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The GPSP Atlas: High-resolution Galactic Photon Survival Probability Atlas for Very-High- and Ultra-High-Energy Gamma-Ray Astronomy

This paper introduces the GPSP Atlas, a high-resolution, publicly available framework that maps Galactic photon survival probabilities across the full parameter space of 1 TeV to 10 PeV using independent ISRF models, revealing structured opacity patterns and extending to Lorentz invariance violation scenarios to support the emerging era of Galactic PeV astronomy.

Original authors: Ekrem Oğuzhan Angüner

Published 2026-08-26

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Ekrem Oğuzhan Angüner

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 sky above us is not empty; it is filled with a faint, ancient glow left over from the birth of the universe, a sea of low-energy light that permeates every corner of our galaxy. When high-energy particles from deep space crash into this sea, they can vanish, transforming into matter and antimatter before they ever reach our telescopes. This process acts like a cosmic fog, obscuring the true nature of the most violent accelerators in the Milky Way. For decades, astronomers have mapped the stars and the gas between them, but they have lacked a precise guide to how this fog thickens or thins depending on where you look and how energetic the incoming light is. Without this map, it is difficult to tell if a distant source is simply dim or if it is actually a powerhouse capable of accelerating particles to energies a million times greater than anything we can create on Earth.

A researcher has now built the first high-resolution atlas to navigate this fog, a tool designed to help astronomers see clearly through the Milky Way's own atmosphere. They call it the Galactic Photon Survival Probability Atlas. Instead of guessing how much light is lost, this atlas provides a detailed, four-dimensional chart that calculates exactly how many gamma-ray photons survive a journey from any point in our galaxy to Earth. The researcher created this map by combining two different, independent models of the galaxy's radiation environment. By running their calculations through both models, they could identify where the maps agree and where they differ, giving astronomers a clear way to measure the uncertainty in their observations. The result is a public database that covers the entire galactic plane, tracking photons from one trillion electron volts up to ten quadrillion electron volts, a range that includes the most extreme energies ever detected from within our own galaxy.

The atlas reveals that the galaxy is not uniformly opaque. In the energy range between 100 and 300 trillion electron volts, the fog is highly structured. The researcher found that the galaxy is significantly more opaque along the directions of its spiral arms, where the density of dust and starlight is highest. If a gamma ray travels along the tangent of a spiral arm, it is much more likely to be absorbed than if it travels toward the galactic center or out toward the edge of the galaxy. However, as the energy of the gamma rays increases beyond one quadrillion electron volts, the nature of the fog changes. At these extreme energies, the specific structure of the galaxy matters less, and the absorption becomes dominated by the uniform background glow of the universe itself. In this high-energy regime, the galaxy becomes nearly isotropic in all directions, though the distance a photon can travel before vanishing still has a limit, reaching a minimum around two to three quadrillion electron volts.

To make these complex calculations useful for real-world astronomy, the researcher introduced a new way of visualizing the data. Instead of showing survival rates as a simple line for a single distance, they mapped the probability of survival across both energy and distance simultaneously. This creates a "phase space" view that shows how the visibility of the galaxy evolves. For example, the map shows that for a source located at the center of the galaxy, the distance at which gamma rays can be seen drops to a minimum around two to three quadrillion electron volts before expanding again at even higher energies, specifically above five to ten quadrillion electron volts. This visualization helps astronomers distinguish between a source that has run out of energy and one that is simply being hidden by the galaxy's fog.

The researcher applied their new atlas to two famous regions of the sky to test its utility. First, they looked at the diffuse gamma-ray glow surrounding the galactic center, a region suspected of harboring a "PeVatron," a natural particle accelerator capable of reaching quadrillion-electron-volt energies. By averaging the survival probability over the entire extended region rather than just a single point, they showed how the observed spectrum is shaped by the varying density of the galactic fog. They found that while the current data is consistent with a powerful accelerator, the strong absorption in this region makes it difficult to confirm the exact maximum energy of the particles without future, more sensitive observations.

