研究POLAR背景下磁化反向冲击动力学的实验平台

吸积过程是高能天体物理学中最重要的现象之一,因为该过程被广泛认为可为多种天体(从恒星到大质量黑洞)提供能量,并且是大量双星系统中的主要辐射源。以辐射能量的形式释放引力能量是一个复杂的物理过程,该过程正是解释天文观测现象的基础。

在从年轻的恒星到活跃的星系核等众多的吸积体系中,来自巴黎综合理工学院的研究小组对那些被认为能形成吸积柱(polar)的系统特别感兴趣。这是一种包含白矮星(WD)的紧密双星系统,吸收来自晚型洛希瓣中的物质来填充次星。这些系统的磁场足够强以至无法形成吸积盘,因此被吸积的物质将堆积到星体的磁极上,最终形成吸积柱。这些物体是形成热核超新星的雏形,标准烛光使研究人员能够测量遥远星系的距离及其宇宙学影响。因此,在研究吸积柱时,研究人员能够为一些宇宙学难题提供部分答案。超音速自由落体吸积物质对白矮星光球的影响能够导致辐射反向冲击的形成,并引起从软X射线到硬X射线的强烈爆发。研究人员目前无法解释天文观测中显示的光度振荡,这可能与冲击前沿的不稳定热振荡或吸积柱中的磁流体力学的不稳定性有关。由于这些系统中的反向冲击位置太靠近白矮星光球(100~1000 km),因此直接观察并不能研究该吸积区域,也无法确定冲击高度和温度等结构参数。这种高能环境的结构也取决于多尺度物理学,它引入了理论和数值模拟的问题。为此,该研究团队开发了一个新的实验平台,将强大的外部磁场(高达15 T)与高功率激光器(数千焦耳)耦合,无需使用平行光管即可发射准直光,并研究了与吸积过程特别是吸积柱相关的磁化反向冲击动力学。相关结果发表在High Power Laser Science and Engineering, Vol. 6, Issue 3, 2018上(B. Albertazzi, et al., Experimental platform for the investigation of magnetized-reverse-shock dynamics in the context of POLAR)。

该课题组的Bruno Albertazzi博士表示:“详细研究这些系统的唯一方法是重现规模化的天体物理实验。”初步研究结果表明,吸积柱中似乎出现了不稳定性,磁化反向冲击的结构也很复杂,这需要在未来的工作中予以证实。

在相互作用开始后75 ns进行的2D磁流体力学辐射闪光模拟图

Experimental platform for the investigation of magnetized-reverse-shock dynamics in the context of POLAR

Accretion processes are among the most important phenomena in high-energy astrophysics as they are widely believed to provide the power supply in several astrophysical objects (from stellar objects to massive black holes), and are the main source of radiation in a large number of interactive binary systems. The release of gravitational energy in the form of radiation energy is a complex physical process but fundamental in interpreting astronomical observations.

Among the numerous accretion systems, from young stellar objects to active galactic nuclei, the research group from Ecole Polytechnique of Paris is particularly interested in those where an accretion column is believed to be formed (polars). They are close binary systems containing a white dwarf (WD) that accretes matter from a late type Roche-lobe filling secondary star. In these systems, the magnetic field is strong enough to prevent the formation of an accretion disk, so matter piles up to the compact object’s magnetic poles, leading to the formation of an accretion column. These objects are potential embryos of thermonuclear supernovae, standard candles that allow us to measure the distance of distant galaxies, and their cosmological repercussions. Therefore, in studying polars the researchers can provide some answers to the cosmological challenges. The impact of the supersonic free-fall accreting matter on the WD photosphere leads to the formation of a radiative reverse shock and gives rise to strong emission from soft to hard x-rays. Astronomical observations showed unexplained luminosity oscillations, which could be related, for example, to unstable thermal oscillations of the shock front or magnetohydrodynamics (MHD) instabilities in the accretion column. As the reverse shock position in these systems is too close to the WD photosphere (~ 100-1000 km), the accretion region is unresolved by direct observations and structural parameters such as the shock height, temperature cannot be defined. The structure of this high-energy environment depends as well on multi-scale physics introducing issues for theoretical and numerical modeling. The members from research group have developed a new experimental platform that couples a strong external magnetic field (up to 15 T) with high-power lasers (∼kJ), enabling to collimate the flow without using a tube and to study the magnetized reverse-shock dynamics related to accretion processes with a particular emphasis on POLAR. Related results are published in High Power Laser Science and Engineering, Vol. 6, Issue 3, 2018 (B. Albertazzi, et al., Experimental platform for the investigation of magnetized-reverse-shock dynamics in the context of POLAR).

“The only way to study these systems in detail is to reproduce a scaled astrophysical experiment” said Dr. Bruno Albertazzi. Preliminary results show that an instability seems to develop in the accretion column and the structure of the magnetized reverse shock seems complex but needs to be confirmed in future work.

2D MHD radiative Flash simulation performed 75 ns after the beginning of the interaction.