T Coronae Borealis, at 0.8 Kpc, Promises First Nova Neutrino Detection

Recurrent novae, stellar explosions on the surfaces of white dwarf stars, are increasingly recognised as powerful particle accelerators, but the precise mechanisms driving this phenomenon remain elusive. Prantik Sarmah, Sovan Chakraborty, and Xilu Wang, from the Chinese Academy of Sciences and the Indian Institute of Technology Guwahati, are leading efforts to predict the multi-messenger signals, namely gamma rays and neutrinos, from the anticipated outburst of T Coronae Borealis. Building on the recent detection of gamma rays from a similar nova, RS Ophiuchi, this research establishes that T Coronae Borealis, being significantly closer to Earth, presents a unique opportunity to detect neutrinos, potentially confirming the presence of hadronic acceleration processes. The team’s modelling explores two distinct acceleration mechanisms and demonstrates that, while gamma-ray detection is likely across multiple observatories, neutrino detection hinges on the specific conditions within the nova system, with magnetic reconnection offering the most promising pathway for a definitive signal.

Novae and the Search for High-Energy Emission

Classical novae are increasingly recognized as potential sources of high-energy particles, including gamma-rays, neutrinos, and cosmic rays, extending beyond their traditional classification as purely optical events. Research focuses on understanding how nova eruptions, particularly those involving magnetic white dwarfs, can accelerate particles to relativistic speeds, and the implications for multi-messenger astronomy. Observing novae across the electromagnetic spectrum, alongside neutrino and cosmic ray detection, is crucial for validating these particle acceleration models. The magnetic field of the white dwarf plays a key role in enhancing particle acceleration and potentially increasing detectable high-energy emission.

Scientists predict gamma-ray emission from novae through both processes involving electrons and protons, with proton-proton interactions favored as they also produce detectable neutrinos. Symbiotic novae, featuring a red giant companion star, are considered particularly promising for high-energy emission due to their system characteristics. Theoretical models of particle acceleration, magnetic reconnection, and radiative processes are used to predict expected signals, despite challenges posed by the infrequent nature of nova outbursts and the need for highly sensitive detectors. Particle acceleration occurs through mechanisms like shock acceleration, where particles gain energy crossing shocks created by expanding nova ejecta, and magnetic reconnection, which releases energy from the magnetic field. Accelerated electrons produce synchrotron radiation, contributing to gamma-ray emission, while accelerated protons collide with other particles, also generating gamma-rays and neutrinos. The research investigates whether novae accelerate particles via hadronic (proton-based) or leptonic (electron-based) processes, a question currently unresolved following observations of the 2021 outburst of RS Ophiuchi. Detailed models of particle acceleration were constructed using two mechanisms: an external shock, formed by the collision of ejected material with the stellar wind, and magnetic reconnection near the white dwarf star. These models predict both gamma-ray and neutrino fluxes, allowing scientists to distinguish between the two acceleration scenarios.

The magnetic reconnection model uniquely predicts that gamma-rays will be absorbed within the nova system, allowing only neutrinos to escape, potentially creating a temporal delay between observed photons and neutrinos. The external shock model predicts detectable gamma-ray signals across multiple facilities, including LHAASO, Fermi-LAT, and others, but offers limited prospects for neutrino detection with IceCube or KM3NeT. To accurately estimate expected fluxes, scientists accounted for the absorption of gamma-rays through interactions with low-energy optical photons, incorporating luminosity and temperature values derived from observations of RS Ophiuchi. While gamma-ray attenuation is typically minimal, it can become significant at higher optical luminosities. Neutrinos are unaffected by absorption, but their potential flavor oscillation during propagation was considered, finding that oscillations are suppressed at higher energies. This work presents the first model-based estimates of gamma-ray and neutrino fluxes expected from T CrB during an outburst, assessing their detectability with current and future observatories. The research focuses on two mechanisms for accelerating protons: an external shock driven by the interaction of ejected material with the red giant wind, and magnetic reconnection near the white dwarf surface. The magnetic reconnection scenario predicts a unique temporal signature, potentially delivering neutrino signals hours before the arrival of photons or neutrinos originating from the external shock.

Calculations demonstrate that gamma-rays are detectable across multiple facilities for a benchmark external shock model, but the predicted neutrino flux remains below the sensitivity limits of IceCube and KM3NeT, mirroring the non-detection of neutrinos from the RS Ophiuchi outburst. The team estimates a maximum proton energy of approximately 1 TeV, limited by dynamical losses within the nova shock. Calculations, calibrated using observations of RS Ophiuchi, provide crucial predictions for the upcoming outburst of T CrB, offering a unique opportunity to observe high-energy neutrinos from a nova and test current astrophysical models. However, the source of these gamma-rays, whether they originate from interactions involving electrons or protons, remains uncertain due to the lack of corresponding neutrino detections. While the external shock model predicts detectable gamma-rays, the prospects for neutrino detection remain limited. Conversely, the magnetic reconnection model offers significantly improved potential for neutrino detection with facilities like IceCube and KM3NeT. Even a non-detection of high-energy neutrinos would provide crucial constraints on existing theories and guide future research into the physics of these energetic events, complementing existing studies of lower-energy neutrinos from novae.