Shocks in nova outflows – I. Thermal emission
Shocks in nova outflows – I. Thermal emission
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DOI：
10.1093/mnras/stu844
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发表时间：
2014-03
影响因子：
4.8
通讯作者：
B. Metzger;R. Hascoet;I. Vurm;A. Beloborodov;L. Chomiuk;J. Sokoloski;T. Nelson
中科院分区：
文献类型：
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作者：
B. Metzger;R. Hascoet;I. Vurm;A. Beloborodov;L. Chomiuk;J. Sokoloski;T. Nelson
越来越多的证据表明新星流出的冲击包括（1）光谱中的多个速度分量； (2) 爆发后几周至几个月开始的硬X射线发射； (3) 早期射电耀斑的时间尺度为数月，超过了自由膨胀的光电离气体的预测；也许最引人注目的是 (4)∼ GeV 伽马射线发射。我们提出了一个一维模型，用于描述快速新星流出和致密外壳 (DES) 之间的激波相互作用及其相关的热 X 射线、光学和无线电发射。较低速度的 DES 可能代表白矮星或周围物质质量损失的早期阶段，与热核失控没有直接关系。当冲击气体的密度最高时，前向激波最初是辐射性的，此时无线电发射源自紧邻激波下游的致密冷却层。我们预测的射电光曲线的特点是，在逐渐降低的频率下，峰值时间急剧上升，峰值时间逐渐降低，峰值亮度温度几乎与频率无关。我们将我们的模型应用于最近产生伽马射线的经典新星V1324 Sco，获得了对早期射电极大值的充分拟合，以合理地假设快速新星流出，并假设DES具有特征速度~ 10 3 km s -1 和质量~ 10 -4 M⊙；前者与之前在新星光谱中观察到的窄线吸收系统的速度一致，而 DES 和快速流出的总喷射质量与通过将晚期射电峰建模为均匀膨胀的光电离气体而独立推断的一致。早期射电光变曲线的快速演化要求 DES 具有陡峭的外部密度剖面，这可能表明白矮星的质量损失很快，从而提供了 DES 因热核失控事件而被驱逐的间接证据。人们发现，早期由 DES 吸收的冲击波中经过再处理的 X 射线对光学/紫外线发射有显着贡献，我们推测这可能是造成新星光学光曲线 s 中先前无法解释的“平台”和次要最大值的原因。
Growing evidence for shocks in nova outflows include (1) mult iple velocity components in the optical spectra; (2) hard X-ray emission starting wee ks to months after the outburst; (3) an early radio flare on timescales of months, in excess of t hat predicted from the freely expanding photo-ionized gas; and, perhaps most dramatically, (4)∼ GeV gamma-ray emission. We present a one dimensional model for the shock interaction between the fast nova outflow and a dense external shell (DES) and its associated th ermal X-ray, optical, and radio emission. The lower velocity DES could represent an earlier stage of mass loss from the white dwarf or ambient material not directly related to the thermonuclear runaway. The forward shock is radiative initially when the density of shocked gas is highest, at which times radio emission originates from the dense cooling layer immediately downstream of the shock. Our predicted radio light curve is characterized by sharper rises to maximum and later peak times at progressively lower frequencies, with a peak brightness temperature that is approximately independent of frequency. We apply our model to the recent gamma-ray producing classical nova V1324 Sco, obtaining an adequate fit to the ear ly radio maximum for reasonable assumptions about the fast nova outflow and assuming the DES possesses a characteristic velocity∼ 10 3 km s −1 and mass∼ few 10 −4 M⊙; the former is consistent with the velocities of narrow line absorption systems observed previously in nova spectra, while the total ejecta mass of the DES and fast outflow is consistent with that inferr ed independently by modeling the late radio peak as uniformly expanding photo-ionized gas. Rapid evolution of the early radio light curves require the DES to possess a steep outer density profile, which may indicate that the onset of mass loss from the white dwarf was rapid, providing indirect evidence that the DES was expelled as the result of the thermonuclear runaway event. Reprocessed X-rays from the shock absorbed by the DES at early times are found to contribute significantly to the optical/UV emission, which we speculate may be responsible for the previously unexplained ‘plateaus’ and secondary maxima in nova optical light curve s.