[论文解读] XMM-Newton observation of 4U 1820-30: Broad band spectrum and the contribution of the cold interstellar medium

E. Costantini, C. Pinto+7 more|UvA-DARE (University of Amsterdam)|Dec 19, 2011

Astrophysical Phenomena and Observations参考文献 72被引用 40

一句话总结

本研究分析了明亮X射线双星4U 1820-30的XMM-Newton X射线谱，重点研究冷星际介质的吸收特征。结果表明，氧和铁的K边吸收最佳拟合为富含镁的硅酸盐与金属铁夹杂物（类似GEMS的尘埃），氧的耗竭因子为0.20，铁的耗竭因子为0.87，且存在轻微电离喷流的初步证据，速度约为1200 km s⁻¹。

ABSTRACT

We present the analysis of the bright X-ray binary 4U 1820-30, based mainly on XMM-Newton-RGS data, but using complementary data from XMM-Epic, Integral, and Chandra-HETG, to investigate different aspects of the source. The broad band continuum is well fitted by a classical combination of black body and Comptonized emission. The continuum shape and the high flux of the source (L/L_Edd\sim0.16) are consistent with a "high state" of the source. We do not find significant evidence of iron emission at energies >=6.4 keV. The soft X-ray spectrum contain a number of absorption features. Here we focus on the cold-mildly ionized gas. The neutral gas column density is N_H\sim1.63x10^21 cm^-2. The detailed study of the oxygen and iron edge reveals that those elements are depleted, defined here as the ratio between dust and the total ISM cold phase, by a factor 0.20\pm0.02 and 0.87\pm0.14, respectively. Using the available dust models, the best fit points to a major contribution of Mg-rich silicates, with metallic iron inclusion. Although we find that a large fraction of Fe is in dust form, the fit shows that Fe-rich silicates are disfavored. The measured Mg:Fe ratio is 2.0\pm0.3. Interestingly, this modeling may point to a well studied dust constituent (GEMS), sometimes proposed as a silicate constituent in our Galaxy. Oxygen and iron are found to be slightly over- and under-abundant, respectively (1.23 and 0.85 times the solar value) along this line of sight. We also report the detection of two absorption lines, tentatively identified as part of an outflow of mildly ionized gas (ξ\sim-0.5) at a velocity of \sim1200 km/s.

研究动机与目标

• 利用高分辨率X射线谱学研究4U 1820-30视线方向上星际尘埃的成分与耗竭情况。
• 确定冷气体与弱电离气体对观测到的X射线吸收特征的贡献。
• 评估氧和铁的吸收边是否与已知尘埃化合物一致，特别是富含镁的硅酸盐与金属铁夹杂物（GEMS）。
• 探讨在O iv和O v线中检测到的疑似喷流吸收体的起源。

提出的方法

• 分析XMM-Newton的RGS和EPIC-pn数据，以建模宽波段X射线连续谱和吸收特征。
• 同时使用INTEGRAL和Chandra-HETG数据，以改善光谱覆盖范围，并约束连续谱和吸收组分。
• 利用当前非晶硅酸盐和金属铁的理论尘埃吸收轮廓，拟合氧和铁的K边特征。
• 通过光致电离和碰撞电离情景建模吸收气体的电离状态，以解释O iv和O v线。
• 将观测到的吸收深度和边形与尘埃模型的预测进行比较，包括GEMS和富铁硅酸盐。
• 利用丰度和耗竭因子推断冷星际介质相中的尘气比和元素分馏情况。

实验结果

研究问题

• RQ1 如何从X射线吸收边推断4U 1820-30视线方向上星际尘埃的化学成分？
• RQ2 氧和铁的耗竭因子与太阳值相比如何？这对尘埃形成和颗粒处理过程有何启示？
• RQ3 观测到的氧和铁K边特征能否由已知尘埃化合物（如富含镁的硅酸盐与金属铁夹杂物，即GEMS）解释？
• RQ4 在约1200 km s⁻¹处检测到的疑似吸收线的起源是什么？气体是否因光致电离或碰撞电离而喷流？
• RQ5 观测到的吸收是否与冷、弱电离星际介质一致？还是可能与源附近局部喷流有关？

主要发现

• 中性氢柱密度测量值为$N_{\rm H} = 1.63 \times 10^{21}$ cm⁻²，与视线方向冷星际介质一致。
• 氧的丰度相对于太阳高出1.23倍，而铁的丰度低至太阳值的0.85倍。
• 氧的耗竭因子为$0.20 \pm 0.02$，表明其大部分被锁在尘埃颗粒中。
• 铁的耗竭因子为$0.87 \pm 0.14$，表明其大量被纳入尘埃中。
• 氧的K边最佳拟合为富含镁的硅酸盐（如顽辉石，MgSiO₃），而铁的K边则由金属铁夹杂物解释，支持类似GEMS的尘埃组成。
• 在约1200 km s⁻¹处检测到疑似喷流吸收体，O iv和O v线可能源于弱电离气体，但其电离机制尚不明确。