depth for these varying ISRF models and fit the hadronic normalization to the LHAASO inner-region data ( $15^\circ < \ell < 125^\circ$ , $|b| < 5^\circ$ ) using $\chi^2$ minimization. The resulting hadronic component is then used to predict the associated $pp$ neutrino flux for the all-sky and the Galactic Ridge ( $|l| < 30^\circ$ , $|b| < 2^\circ$ ).

Key Results

Impact on Diffuse Gamma-Ray Fit: The alternative ISRF models, which significantly increase IR photon densities in the inner Galaxy, affect the LHAASO diffuse fit only modestly. The analysis excludes the Galactic Center direction and applies source masks, limiting the sensitivity to the innermost radial variations. The fit quality ( $\chi^2_\nu$ ) remains comparable across models, with the baseline model yielding $\chi^2_\nu \approx 0.5$ and the most extreme profile (M4) yielding $\chi^2_\nu \approx 0.8$ .
Leptonic Contribution: The unresolved PWN component accounts for approximately 20% of the diffuse flux at a few TeV, rising to $\sim30\%$ at tens of TeV, and decreasing to $\sim10\%$ at the highest energies in the baseline model. This is consistent with data-driven estimates.
Neutrino Predictions: Using the hadronic normalization derived from the LHAASO fit, the predicted all-sky neutrino flux is consistent with IceCube measurements. The predicted neutrino flux from the Galactic Ridge is compatible with current constraints from ANTARES and KM3NeT, lying below the ANTARES best-fit flux by a factor of roughly two.
Inner-Galaxy Point Sources: While the diffuse fit is robust against ISRF variations, the authors find that enhanced IR profiles significantly modify the spectral energy distribution (SED) of point sources near the Central Molecular Zone (CMZ). In IC-dominated scenarios, centrally peaked IR profiles can cause spectral hardening in the WCDA band (1–25 TeV) but lead to sharp attenuation (cutoffs) above $\sim10$ TeV due to increased $\gamma\gamma$ opacity.

Significance and Claims
The paper claims that current LHAASO diffuse measurements, due to their longitude coverage and source masking, are not sufficiently sensitive to tightly constrain the radial distribution of the ISRF in the inner Galaxy. Consequently, the hadronic normalization inferred from LHAASO data remains robust against these specific ISRF uncertainties. However, the authors emphasize that these uncertainties are critical for interpreting individual sources near the Galactic Center. The study concludes that future multi-messenger observations, specifically combining KM3NeT neutrino data with gamma-ray measurements of individual sources, will be necessary to disentangle the hadronic component and constrain the radial distribution of the ISRF near the Galactic Center. The work also highlights that the outer-Galaxy flux is underpredicted by the baseline model, suggesting potential missing components such as additional source populations or modifications to the CR source distribution.