高能暂现源 · 每日文献综述

At every scale they occupy, magnetic fields affect various phenomena, including star formation, cosmic ray transport, charged particle acceleration, space weather, transport in planetary atmospheres, and laboratory plasmas. These fields are often generated and sustained by turbulent flows in a process called the dynamo. In 1955, E. N. Parker parameterized the effects of small-scale turbulence to propose a mean-field dynamo theory. The widely used theory reproduces observed large-scale fields but suffers from difficulty in tuning parameters as they are not justified from first principles: Studies of turbulent flows show tangled magnetic fields, which are folded and fragmented into small-scale structures due to shear-flow straining. Here, considering a shear flow that is unstable and driven, we develop analytic theory and perform three-dimensional (3D), advanced computer simulations of turbulence with up to 4096 x 4096 x 8192 grid points, showing ab initio generation of quasi-periodic, large-scale magnetic fields. The generation occurs via the mean-vorticity effect---an additional mean-field dynamo process postulated in 1990. Crucial to this dynamo is the prior generation of large-scale 3D jets, robustly produced as topologically protected and exact nonlinear solutions of the magnetohydrodynamic equations. The jet-driven dynamo applies to shear-driven laboratory and astrophysical systems. These include binary neutron star mergers, where the reported dynamo likely operates on microsecond timescales to produce in milliseconds some of the strongest magnetic fields in the Universe, providing signals for multimessenger astronomy.

Gravitational wave detectors produce time series of the gravitational wave strain coadded with instrument noise. For evenly sampled data, such as from laser interferometers, it has been traditional to Fourier transform the data and perform analyses in the frequency domain. The motivation being that the Fourier domain noise covariance matrix will be diagonal if the noise properties are constant in time, which greatly simplifies and accelerates the analysis. However, if the noise is nonstationary this advantage is lost. It has been proposed that the time-frequency or wavelet domain is better suited for studying nonstationary noise, at least when the time variation is suitably slow, since then the wavelet domain noise covariance matrix is, to a good approximation, diagonal. Here we investigate the conditions under which the diagonal approximation is appropriate for the case of the Wilson-Daubechies-Meyer (WDM) wavelet packet basis, which is seeing increased use in gravitational wave data analysis. We show that so long as the noise varies slowly across a wavelet pixel, in both time and frequency, the WDM noise correlation matrix is well approximated as diagonal. The off-diagonal terms are proportional to the time and frequency derivatives of the dynamic spectral model. The same general picture should apply to other discrete wavelet transforms with wavelet filters that are suitably compact in time and frequency. Strategies for handling data with rapidly varying noise that violate these assumptions are discussed.

The detected Gaia systems hosting compact objects challenge standard models of binary star evolution. In particular, if the observed black hole (BH) systems evolved in isolation, they are expected to have undergone a mass transfer phase. Given their highly unequal masses, such mass transfer is dynamically unstable within standard models, leading to a stellar merger or a short-period binary. In contrast, the observed systems have much wider orbits than predicted, making their formation within conventional evolutionary frameworks difficult to reconcile. Using detailed binary evolution calculations, we test whether nonconservative mass transfer, in which most of the mass is lost from the system carrying the specific angular momentum of the donor's center of mass, can explain the properties of two Gaia BH systems. This mass-loss geometry differs from standard isotropic re-emission from the accretor's vicinity. We find that our mass-loss geometry model reproduces the orbital periods of the two Gaia BH systems remarkably well over a wide range of initial conditions, offering a plausible formation pathway. We speculate this may point to enhanced eruptive mass loss, potentially driven by high-opacity subsurface layers in the donor prior to Roche-lobe overflow, consistent with preferentially bipolar outflows observed in luminous blue variables. Alternatively, it may indicate the need for more sophisticated mass-transfer prescriptions that account for highly unequal Roche-lobe sizes, subsynchronous rotation, and possible self-accretion. Similar mechanisms may operate in other post-mass-transfer systems facing analogous evolutionary challenges, including Gaia neutron-star and white-dwarf binaries, stripped-envelope Wolf─Rayet stars, and low-mass X-ray binaries.