Galactic Microquasar and Supernova Remnants Imprinting on Diffuse Neutrino and Gamma-Ray Sky

Deep in the heart of our Milky Way, a constant, invisible rain of particles falls from the stars. These are cosmic rays, high-speed protons and atomic nuclei that travel across the galaxy at nearly the speed of light. For decades, scientists have known that these particles are born in violent cosmic events, such as the explosions of massive stars, but the exact mix of sources and how they travel to us has remained a puzzle. When these cosmic rays crash into the thin gas that fills the space between stars, they create a secondary shower of particles, including ghostly neutrinos and high-energy gamma rays. Detecting these secondary signals is like seeing the splash of a stone thrown into a dark pond; it tells us the stone was there, even if we cannot see the thrower. Understanding the origin of this diffuse glow is crucial because it forms the background noise against which astronomers must search for specific, powerful sources. If we cannot accurately map this background, we cannot distinguish the faint whispers of new cosmic accelerators from the roar of the galaxy itself.

A team of researchers has now built a new, detailed map of this cosmic background, revealing that the story of the galaxy's high-energy sky is told by two distinct groups of cosmic engines working in tandem. In a study published in August 2026, Shiqi Yu and Bing Theodore Zhang developed a computer model that tracks how cosmic rays from two different types of sources—supernova remnants and microquasars—spread through the galaxy and interact with interstellar gas. Supernova remnants are the expanding shells of debris left behind when massive stars explode, while microquasars are systems where a black hole or neutron star pulls matter from a companion star, shooting out powerful jets of particles. The researchers found that these two populations do not contribute equally across all energies. Instead, they dominate different parts of the energy spectrum, creating a seamless transition that explains recent observations from the IceCube Neutrino Observatory and the LHAASO gamma-ray telescope.

The team's approach was to treat these two source populations as independent actors, each with its own rules for how fast it accelerates particles and how far those particles can travel. They simulated the movement of cosmic rays over millions of years, anchoring their calculations to precise measurements of cosmic rays detected near Earth by the DAMPE and LHAASO instruments. By feeding these local measurements into a three-dimensional model of the galaxy's gas distribution, they could predict how many neutrinos and gamma rays should be produced in every direction. The results showed a clear division of labor. At lower energies, below about 10 trillion electron volts, the diffuse glow is almost entirely the work of supernova remnants. These ancient stellar explosions have been seeding the galaxy with cosmic rays for eons, creating a steady, widespread background. However, as the energy rises above this threshold, the contribution from supernova remnants fades, and a new player takes the lead: microquasars.

This shift is significant because it solves a long-standing problem regarding how the galaxy accelerates particles to the highest known energies. While supernova remnants are excellent at accelerating particles, they may struggle to push them beyond a certain limit. Microquasars, with their intense magnetic fields and relativistic jets, appear capable of acting as "PeVatrons," accelerating particles to energies a thousand times higher than those produced by the Large Hadron Collider. The model suggests that the hardening of the cosmic ray spectrum at high energies is not a mystery, but a natural consequence of microquasars taking over the job. When the researchers compared their predictions to the actual data collected by IceCube over twelve years, the match was remarkably close. Their model successfully reproduced the observed flux of neutrinos, providing a physically motivated baseline that accounts for the contributions of both source types without needing to invent new physics.

The same logic applied to gamma rays, the high-energy cousins of visible light. The researchers found that their model, which included the effects of microquasars, aligned well with observations from LHAASO, particularly at energies above 100 trillion electron volts. At these extreme energies, the gamma-ray sky is dominated by the hadronic interactions of protons from microquasars, creating a clean signal that is less contaminated by other processes. Below this threshold, the data showed a slight excess that the model did not fully capture, likely due to electrons scattering light or faint, unresolved sources that are too dim to be seen individually. This residual gap highlights where future observations can refine the picture, but the core finding remains robust: the high-energy diffuse sky is a composite of two distinct populations.

The study also revealed how the structure of the galaxy shapes what we see. Because neutrinos are only produced when cosmic rays collide with dense pockets of gas, the resulting map of neutrino emission is not a smooth, uniform glow. Instead, it traces the filamentary, clumpy structure of the interstellar medium, creating localized "hot spots" where the gas is densest. These spots do not necessarily mark the location of an active accelerator; rather, they show where the steady sea of cosmic rays, flowing from all directions, happens to hit the most matter. This distinction is vital for astronomers searching for new point sources, as it helps them separate the true signal of a specific object from the complex, textured background of the galaxy.

By decoupling the contributions of supernova remnants and microquasars, the researchers have provided a flexible framework that can be tested and updated as new data arrives. Their work suggests that the diffuse background is not a single, monolithic entity but a layered tapestry woven from different cosmic threads. This understanding offers a solid foundation for the next generation of multi-messenger observatories, which will use neutrinos and gamma rays to peer deeper into the galaxy. With a clearer picture of the background, these instruments will be better equipped to identify the most powerful accelerators in the universe, turning the faint, diffuse glow of the Milky Way into a precise map of its most violent and energetic processes.