Detection of Highly Ionized O and Ne Absorption Lines in the X-Ray Spectrum of 4U 1820–303 in the Globular Cluster NGC 6624
Detection of Highly Ionized O and Ne Absorption Lines in the X-Ray Spectrum of 4U 1820–303 in the Globular Cluster NGC 6624
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DOI：
10.1086/381087
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发表时间：
2003-10
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影响因子：
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通讯作者：
K. Futamoto;K. Mitsuda;Y. Takei;R. Fujimoto;N. Yamasaki
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作者：
K. Futamoto;K. Mitsuda;Y. Takei;R. Fujimoto;N. Yamasaki
我们在用Chandra LETG观测的两个低质量X射线双星4U 1820-303和Cyg X-2的能谱中寻找了高电离O和Ne的吸收线，并检测到4U 1820-303的O VII，O VIII和Ne IX吸收线。O VII Kα线的等效宽度为1.19 eV（90%误差），显著性为6.5 σ。对于Cyg X-2，没有检测到吸收线，O VII Kα的等效宽度的90%上限为1.06 eV。本征谱线宽度没有得到解析，对于4U 1820-303的O VII Kα谱线，得到了对应于速度色散的上限值B = 420 km s-1。假设B的几个不同值，根据生长曲线分析估计离子柱密度。在4U 1820-303中观察到的吸收线可能是由于热的星际介质，因为如果吸收柱靠近双星系统，O将被完全光电离。根据O VII Kα和Kβ线、Ne/O丰度比和H柱密度之间的一致性，将速度频散限制在B = 200-420 km s-1。估计平均温度和O VII密度分别为log T（K）= 6.2-6.3和n =（0.7-2.3）× 10-6 cm-3。两个源的O VII柱密度的差异可能与银河系隆起区软X射线背景（SXB）的增强有关。使用多面体模型的热气体占SXB，我们校正的密度梯度和估计的中平面O VII密度在太阳附近。然后利用活动星系核（AGN）的吸收线估算了热气体的标度高度。这表明，活动星系核的吸收线和高纬度SXB发射线的一个显着的部分可以解释在我们的银河系的热气体。
We searched for absorption lines of highly ionized O and Ne in the energy spectra of two low-mass X-ray binaries, 4U 1820-303 in the globular cluster NGC 6624 and Cyg X-2, observed with the Chandra LETG, and detected O VII, O VIII, and Ne IX absorption lines for 4U 1820-303. The equivalent width of the O VII Kα line was 1.19 eV (90% errors), and the significance was 6.5 σ. Absorption lines were not detected for Cyg X-2 with a 90% upper limit on the equivalent width of 1.06 eV for O VII Kα. The intrinsic line width was not resolved, and an upper limit corresponding to a velocity dispersion of b = 420 km s-1 was obtained for the O VII Kα line of 4U 1820-303. The ion column densities were estimated from the curve-of-growth analysis, assuming several different values of b. The absorption lines observed in 4U 1820-303 are likely due to hot interstellar medium, because O will be fully photoionized if the absorbing column is located close to the binary system. The velocity dispersion is restricted to b = 200-420 km s-1 from consistency between O VII Kα and Kβ lines, the Ne/O abundance ratio, and H column density. The average temperature and the O VII density are estimated to be log T(K) = 6.2-6.3 and n = (0.7-2.3) × 10-6 cm-3, respectively. The difference of O VII column densities for the two sources may be connected to the enhancement of the soft X-ray background (SXB) toward the Galactic bulge region. Using the polytrope model of hot gas to account for the SXB, we corrected for the density gradient and estimated the midplane O VII density at the solar neighborhood. The scale height of hot gas is then estimated using the active galactic nuclei (AGN) absorption lines. It is suggested that a significant portion of both the AGN absorption lines and the high-latitude SXB emission lines can be explained by the hot gas in our Galaxy.