High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: This work is concerned with advancing multi-fluid models in General Relativity, and in particular focuses on the modelling of dissipative fluids and turbulent flows. Such models are required for an accurate description of neutron star phenomenology, and binary neutron star mergers in particular. In fact, the advent of multi-messenger astronomy offers exciting prospects for exploring the extreme physics at play during such cosmic fireworks. We first focus on modelling dissipative fluids in relativity, and explore the arguably unique model that is ideally suited for describing dissipative multi-fluids in General Relativity. Modelling single fluids in relativity is already a hard task, but for neutron stars it is easy to argue that we need to understand even more complicated settings: the presence of superfluid/superconducting mixtures, for example, means that we need to go beyond single-fluid descriptions. We then consider turbulent flows and focus on how to perform "filtering" in a curved spacetime setting. We do so as most recent turbulent models in a Newtonian setting are based on the notion of spatial filtering. As the same strategy is beginning to be applied in numerical relativity, we focus on the foundational underpinnings and propose a novel scheme for carrying out filtering, ensuring consistency with the tenets of General Relativity. Finally, we discuss two applications of relevance for binary neutron star mergers. We focus on the modelling of ($\beta$-)reactions in neutron star simulations, and provide a discussion of the magneto-rotational instability that is suited to highly dynamical environments like mergers. We focus on these two problems as reactions are expected to source the dominant dissipative contribution to the overall dynamics, while the magneto-rotational instability is considered crucial for sustaining the development of turbulence in mergers.

Abstract: A fast artificial neural network is developed for the prediction of cosmic ray transport in turbulent astrophysical magnetic fields. The setup is trained and tested on bespoke datasets that are constructed with the aid of test-particle numerical simulations of relativistic cosmic ray dynamics in synthetic stochastic fields. The neural network uses, as input, particle and field properties and estimates transport coefficients 10^7 faster than standard numerical simulations with an overall error of ~5% .

Abstract: We present an axisymmetric failed supernova simulation beyond black hole formation, for the first time with numerical relativity and two-moment multi energy neutrino transport. To ensure stable numerical evolution, we use an excision method for neutrino radiation-hydrodynamics within the inner part of black hole domain. We demonstrate that our excision method is capable to stably evolve the radiation-hydrodynamics in dynamical black hole spacetime. As a remarkable signature of the final moment of PNS, we find the emergence of high energy neutrinos. Those high energy neutrinos are associated with the proto-neutron star shock surface being swallowed by the central black hole and could be a possible observable of failed supernovae.

Abstract: Binary coalescences are known sources of gravitational waves (GWs) and they encompass combinations of black holes (BHs) and neutron stars (NSs). Here we show that when BHs are embedded in magnetic fields ($B$s) larger than approximately $10^{10}$ G, charged particles colliding around their event horizons can easily have center-of-mass energies in the range of ultra-high energies ($\gtrsim 10^{18}$ eV) and escape. Such B-embedding and high-energy particles can take place in BH-NS binaries, or even in BH-BH binaries with one of the BHs being charged (with charge-to-mass ratios as small as $10^{-5}$, which do not change GW waveforms) and having a residual accretion disk. Ultra-high center-of-mass energies for particle collisions arise for basically any rotation parameter of the BH when $B \gtrsim 10^{10}$ G, meaning that it should be a common aspect in binaries, especially in BH-NS ones given the natural presence of a $B$ onto the BH and charged particles due to the NS's magnetosphere. We estimate that up to millions of ultra-high center-of-mass collisions may happen before the merger of BH-BH and BH-NS binaries. Thus, binary coalescences can also be efficient sources of ultra-high energy cosmic rays (UHECRs) and constraints to NS/BH parameters would be possible if UHECRs are detected along with GWs.

Abstract: We present the first multi-band optical light curves of PSR J1622-0315, among the most compact known redback binary millisecond pulsars, with an orbital period Porb=3.9 h. We find a flux modulation with two maxima per orbital cycle and a peak-to-peak amplitude of about 0.3 mag, which we attribute to the ellipsoidal shape of the tidally distorted companion star. The optical colours imply a late-F to early-G spectral type companion and do not show any detectable temperature changes along the orbit. This suggests that the irradiation of the star's inner face by the pulsar wind is unexpectedly missing despite its short orbital period. To interpret these results, we introduce a new parameter fsd, defined as the ratio between the pulsar wind flux intercepted by the companion star and the companion intrinsic flux. This flux ratio fsd, which depends on the spin-down luminosity of the pulsar, the base temperature of the companion and the orbital period, can be used to quantify the effect of the pulsar wind on the companion star and turns out to be the most important factor in determining whether the companion is irradiated or not. We find that the transition between these two regimes occurs at fsd=2-4 and that the value for PSR J1622-0315 is fsd=0.7, placing it firmly in the non-irradiated regime.