They also turned their attention to Cygnus X-3, a binary star system that recently emitted gamma rays up to four quadrillion electron volts. Because this system lies in a different part of the galaxy, the fog it passes through is less dense than in the galactic center. The researcher used their atlas to correct the observed spectrum for absorption, revealing that the intrinsic light from the source is even harder and more energetic than previously thought. This correction is crucial for understanding the physical processes at work in the binary system. Furthermore, they used the atlas to explore a hypothetical scenario where the laws of physics might be slightly different at these extreme energies. If the speed of light were to vary slightly for the highest-energy particles, the galaxy would become even more transparent, allowing us to see sources that should otherwise be invisible. The atlas provides the framework to test these ideas against future observations, offering a way to probe the fundamental laws of nature using the galaxy itself as a laboratory.

The work represents a significant step forward in the emerging era of ultra-high-energy gamma-ray astronomy. As new telescopes come online with the ability to detect these rare, high-energy photons, the need for a standardized way to correct for galactic absorption will only grow. This atlas provides that standard, offering a robust foundation for studying the most extreme particle accelerators in the universe, mapping the diffuse glow of the galaxy, and searching for signs of new physics. By making the data and the tools publicly available, the researcher has ensured that the entire scientific community can use this map to look deeper into the cosmos than ever before.

Technical Summary: The GPSP Atlas

Problem Statement
The emergence of Galactic PeV astronomy, driven by wide-field observatories like LHAASO and HAWC, has made the correction for photon–photon ( $\gamma\gamma$ ) absorption a critical component of gamma-ray data analysis. As gamma rays propagate through the interstellar medium (ISM), they undergo attenuation via pair production ( $\gamma\gamma \to e^+e^-$ ) when interacting with background photon fields, primarily the Interstellar Radiation Field (ISRF) and the Cosmic Microwave Background (CMB). This absorption creates a "Galactic fog" that suppresses observed spectra, particularly in the 30–200 TeV range (dominated by IR/optical ISRF) and near PeV energies (dominated by CMB).

While individual absorption corrections have been applied to specific sources, the field lacks a unified, high-resolution framework to standardize the analysis of the rapidly growing population of Very-High-Energy (VHE) and Ultra-High-Energy (UHE) sources. Furthermore, reconstructing intrinsic spectra to identify Galactic PeVatrons (sources accelerating particles to PeV energies) or testing fundamental physics, such as Lorentz Invariance Violation (LIV), requires precise knowledge of the survival probability $P(E_\gamma, l, b, d)$ across the full parameter space of energy, Galactic coordinates, and distance.

Methodology
The author constructs the Galactic Photon Survival Probability (GPSP) Atlas, a high-resolution 4D lookup table designed to provide precomputed survival probabilities for arbitrary source configurations.

Physics Framework: The attenuation is calculated using the optical depth $\tau$ , derived from the Breit–Wheeler cross section for pair production. The calculation assumes straight-line propagation and neglects secondary electromagnetic cascade effects, focusing on the attenuation of the primary gamma-ray component.
ISRF Models: To assess systematic uncertainties, the atlas is built using two independent, publicly available GALPROP-based ISRF models:
- R12 (Robitaille et al. 2012): Features pronounced spiral-arm structures and an inner dust "hole."
- F98 (Freudenreich 1998): Predicts a smoother, more centrally concentrated dust distribution with a non-axisymmetric bar.
  Both models are combined with the isotropic CMB.
Grid Configuration: The atlas covers a 4D grid defined by:
- Galactic Longitude ( $l$ ): $0^\circ$ to $359.9^\circ$ (step $0.1^\circ$ ).
- Galactic Latitude ( $b$ ): $-5^\circ$ to $+5^\circ$ (step $0.1^\circ$ ).
- Distance ( $d$ ): 0.1 kpc to 20.0 kpc (step 0.1 kpc).
- Energy ( $E_\gamma$ ): 1 TeV to 10 PeV (81 logarithmic bins).
  This results in $\sim 5.89 \times 10^9$ data entries.
LIV Extension: A companion atlas, GPSP-LIV, is constructed to investigate subluminal Lorentz Invariance Violation. This version incorporates modified pair production kinematics where the dispersion relation is altered by a LIV scale parameter $\Lambda$ . It covers a 5D parameter space including the LIV strength ( $\log_{10}(\Lambda/E_{Pl})$ from $-4.5$ to $-2.0$).
Data Format: The atlases are released in FITS format, compatible with standard analysis tools like Gammapy, containing 32-bit floating-point survival probabilities.

