The Hybrid Nature of a PeVatron: How Pulsar J1849-0001 Powers Record Gamma-Ray Emission ✦
Context
The search for galactic sources capable of accelerating particles to energies around peta-electronvolts is one of the central tasks of modern astrophysics. Key candidates are neutron stars, especially pulsars, discovered by Jocelyn Bell Burnell and predicted by Fritz Zwicky. Their powerful winds create plerions—nebulae often formed after supernova explosions.
Methods
Using the GAMERA code and data from spectroscopy (measurements of radiation flux at different wavelengths) and photometry (brightness measurements), scientists built three models to explain the broadband spectrum of the source. In the hybrid model, plerion electrons scatter cosmic microwave background radiation (inverse Compton effect), while protons accelerated to PeV energies collide with hydrogen nuclei in a neighboring molecular cloud, producing neutral pions.
Results
The purely leptonic model, requiring an extreme cutoff of the electron spectrum at 600 TeV, and the hadronic model, underestimating the most energetic gamma-ray flux, proved untenable upon detailed comparison with data. The hybrid scenario, in which the cosmic-ray diffusion coefficient is suppressed to ~1% of the galactic value and the magnetic field in the nebula is 4 μG, perfectly reproduces the entire spectrum, including photons with energies of ~2 PeV. Remarkably, the most energetic emission spatially coincides with a molecular hydrogen cloud, which is natural for a hadronic origin.
Implications
The work confirms that pulsars within old supernova remnants can serve as real PeVatrons. This calls for a reassessment of the contribution of such objects to the overall budget of galactic cosmic rays and highlights the importance of hybrid models.
Future development
Future observations with spectroscopic telescopes and neutrino observatories such as NEON will finally disentangle the leptonic and hadronic channels. If a neutrino signal is detected, it will be direct evidence of hadronic processes—a kind of 'signature' of the accelerator.
Impact
The results will impact our understanding of the evolution of neutron stars and their environments, as well as strategies for searching for other galactic PeVatrons.
Next steps
Next steps include modeling the co-evolution of the pulsar and the molecular cloud, as well as calculating neutrino signals for future detectors.
Key open problems
The study is directly connected to the unsolved problem of the origin of cosmic rays in the knee region of the spectrum (energies ~1-10 PeV) and to the question of which objects dominate particle acceleration in the Galaxy.
🎯 The energy of 2 PeV is 200 times higher than the maximum proton energy at the Large Hadron Collider—such natural accelerators are still unattainable in laboratories.
Key numbers
- Maximum gamma-ray energy: ~2 PeV
- Distance to the pulsar: ~7 kpc
- Magnetic field in the nebula: ~4 μG
- Mass of the nearby molecular cloud: ~4×10^4 solar masses
- Diffusion coefficient: ~1% of the average galactic value