太阳物理研究组知识库

软γ重复暴（soft gamma-ray repeater, SGR）被认为产生于磁中子星。发生在SGR上的巨耀发在短时标内释放出大于1039J的巨大能量,被认为是宇宙中已知最强的能量释放过程之一,其剧烈程度仅次于超新星爆发和γ暴。详细介绍了几种磁球层理论模型,并重点介绍了以太阳爆发日冕物质抛射灾变理论为基础建立的磁中子星巨耀发的磁流体力学的半解析模型。在模型中,板块的转动或错位造成磁球层内磁场的扭缠,从而导致磁通量绳的形成和磁能缓慢的积累。当积累的能量超过阈值,系统就会失去平衡,然后产生爆发并释放能量。用该模型计算的SGR 1806-20, SGR 0526-66和SGR 1900+14这3次巨耀发的光变曲线都与观测基本相符。此外,有关磁中子星巨耀发的磁流体动力学的数值模拟工作也得到了全面的展开,通过求解各种初始条件和边界条件下的磁流体力学方程组,计算机的数值模拟可以得到磁中子星巨耀发过程中的磁场形态演化和内部精细结构。 

Giant flares (GF) observed from the soft gamma-ray repeater (SGR) are generally believed to take place on the magnetar, a neutron star with an extremely strong magnetic field (1010 ∼ 1011 T), and release huge amount of energy (> 1039 J) in a very short time interval (a few tens of seconds). They are thought to be the most energetic phenomenon in the universe next to the supernova and the gamma-ray burst. Currently, it is usually thought that the energy driving the GF from the SGR is from the magnetic field in the magnetosphere of the magnetar, the eruption is triggered by the catastrophic loss of equilibrium in the magnetic structure as a result of various instabilities in the structure, and magnetic reconnection subsequently converts most of the magnetic energy into heat and kinetic energy of the plasma involved in the eruption. Meanwhile, a small portion of the magnetic energy is used to accelerate charged particles as well.

In this review, we introduce the physical properties of magnetars and the observations of GF on SGR. So far, two types of models for GF exist, including the crust model and the
magnetosphere model, distinguishing from one another by the region where the energy is stored prior to GF. The former assumes that the energy is stored in the crust of the neutron star, and the latter assumes that thestorage occurs in the magnetosphere. In the crust model, a giantflare is caused by a sudden untwisting of the internal magnetic field and subsequently a large and quick rotational displacement takes place. Alternatively, in the magnetosphere model, the magnetic energy is slowly stored in the magnetosphere until the system reaches a critical state and loses the mechanical equilibrium, triggering the eruption. Observations of the giant flare from SGR 1806−20 on 2004 December 27 showed that it lasted a very short rise time (0.25 ms). This time interval of the eruption is short compared to the timescale required for the crust model and is more consistent with the magnetosphere model.

In this paper, some theoretical magnetosphere models are introduced. The magnetohydrodynamical(MHD) model for magnetar giant flares in the framework of the catastrophe model for the coronal mass ejection from the Sun is focused on. In this model, the rotation and/or displacement of the crust causes the field to twist, compress, stretch, and deform, leading to the formation of a magnetic flux rope in the magnetosphere, as well as slow accumulation of the magnetic energy in the related configuration. When the energy and helicity stored in the configuration reach a threshold, the system loses its equilibrium, the flux rope is ejected outward in a catastrophic way, and magnetic reconnection helps the catastrophe develop to a plausible eruption. The calculated light curves of SGR 1806−20, SGR 0526−66, and SGR 1900+14 are in good agreement with the observed light curves of these giant flares. In addition, some MHD simulations about the GF are introduced. Solving the MHD equations for different initial and boundary conditions, the GF can be duplicated in the numerical experiment, the evolution of magnetic configuration, including the current sheet where magnetic reconnection takes place, for the eruption could then be further obtained in detail.