Burst oscillations during the rising phases of thermonuclear X-ray bursts are usually believed to originate from flame spreading on the neutron star surface. However, the decrease of fractional oscillation amplitude with rise time, which provides a main observational support for the flame spreading model, have so far been reported from only a few bursts. Moreover, the non-detection and intermittent detections of rise oscillations from many bursts are not yet understood considering the flame spreading scenario. Here, we report the decreasing trend of fractional oscillation amplitude from an extensive analysis of a large sample of Rossi X-ray Timing Explorer Proportional Counter Array bursts from ten neutron star low-mass X-ray binaries. This trend is 99.99% significant for the best case, which provides, to the best of our knowledge, by far the strongest evidence of such trend. Moreover, it is important to note that an opposite trend is not found from any of the bursts. The concave shape of the fractional amplitude profiles for all the bursts suggests latitude-dependent flame speeds, possibly due to the effects of the Coriolis force. We also systematically study the roles of low fractional amplitude and low count rate for non-detection and intermittent detections of rise oscillations, and attempt to understand them within the flame spreading scenario. Our results support a weak turbulent viscosity for flame spreading, and imply that burst rise oscillations originate from an expanding hot spot, thus making these oscillations a more reliable tool to constrain the neutron star equations of state.
Drawing the line between explosion and collapse in electron-capture supernovae -- I. Impact of conductive flame speeds and ignition conditions on the explosion mechanism
Heidelberger Institut für Theoretische Studien（海德堡理论研究所）；Theoretical Division, Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室理论部）；Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik（海德堡大学天文学中心理论天体物理研究所）；Max-Planck-Institut für Astrophysik（马克斯·普朗克天体物理学研究所）；Astrophysics Research Centre, Lennard-Jones Laboratories, Keele University（基尔大学天体物理研究中心）；Kavli IPMU (WPI), The University of Tokyo（东京大学 Kavli 宇宙粒子与宇宙学起源前沿国际研究中心（WPI））
Electron Capture Supernovae of Super-AGB Stars: Sensitivity on Input Physics
主序列上的$\sim$ 8 - 10 $M_{\odot}$恒星形成强电子简并O+Ne+Mg核心并成为超级AGB星。如果这样的O+Ne+Mg核增长到1.38 $M_\odot$，则发生在$^{20}$ Ne $(e,ν_e)^{20}$ F $(e,ν_e)^{20}$ O上的电子捕获，并点燃中心周围的O-Ne爆燃。在本文中，我们进行二维流体动力学模拟的O-Ne火焰的传播，看看这样的火焰是否会导致崩溃的O+Ne+Mg核心，由于随后的电子捕获背后的火焰或引发热核爆炸。我们提出了一系列的模型，探讨结果如何依赖于模型参数的中心密度范围从$10^{9.80}$到$10^{10.20}$ g cm $^{-3}$，火焰结构的中心和偏心点火内核，特殊和广义相对论效应，湍流火焰速度公式和层流燃烧阶段的处理。我们发现，从恒星演化模型中得到的O+Ne+Mg核有很高的坍缩成中子星的倾向。我们得到了电子俘获坍缩与热核爆炸之间的分歧。我们讨论了这类超新星在核合成中的意义和可能的观测信号。