Key Contributions

Unified Framework: The first Galactic framework describing gamma-ray survival probabilities over the full parameter space $P(l, b, d, E_\gamma)$ between 1 TeV and 10 PeV.
Systematic Uncertainty Quantification: By providing parallel atlases based on R12 and F98 models, the work allows for a direct assessment of ISRF-related systematic uncertainties, which are found to be most significant in the inner Galaxy and at energies where IR interactions dominate.
Novel Visualization: Introduction of the "gamma-ray survival phase space," a 2D energy–distance representation that visualizes Galactic transparency and horizons, extending beyond traditional 1D profiles.
LIV Integration: The GPSP-LIV atlas is the first large-scale Galactic photon survival atlas incorporating LIV-modified pair production cross sections, enabling systematic studies of LIV effects on Galactic propagation.

Results

Galactic Transparency Maps: The maps reveal that Galactic opacity is highly structured. In the $\sim 100–300$ TeV regime, attenuation is enhanced along spiral-arm tangent directions (e.g., Norma and Scutum–Crux arms) due to higher IR photon densities and longer path lengths through dense regions.
Energy Dependence: As energy increases toward the PeV regime, attenuation becomes increasingly dominated by the isotropic CMB. Consequently, the survival probability maps transition from anisotropic, structured patterns to nearly isotropic distributions.
Gamma-Ray Horizons: The horizon distance (defined by $P_{surv} \approx 0.37$ ) reaches a minimum of $\sim 7$ kpc at $\sim 2–3$ PeV before expanding again at higher energies. The Galactic Anti-Center remains the most transparent direction.
Model Differences: The fractional deviation between R12 and F98 models peaks at $\sim 9\%$ (up to $\sim 13\%$ in the central plane) in the 50–200 TeV range but drops to $<1\%$ at PeV energies where CMB dominates.
LIV Effects: Subluminal LIV scenarios suppress the pair production cross section and shift the kinematic threshold, significantly increasing Galactic transparency at PeV energies. Sources emitting beyond the standard gamma-ray horizon in the 2–3 PeV range are identified as promising targets for probing LIV.
Applications:
- Galactic Center (GC): Analysis of the GC diffuse emission shows that while attenuation is moderate at H.E.S.S. energies, it introduces significant spectral steepening at UHE energies, potentially masking intrinsic PeVatron signatures.
- Cygnus X-3: Reconstruction of the intrinsic spectrum for Cygnus X-3 demonstrates that ISRF model differences (R12 vs. F98 vs. P17) lead to variations in the "IR shoulder" feature and the energy of minimum survival probability, though current measurement uncertainties dominate over these systematic differences.

Significance and Claims
The paper claims that the GPSP and GPSP-LIV atlases establish a publicly available foundation for studies of Galactic PeVatrons, diffuse gamma-ray emission, and fundamental physics searches. The author emphasizes that as gamma-ray astronomy enters the PeV domain, attenuation corrections will become a standard component of UHE analyses.

The work is presented as a tool to:

Standardize the analysis of the growing UHE source population.
Distinguish between intrinsic acceleration limits and absorption-induced spectral features.
Provide a systematic framework for testing LIV scenarios by comparing observed spectra against LIV-modified propagation models.

The author remains modest regarding LIV constraints, noting that the current analysis serves as a "proof-of-principle" demonstration. They state that robust LIV constraints will likely require population studies with well-characterized source properties and full likelihood-based analyses, rather than individual source detections alone. Similarly, they acknowledge that the current framework neglects secondary cascade effects and anisotropic ISRF treatments, which are planned for future iterations.

Technical Summary: The GPSP Atlas

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