Off-axis synchrotron light curves from full-time-domain moving-mesh simulations of jets from massive stars

We present full-time-domain, moving-mesh, relativistic hydrodynamic simulations of jets launched from the center of a massive progenitor star and compute the resulting synchrotron light curves for observers at a range of viewing angles. We follow jet evolution from ignition inside the stellar center, propagation in the stellar envelope and breakout from the stellar surface, then through the coasting and deceleration phases. The jet compresses into a thin shell, sweeps up the circumstellar medium, and eventually enters the Newtonian phase. The jets naturally develop angular and radial structure due to hydro-dynamical interaction with surrounding gas. The calculated synchrotron light curves cover the observed temporal range of prompt to late afterglow phases of long gamma-ray bursts (LGRBs). The on-axis light curves exhibit an early emission pulse originating in shock-heated stellar material, followed by a shallow decay and a later steeper decay. The off-axis light curves rise earlier than previously expected for top-hat jet models -- on a time scale of seconds to minutes after jet breakout, and decay afterwards. Sometimes the off-axis light curves have later re-brightening components that can be contemporaneous with SNe Ic-bl emission. Our calculations may shed light on the structure of GRB outflows in the afterglow stage. The off-axis light curves from full-time-domain simulations advocate new light curve templates for the search of off-axis/orphan afterglows

Numerical simulations of the jet dynamics and synchrotron radiation of binary neutron star merger event GW170817/GRB170817A

We present numerical simulations of energetic flows propagating through the debris cloud of a binary neutron star (BNS) merger. Starting from the scale of the central engine, we use a moving-mesh hydrodynamics code to simulate the complete dynamical evolution of the produced relativistic jets. We compute synchrotron emission directly from the simulations and present multi-band light curves of the early (sub-day) through late (weeks to years) afterglow stages. Our work systematically compares two distinct models for the central engine, referred to as the narrow and wide engine scenario, which is associated with a successful structured jet and a quasi-isotropic explosion respectively. Both engine models naturally evolve angular and radial structure through hydrodynamical interaction with the merger debris cloud. They both also result in a relativistic blast wave capable of producing the observed multi-band afterglow data. However, we find that the narrow and wide engine scenario might be differentiated by a new emission component that we refer to as a merger flash. This component is a consequence of applying the synchrotron radiation model to the shocked optically thin merger cloud. Such modeling is appropriate if injection of non-thermal electrons is sustained in the breakout relativistic shell, for example by internal shocks or magnetic reconnection. The rapidly declining signature may be detectable for future BNS mergers during the first minutes to day following the GW chirp. Furthermore, its non-detection for the GRB170817A event may disfavor the wide, quasi-isotropic explosion model

An Apparent Redshift Dependence of Quasar Continuum: Implication for Cosmic Dust Extinction?

We investigate the luminosity and redshift dependence of the quasar continuum by means of composite spectrum using a large non-BAL radio-quiet quasar sample drawn from the Sloan Digital Sky Survey. Quasar continuum slopes in the UV-Opt band are measured at two different wavelength ranges, i.e., $\alpha_{\nu12}$ ($1000\sim 2000 \rm\AA$) and $\alpha_{\nu24}$ ($2000 \sim 4000 \rm\AA$) derived from power law fitting. Generally, the UV spectra slope becomes harder (higher $\alpha_{\nu}$) towards higher bolometric luminosity. On the other hand, when quasars are further grouped into luminosity bins, we find both $\alpha_{\nu12}$ and $\alpha_{\nu24}$ show significant anti-correlation with redshift (i.e., quasar continuum becomes redder towards higher redshift). We suggest that the cosmic dust extinction is very likely the cause of this observed $\alpha_\nu-z$ relation. We build a simple cosmic dust extinction model to quantify the observed reddening tendency and find an effective dust density $n\sigma_v \sim 10^{-5}h~\rm Mpc^{-1}$ at $z<1.5$. The other possibilities that could produce such a reddening effect have also been discussed.Comment: 6 pages, 5 figures; published in ApJ

Responsible Task Automation: Empowering Large Language Models as Responsible Task Automators

