Next-Generation Gamma-Ray Telescope Set to Decode the Strange Halos Around Pulsars

Some of the most enigmatic objects in the Milky Way are not the neutron stars themselves, but the vast, glowing shrouds that surround them. When astronomers pointed the High-Altitude Water Cherenkov (HAWC) observatory at the sky in 2017, they discovered bright, sprawling clouds of multi-teraelectronvolt gamma rays enveloping the Geminga and Monogem pulsars, two dead stars lying a few hundred parsecs from Earth. These structures, now known as TeV halos, stretch out to roughly five degrees on the sky, corresponding to about 25 parsecs of space, and they have since been recognized as an almost universal accessory of middle-aged pulsars. A new theoretical study published in The European Physical Journal C by Dan Hooper, Eleonora Pinetti, and Alexander Sokolenko argues that the forthcoming Cherenkov Telescope Array (CTA) will finally be able to pry open the physics of these halos, distinguishing between models that all existing data cannot tell apart.

The mystery at the heart of TeV halos lies in how their parent pulsars inject energy into their surroundings. Pulsars are rapidly spinning neutron stars whose rotation gradually slows, releasing enormous quantities of rotational kinetic energy. According to the HAWC measurements, Geminga and Monogem each convert roughly ten percent of their total spin-down power into ultra-relativistic electron-positron pairs. These charged particles then race outward through the interstellar medium, colliding with ambient starlight, dust emission, and the cosmic microwave background. Each such collision, an inverse Compton scattering event, boosts the low-energy photon to teraelectronvolt energies, painting the halo in gamma rays that ground-based telescopes can detect. The intensity of the emission thus serves as a calorimeter, revealing how efficiently the pulsar converts its fading spin into exotic matter.

Yet the most startling revelation from the halos concerns not the pulsars but the medium around them. Cosmic rays normally diffuse through the Galaxy relatively efficiently, with a diffusion coefficient of about 4 x 10^28 square centimeters per second. But the observed size of the Geminga and Monogem halos demands a diffusion coefficient near these sources that is hundreds of times smaller, around 10^26 square centimeters per second. Were diffusion as rapid near Geminga as it is in the general interstellar medium, the inverse Compton glow produced by 35-teraelectronvolt electrons would extend roughly 200 parsecs, spanning some 60 degrees on the sky, far beyond the modest two-degree extension actually measured by HAWC and its predecessor Milagro. Something in the immediate vicinity of these pulsars tampers with the propagation of cosmic rays, and no one yet knows exactly what or how.

Several physical pictures have been proposed to explain the suppressed diffusion, ranging from self-generated turbulence, in which the escaping electrons themselves stir up magnetic fluctuations that trap them, to two-zone models in which a slow-diffusion bubble surrounds the pulsar and gives way to normal interstellar conditions at some boundary radius. To discriminate among these scenarios, astronomers need far more precise measurements of both the energy spectrum and the angular distribution of halo gamma rays. This is precisely where the Cherenkov Telescope Array enters the picture. CTA, the flagship next-generation observatory for very-high-energy gamma-ray astronomy, will span an energy range from 20 gigaelectronvolts to 300 teraelectronvolts with an energy resolution better than ten percent and angular resolution far exceeding existing instruments. Its two arrays, CTA North with 4 large and 9 medium telescopes and CTA South with 4 large, 14 medium, and 37 small telescopes, will together survey the sky with unprecedented sensitivity.

In their study, the researchers built a detailed theoretical framework describing how electrons injected by a pulsar propagate and radiate. They modeled the pulsar as a point source emitting electrons with a power-law spectrum characterized by an index alpha and an exponential cutoff energy, and they tied the injection rate to the pulsar’s spin-down luminosity, which decays over a characteristic timescale tau governed by the braking index n. As the electrons stream outward, they lose energy through synchrotron radiation in the ambient 3-microgauss magnetic field and through inverse Compton scattering off three radiation fields: the cosmic microwave background, infrared dust emission, and optical starlight. At the highest energies, the scattering enters the Klein-Nishina regime, where the cross section is suppressed relative to the classical Thomson limit, a subtlety the authors treated explicitly in their calculations.

