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1. Small-scale Magnetic Fields Are Critical to Shaping Solar Gamma-Ray Emission

   https://doi.org/10.3847/1538-4357/ad158f  

   Li_李, Jung-Tsung 融宗 ; Beacom, John F ; Griffith, Spencer ; Peter, Annika_H G ( January 2024 , The Astrophysical Journal)

   The Sun is a bright gamma-ray source due to hadronic cosmic-ray interactions with solar gas. While it is known that incoming cosmic rays must generally first be reflected by solar magnetic fields to produce outgoing gamma rays, theoretical models have yet to reproduce the observed spectra. We introduce a simplified model of the solar magnetic fields that captures the main elements relevant to gamma-ray production. These are a flux tube, representing the network elements, and a flux sheet, representing the intergranular sheets. Both the tube and sheet have a horizontal size of order 100 km and serve as sites where cosmic rays are reflected and gamma rays are produced. While our simplified double-structure model does not capture all the complexities of the solar-surface magnetic fields, such as Alfvén turbulence from wave interactions or magnetic fluctuations from convection motions, it improves on previous models by reasonably producing both the hard spectrum seen by Fermi Large Area Telescope at 1–200 GeV and the considerably softer spectrum seen by the High Altitude Water Cherenkov Observatory (HAWC) at near 10³GeV. We show that lower-energy (≲10 GeV) gamma rays are primarily produced in the network elements and higher-energy (≳few × 10 GeV) gamma rays in the intergranular sheets. Notably, the spectrum softening observed by HAWC results from the limited effectiveness of capturing and reflecting ∼10⁴GeV cosmic rays by the finite-sized intergranular sheets. Our study is important for understanding cosmic-ray transport in the solar atmosphere and will lead to insights into small-scale magnetic fields at the photosphere.

    

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2. Core-collapse supernovae in dense environments – particle acceleration and non-thermal emission

   https://doi.org/10.1093/mnras/stac2234  

   Brose, R. ; Sushch, I. ; Mackey, J. ( August 2022 , Monthly Notices of the Royal Astronomical Society)

   Supernova remnants (SNRs) are known to accelerate cosmic rays from the detection of non-thermal emission in radio waves, X-rays, and gamma-rays. However, the ability to accelerate cosmic rays up to PeV energies has yet to be demonstrated. The presence of cut-offs in the gamma-ray spectra of several young SNRs led to the idea that PeV energies might only be achieved during the first years of a remnant’s evolution. We use our time-dependent acceleration-code RATPaC to study the acceleration of cosmic rays in supernovae expanding into dense environments around massive stars. We performed spherically symmetric one-dimensional (1D) simulations in which we simultaneously solve the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. We investigated typical circumstellar-medium (CSM) parameters expected around red supergiant (RSG) and luminous blue variable (LBV) stars for freely expanding winds and accounted for the strong γγ absorption in the first days after explosion. The maximum achievable particle energy is limited to below $600\,$TeV even for the largest considered values of the magnetic field and mass-loss rates. The maximum energy is not expected to surpass $\approx 200\,$ and $\approx 70\,$TeV for LBVs and RSGs that experience moderate mass-loss prior to the explosion. We find gamma-ray peak-luminosities consistent with current upper limits and evaluate that current-generation instruments are able to detect the gamma-rays from Type-IIP explosions at distances up to $\approx 60\,$ kpc and Type-IIn explosions up to $\approx 1.0\,$ Mpc. We also find a good agreement between the thermal X-ray and radio synchrotron emission predicted by our models with a range of observations.

    

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3. Dark Matter In Extreme Astrophysical Environments

   Baryakhtar, M. ; Caputo, R. ; Croon, D. ; Perez, K. ; Berti, E. ; Bramante, J. ; Buschmann, M. ; Brito, R. ; Cole, P.S. ; Coogan, A. ; et al ( March 2022 , ArXivorg)

   Exploring dark matter via observations of extreme astrophysical environments -- defined here as heavy compact objects such as white dwarfs, neutron stars, and black holes, as well as supernovae and compact object merger events -- has been a major field of growth since the last Snowmass process. Theoretical work has highlighted the utility of current and near-future observatories to constrain novel dark matter parameter space across the full mass range. This includes gravitational wave instruments and observatories spanning the electromagnetic spectrum, from radio to gamma-rays. While recent searches already provide leading sensitivity to various dark matter models, this work also highlights the need for theoretical astrophysics research to better constrain the properties of these extreme astrophysical systems. The unique potential of these search signatures to probe dark matter adds motivation to proposed next-generation astronomical and gravitational wave instruments. Note: Contribution to Snowmass 2021 -- CF3. Dark Matter: Cosmic Probes 

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4. High‐Energy Neutrinos from the Cosmos

   https://doi.org/10.1002/andp.202100309  

   Halzen, Francis ( September 2021 , Annalen der Physik)

   The IceCube project transformed a cubic kilometer of transparent natural Antarctic ice into a Cherenkov detector. It discovered PeV‐energy neutrinos originating beyond our galaxy with an energy flux that is comparable to that of GeV‐energy gamma rays and EeV‐energy cosmic rays. These neutrinos provide the only unobstructed view of the cosmic accelerators that power the highest energy radiation reaching us from the universe. The results from IceCube's first decade of operations, foremost the measurement of the diffuse neutrino flux from the universe using multiple techniques is reviewed. The multimessenger data that identified the supermassive black hole TXS 0506+056 as a source of cosmic neutrinos is subsequently reviewed and attention is drawn to accumulating indications that cosmic neutrinos are associated with gamma‐ray‐obscured active galaxies, that is, the energy in gamma rays that accompanies cosmic neutrinos emerges at MeV energies, or below. Reaching beyond 10 PeV energy, cosmic neutrinos provide a natural beam to study neutrinos themselves.

    

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5. Oscillations of highly magnetized non-rotating neutron stars

   https://doi.org/10.1038/s42005-022-01112-w  

   Leung, Man Yin ; Yip, Anson Ka Long ; Cheong, Patrick Chi-Kit ; Li, Tjonnie Guang Feng ( December 2022 , Communications Physics)

   Highly magnetized neutron stars are promising candidates to explain some of the most peculiar astronomical phenomena, for instance, fast radio bursts, gamma-ray bursts, and superluminous supernovae. Pulsations of these highly magnetized neutron stars are also speculated to produce detectable gravitational waves. In addition, pulsations are important probes of the structure and equation of state of the neutron stars. The major challenge in studying the pulsations of highly magnetized neutron stars is the demanding numerical cost of consistently solving the nonlinear Einstein and Maxwell equations under minimum assumptions. With the recent breakthroughs in numerical solvers, we investigate pulsation modes of non-rotating neutron stars which harbour strong purely toroidal magnetic fields of 10¹⁵⁻¹⁷G through two-dimensional axisymmetric general-relativistic magnetohydrodynamics simulations. We show that stellar oscillations are insensitive to magnetization effects until the magnetic to binding energy ratio goes beyond 10%, where the pulsation mode frequencies are strongly suppressed. We further show that this is the direct consequence of the decrease in stellar compactness when the extreme magnetic fields introduce strong deformations of the neutron stars.

    

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