The recent success of Large Language Models (LLMs) signifies an impressive stride towards artificial general intelligence. They have shown a promising prospect in automatically completing tasks upon user instructions, functioning as brain-like coordinators. The associated risks will be revealed as we delegate an increasing number of tasks to machines for automated completion. A big question emerges: how can we make machines behave responsibly when helping humans automate tasks as personal copilots? In this paper, we explore this question in depth from the perspectives of feasibility, completeness and security. In specific, we present Responsible Task Automation (ResponsibleTA) as a fundamental framework to facilitate responsible collaboration between LLM-based coordinators and executors for task automation with three empowered capabilities: 1) predicting the feasibility of the commands for executors; 2) verifying the completeness of executors; 3) enhancing the security (e.g., the protection of users' privacy). We further propose and compare two paradigms for implementing the first two capabilities. One is to leverage the generic knowledge of LLMs themselves via prompt engineering while the other is to adopt domain-specific learnable models. Moreover, we introduce a local memory mechanism for achieving the third capability. We evaluate our proposed ResponsibleTA on UI task automation and hope it could bring more attentions to ensuring LLMs more responsible in diverse scenarios. The research project homepage is at https://task-automation-research.github.io/responsible_task_automation

The Stokes-Einstein Relation at Moderate Schmidt Number

The Stokes-Einstein relation for the self-diffusion coefficient of a spherical particle suspended in an incompressible fluid is an asymptotic result in the limit of large Schmidt number, that is, when momentum diffuses much faster than the particle. When the Schmidt number is moderate, which happens in most particle methods for hydrodynamics, deviations from the Stokes-Einstein prediction are expected. We study these corrections computationally using a recently-developed minimally-resolved method for coupling particles to an incompressible fluctuating fluid in both two and three dimensions. We find that for moderate Schmidt numbers the diffusion coefficient is reduced relative to the Stokes-Einstein prediction by an amount inversely proportional to the Schmidt number in both two and three dimensions. We find, however, that the Einstein formula is obeyed at all Schmidt numbers, consistent with linear response theory. The numerical data is in good agreement with an approximate self-consistent theory, which can be used to estimate finite-Schmidt number corrections in a variety of methods. Our results indicate that the corrections to the Stokes-Einstein formula come primarily from the fact that the particle itself diffuses together with the momentum. Our study separates effects coming from corrections to no-slip hydrodynamics from those of finite separation of time scales, allowing for a better understanding of widely observed deviations from the Stokes-Einstein prediction in particle methods such as molecular dynamics.Comment: Submitte

Thermal analysis of FeCoCu pre-alloyed powders used for diamond tools

By simulating the pressureless sintering process, the thermal effects of FeCoCu pre-alloyed powders have been investigated. According to the notions of the Kissinger method, the activation energies in the expansion-shrinkage conversion stage are analyzed. Results show that with Fe content increasing, the specimens’ specific heat capacity values present the increasing trend. The 25 %Fe–15 %Co–60 %Cu specimens have negative enthalpy value at 10 and 20°C/min heating rate but positive values at 30 °С/min. For the specimens with lower Cu content, the enthalpies are always positive. It is established that both the specific heat capacity and enthalpy are larger when at higher heating rates. The activation energy of the 65 %Fe–15 %Co–20 %Cu specimens is 10 times higher than that of the 25 %Fe–15 %Co–60 %Cu specimens and the 45 %Fe–15 %Co–40 %Cu specimens.При моделюванні процесу спікання без тиску досліджено термічні ефекти в попередньо легованих порошках FeCoCu. З використанням методу Кіссінджера проаналізовано енергію активації на стадії розширення–усадка. Результати показують, що при збільшенні вмісту Fe значення питомої теплоємності демонструють тенденцію до зростання. Зразки 25 %Fe–15 %Co–60 %Cu мають негативні значення ентальпії при швидкості нагріву 10 ° і 20 °С/хв, але позитивні при 30 °С/хв. Для зразків з меншим вмістом Cu ентальпія завжди позитивна. Встановлено, що питома теплоємність і ентальпія більші при більш високіх швидкостях нагрівання. Енергія активації зразків 65 %Fe–15 %Co–20 %Cu у 10 разів вища, ніж зразків 25 %Fe–15 %Co–60 %Cu і 45 %Fe–15 %Co–40 %Cu.При моделировании процесса спекания без приложения давления исследованы термические эффекты в предварительно легированных порошках FeCoCu. С использованием метода Киссинджера проанализирована энергия активации на стадии расширение–усадка. Результаты показывают, что с увеличением содержания железа значения удельной теплоемкости образцов демонстрируют тенденцию к повышению. Образцы 25 %Fe–15 %Co–60 %Cu имеют отрицательные значения энтальпии при скорости нагрева 10 и 20 °С/мин, но положительные при 30 °С/мин. Для образцов с меньшим содержанием Cu энтальпия всегда положительна. Установлено, что удельная теплоемкость и энтальпия больше при более высоких скоростях нагрева. Энергия активации образцов 65 %Fe–15 %Co–20 %Cu в 10 раз выше, чем образцов 25 %Fe–15 %Co–60 %Cu и 45 %Fe–15 %Co–40 %Cu