Title: Chandra X-ray spectroscopy of a clear dip in GX 13+1 Show Chinese title

Title: 清晰消光现象的GX 13+1的钱德拉X射线光谱学研究

Abstract: The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of $\sim$ 400 km s$^{-1}$ and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. The 1.0-25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting $\sim$ 450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge, that is relatively small compared with the dimensions of the outer disk radius. Show Chinese abstract

Abstract: GX 13+1 是一个持久且明亮的银河系X射线双星系统，其中包含一个吸积中子星。它显示出高度电离的吸收特征，蓝移速度为$\sim$400 km s$^{-1}$，且其外流质量率与吸积率相似。许多其他X射线源也表现出温暖吸收特征，并且它们都同时显示出周期性的下陷行为。最近，通过长期X射线折叠光曲线，也确定了GX 13+1的下陷周期性，从而在存档的Chandra观测中清楚地识别出其中一个周期性下陷。我们首次对2010年在存档的Chandra观测中发现的GX 13+1的周期性下陷进行了光谱特征描述。我们使用了Chandra/HETGS数据（1.0-10 keV能段）和同时期RXTE/PCA数据（3.5-25 keV能段）来分析宽带X射线谱。我们采用了不同的连续谱模型来描述连续发射，并利用XSTAR推导出的温暖吸收器成分来限制高度电离的吸收特征。1.0-25 keV的连续发射与软吸积盘发射模型以及光学厚度较高的硬康普顿化成分一致。下陷事件持续时间约为$\sim$450秒，通过中性吸收剂列密度增加而光谱解析，然而，我们并未发现温暖吸收剂的列密度和电离参数相对于非下陷谱有显著变化。我们认为，与其他同类源相比，其非常低的下陷循环率可以归因于其长轨道周期以及相对较小且主要为中性的隆起，与外部盘半径的尺寸相比。