吸积柱模型的拟合策略及其对Cen X-3宽带谱的应用
Fitting strategies of accretion column models and application to the broadband spectrum of Cen X-3
摘要：
由于中子星双星吸积柱内辐射转移模型的复杂性，它们的X射线谱通常仍用唯象模型（如截止幂律）来描述。虽然这些模型的行为是很好的理解，他们允许不同的源和研究源行为的比较，在何种程度上可以从模型参数导出的基础物理是非常有限的。近年来，一些物理动机的光谱模型已经被开发出来，以克服这些限制。然而，它们的应用通常在计算上要昂贵得多，并且它们需要大量难以约束的参数。以前的工作已经提出了一个解析解的辐射转移方程内的吸积柱假设的速度分布是线性的光学深度。这种解决方案的实现既快速又准确，足以拟合观察到的光谱，可作为XSPEC中的模型。这种实现的主要困难在于，某些解决方案违反了能量守恒，因此必须被用户拒绝。我们提出了一种新的拟合策略，确保能量守恒过程中的$χ^2$ -最小化，大大简化了模型的应用。我们证明了这种方法以及可能的参数退化的研究与一个全面的马尔可夫链蒙特卡罗分析的完整的参数空间相结合的NuSTAR和Swift/XRT数据集的岑X-3。派生的吸积流结构的特点是一个小的柱半径为$\sim$ 63米和光谱占主导地位的散装康普顿化的韧致辐射种子光子，与以前的研究一致。
英文摘要：
Due to the complexity of modeling the radiative transfer inside the accretion columns of neutron star binaries, their X-ray spectra are still commonly described with phenomenological models, for example, a cutoff power law. While the behavior of these models is well understood and they allow for a comparison of different sources and studying source behavior, the extent to which the underlying physics can be derived from the model parameters is very limited. During recent years, several physically motivated spectral models have been developed to overcome these limitations. Their application, however, is generally computationally much more expensive and they require a high number of parameters which are difficult to constrain. Previous works have presented an analytical solution to the radiative transfer equation inside the accretion column assuming a velocity profile that is linear in the optical depth. An implementation of this solution that is both fast and accurate enough to be fitted to observed spectra is available as a model in XSPEC. The main difficulty of this implementation is that some solutions violate energy conservation and therefore have to be rejected by the user. We propose a novel fitting strategy that ensures energy conservation during the $χ^2$-minimization which simplifies the application of the model considerably. We demonstrate this approach as well a study of possible parameter degeneracies with a comprehensive Markov-chain Monte Carlo analysis of the complete parameter space for a combined NuSTAR and Swift/XRT dataset of Cen X-3. The derived accretion-flow structure features a small column radius of $\sim$63 m and a spectrum dominated by bulk-Comptonization of bremsstrahlung seed photons, in agreement with previous studies.
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