The diffusion itself was modeled as a two-zone structure, with a suppressed coefficient D0 within a halo radius rh transitioning to the standard interstellar value beyond. The team adopted a default set of parameters: alpha equals 1.8, D0 equals 10^26 square centimeters per second, an energy dependence parameter delta of 0.33, a braking index of 3, a spin-down timescale of 12 kiloyears, a halo radius of 30 parsecs, an electron cutoff of 500 teraelectronvolts, and a conversion efficiency eta of 0.25, appropriate for Geminga at its measured distance of 250 parsecs and age of 340 kiloyears. Varying each parameter in turn, they computed how the resulting gamma-ray spectrum and surface brightness profile would change, comparing their predictions against the existing measurements from HAWC and HESS. A striking pattern emerged: while the braking index and spin-down timescale leave almost no imprint on the high-energy spectrum or the brightness profile, the diffusion parameters, the injected spectral index, the cutoff energy, and the halo radius all leave measurable signatures, and some affect the morphology differently at different photon energies.

To translate these theoretical distinctions into concrete observational forecasts, the team used the publicly available Gammapy software together with the prod5 instrument response function for the CTA-North array. They simulated 50 hours of observation of Geminga, dividing a 5-degree by 5-degree field of view into angular bins of 0.05 degrees on a side and twenty logarithmically spaced energy bins between 0.03 and 100 teraelectronvolts. Each mock dataset contained both the predicted gamma-ray signal and the residual cosmic-ray background. The extended nature of the halo inevitably degrades CTA’s sensitivity compared to a point source of the same flux, but the simulation showed that the observatory would nevertheless measure the halo’s spectrum with such precision that it could easily distinguish between an injected spectral index of 1.5, 1.8, or 2.0, and could identify a low cutoff energy of 50 teraelectronvolts or an energy-independent diffusion scenario with delta equal to zero.

The forecast grew even more powerful when the authors combined spectral and spatial information. By converting the quality of fit between competing models into statistical significances, they found that after only 50 hours, CTA could distinguish a wide swath of currently viable TeV halo models at the coveted five-sigma level or beyond. Among the discriminated scenarios are models with a small halo radius of 10 parsecs, an inflated diffusion coefficient of 10^27 square centimeters per second, and several combinations of spectral and diffusion parameters that the gamma-ray spectrum alone cannot separate. Only a few degeneracies survive, such as that between the default model and one with an energy dependence delta of 0.5, and the measurements remain relatively insensitive to the precise values of the spin-down timescale and braking index.

The stakes of these measurements extend well beyond pulsar astrophysics. The confirmation that Geminga and Monogem channel roughly a tenth of their spin-down power into high-energy electrons and positrons lends strong support to the idea that pulsars are responsible for the puzzling excess of cosmic-ray positrons reported by the PAMELA and AMS-02 experiments, a long-standing anomaly in cosmic-ray physics. Moreover, because a pulsar’s radio beam is visible only to observers aligned with its magnetic axis, while its TeV halo glows isotropically, halos offer a way to discover the many pulsars whose radio beams never sweep across the Earth. Of the roughly 3400 pulsars catalogued to date, the vast majority were found at radio wavelengths, suggesting that a large hidden population awaits detection through its halo emission.

The study also leaves the door open to further complications. Recent observations by HAWC and LHAASO have hinted that the emission around Geminga may be anisotropic, potentially reflecting directional particle transport along ordered magnetic fields. Although Hooper and his colleagues restricted their analysis to spherically symmetric halo models, they note that CTA’s superior angular resolution makes it well suited to testing such asymmetric features in the future. As the arrays in both hemispheres come online and begin their surveys, the humble gamma-ray glow surrounding dead stars may transform from a curiosity into one of the sharpest probes available for the physics of cosmic-ray transport, the evolution of neutron stars, and perhaps even the origin of some of the most energetic particles ever detected at Earth.

Subject of Research: Forecasting the ability of the Cherenkov Telescope Array to probe cosmic-ray diffusion and particle injection in pulsar TeV halos

Article Title: Unraveling TeV halos with the Cherenkov Telescope Array

Article References: Hooper, D., Pinetti, E., & Sokolenko, A. (2026). Unraveling TeV halos with the Cherenkov Telescope Array. The European Physical Journal C, 86(9), Article 1048. https://doi.org/10.1140/epjc/s10052-026-16259-x

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16259-x

Keywords: TeV halos, pulsars, Cherenkov Telescope Array, Geminga, cosmic rays, gamma-ray astronomy, inverse Compton scattering, diffusion, HAWC, HESS, LHAASO, neutron stars