Tidally Disrupted Stars as Potential Sources of Ultra-High Energy Cosmic Rays and Neutrinos
The paper "Tidally Disrupted Stars as a Possible Origin of both Cosmic Rays and Neutrinos at the Highest Energies" explores the hypothesis that Tidal Disruption Events (TDEs) involving massive or supermassive black holes may account for the observed fluxes of Ultra-High Energy Cosmic Rays (UHECRs) and PeV scale neutrinos. Specifically, it examines the generation of these phenomena in scenarios where stars with heavier compositions, such as carbon-oxygen white dwarfs, are disrupted. The paper proposes that jetted TDEs, through photo-hadronic interactions, can serve as simultaneous sources of UHECRs and neutrinos, providing an alternative astrophysical candidate to hitherto considered sources like Gamma-Ray Bursts (GRBs) and Blazars, which previous data analyses do not favor.
High-energy astrophysical neutrinos have been detected at about 0.1-1 PeV energies, presumably originating from sources beyond our galaxy. The neutrinos' association with cosmic rays originates from similar energy budgeting considerations implying a common source is plausible. However, identification of such sources has been elusive, motivating the exploration of alternative candidates such as TDEs. In the context of TDEs, a star disrupted by the strong gravitational influence of a nearby black hole forms an accretion disk, which can potentially launch a relativistic jet. This jet is capable of accelerating protons or nuclei to ultra-high energies, giving rise to neutrinos as secondary products.
The paper provides an extensive numerical simulation to demonstrate that TDE jets can account for observed UHECR and neutrino data. The authors employ models that simulate the nuclear cascade in the jet induced by photo-hadronic interactions, leveraging techniques previously successful in explaining phenomena attributed to GRBs. Their methodology is grounded in established physics, modeling both the TDE jet emissions and UHECR propagation through extragalactic space. By adopting parameters informed by observations of specific jetted TDEs such as Swift J1644+57, and assuming stars with mid-to-heavy compositions (notably carbon-oxygen compositions akin to white dwarfs), they illustrate how such a model can fit both the UHECR and neutrino data.
The paper finds that a nuclear cascade develops optimally in the jet when the TDE promotes sufficient radiation density, resulting in efficient photo-nuclear interactions and subsequent disintegration of nuclei. For instance, they simulate scenarios with pure 14N injection in the jet, which is representative of a carbon-oxygen composition. Their simulations indicate that with this specific nuclear injection composition, both UHECRs and astrophysical neutrino observations can be explained coherently, contingent on an adequate local rate of jetted TDEs and appropriate jet baryonic loadings.
Key numerical results from the paper indicate that certain parameter choices, such as luminosity and jet production radius, allow for compatibility with UHECR and neutrino observations. The optimal parameter space suggests a nuclear survival regime where observed results align with expectations. Notably, the best fit to the data suggests a parameter space where TDE jets are not optically thick to interactions, meaning that only partial disintegration occurs for high-energy nuclei.
While the model holds promise, the authors discuss its limitations and the need for further investigations. The evolution of these events with redshift, potential discrepancies in nuclear cross-section data, and the uncertainties related to the extragalactic background light are among areas requiring further scrutiny. Moreover, they propose potential variations on the model such as different jet compositions and alternative rates of these events over cosmic time.
The paper's implications offer significant theoretical and practical bearings. By advocating for TDEs as potential sources of the highest energy cosmic rays and neutrinos, it challenges prior assumptions regarding astrophysical accelerators and posits new directions for both observational astronomy and theoretical modeling. Future research facilitated by more observational data of jetted TDEs, along with advancements in multi-messenger astronomy, will be essential in further substantiating or refuting the viability of this model, potentially significantly impacting our understanding of cosmic ray origins.