Cosmic energy sources
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Ultrahigh-Energy Cosmic Rays: Source Candidates and Constraints
Active Galactic Nuclei, Supermassive Black Holes, and Relativistic Jets
Active Galactic Nuclei (AGN) and their relativistic jets are among the most promising sources of ultrahigh-energy cosmic rays (UHECRs). AGNs can accelerate particles through mechanisms such as first-order Fermi acceleration at shocks and stochastic shear acceleration in large-scale jets. Observational data show a correlation between the arrival directions of the highest-energy cosmic rays and the positions of nearby AGNs, supporting the idea that these objects are significant contributors to the UHECR population. Additionally, supermassive black holes (SMBHs) at the centers of galaxies may accelerate particles to extreme energies via processes like the magnetic Penrose process, which can energize protons to above 10^20 eV without requiring fine-tuned conditions or extended acceleration zones. These findings suggest that AGNs and SMBHs, especially those in nearby radio galaxies, are strong candidates for the sources of UHECRs Tursunov2020Abraham2007Rieger2022.
Binary Neutron Star Mergers and Pulsars
Recent research proposes that binary neutron star (BNS) mergers could be responsible for producing UHECRs. The jets generated during these mergers are thought to be nearly identical due to the narrow range of neutron star masses, which could explain the observed narrow rigidity range of UHECRs. This scenario also predicts possible coincidences between high-energy neutrinos and gravitational waves, providing a testable signature for this source class. New-born pulsars have also been considered as potential UHECR sources, but current neutrino observations place strong constraints on their contribution Aartsen2016Farrar2024.
Gamma-Ray Bursts and PeVatrons
Gamma-ray bursts (GRBs) have long been considered as possible sources of UHECRs. However, models assuming a pure proton composition and internal shock acceleration face challenges, such as requiring unrealistically high energy budgets and producing neutrino fluxes that exceed observational limits. While GRBs may still contribute to UHECRs under certain escape scenarios, their role is likely subdominant compared to other sources. In the Milky Way, so-called "PeVatrons"—cosmic accelerators capable of producing particles up to petaelectronvolt energies—have been identified, but their exact nature remains uncertain. Candidates include pulsar wind nebulae, supernova remnants, and star-forming regions, though only the Crab Nebula has been firmly identified as a PeVatron Aartsen2016Cao2021Baerwald2014.
Multimessenger Constraints and Source Evolution
A multimessenger approach, combining data from cosmic rays, neutrinos, and gamma rays, helps constrain the properties and evolution of UHECR sources. Models that fit the observed spectrum and composition often require sources to accelerate a mix of nuclear masses or a Milky Way-like composition, with the nearest source being tens of megaparsecs away. The energy generation rate and spectral hardness of cosmic rays suggest either reacceleration within the galaxy or the presence of extragalactic sources with hard spectra. Notably, the lack of a strong cosmological evolution in the observed neutrino flux disfavors source classes with rapid evolution, such as AGNs and GRBs, if UHECRs are proton-dominated Aartsen2016Muzio2019Murase2018.
Gamma-Ray Dim Sources and Unresolved Populations
Recent studies indicate that the sources of the highest-energy cosmic rays may not be prominent gamma-ray emitters. Simulations show that known gamma-ray sources cannot account for the observed anisotropy in UHECR arrival directions, implying the existence of a population of UHECR sources that are either gamma-ray dim or unresolved by current telescopes .
Conclusion
The search for the origins of cosmic energy sources, especially UHECRs, points to a diverse set of astrophysical objects, including AGNs, SMBHs, BNS mergers, and possibly GRBs and Galactic PeVatrons. Multimessenger observations and improved modeling continue to refine our understanding, but current evidence suggests that the most powerful cosmic accelerators are likely extragalactic, with some remaining hidden from gamma-ray observations. The identification and characterization of these sources remain a central challenge in high-energy astrophysics Aartsen2016Farrar2024Tursunov2020+7 MORE.
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