用高角分辨的X射线点源的晕研究星际尘埃的空间分布
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摘要
当X射线穿越星际介质时会被星际尘埃颗粒吸收和散射,散射角度较小的光子就会在原来的X射线周围形成一个弥散的图像,这就是“晕”。散射的强度与被散射光子的能量、散射光深、尘埃颗粒的成分和大小分布以及尘埃颗粒在视线方向上的空间分布都有关系,因此X射线散射“晕”也就成为研究星际尘埃颗粒大小分布以及空间分布的一种重要手段。Chandra卫星上的ACIS具有很高的角分辨率、良好的能量分辨率和较宽的能量范围,因此成为最理想的研究X射线晕的仪器。但在标准的TE观测模式下,强源的光子通常在源的中心造成严重的堆积效应,使我们不能直接得到高角分辨率的晕的形状。
因此我们首先改进了Yao等人提出的从HETG/ACIS的CC模式的0阶数据或者用TE模式中的1阶数据提取小角度处的晕的径向分布的方法,并重新分析了Chandra观测的Cygnus X-1数据。然后我们用这种方法得到了17个X射线点源的晕的形状,再利用星际尘埃模型WD01和MRN去拟合晕的强度分布,得到了氢的散射柱密度以及星际尘埃的空间分布,从WD01模型和MRN模型中拟合出来的氢的散射柱密度有很好的线性相关性,并且它们与能谱拟合中得到的氢的吸收柱密度也有很好的线性关系,它们的关系如下:NH,WD01 =(0.720±0.009)×N_(H,abs) + (0.051±0.013)和N_(H,MRN) = (1.156±0.016)×N_(H,abs) +(0.062±0.024),单位为10~(22) cm~(-2)。利用1.5角秒到60角秒之间的晕我们得到了晕的相对强度(FHI)与NH的统计关系。WD01模型和MRN模型的拟合结果都显示在点源附近有相对较高密度的星际尘埃存在,我们认为这是在X射线双星附近存在分子云的有力证据。我们发现扣除点源附近(x = 0.99 ? 1.0)的氢后的氢的散射柱密度与点源的X射线光子所经过的有效散射距离有一个很好的线性关系,而点源附近的氢的散射柱密度与有效散射距离则没有明显的相关性,说明点源附近的星际尘埃与银河系的尘埃盘没有必然联系。我们还估算了两颗特殊的X射线脉冲星Vela X-1和GX 301-2附近的星风结构以及密度等。
最后我们提出用X射线暂现源的晕来研究星际尘埃的立体结构,用更高角分辨率的X射线散射晕来研究点源附近星际尘埃的分布,甚至是星风的结构,以及利用点源附近的尘埃柱密度的统计分布来验证大质量恒星不经过超新星爆发而直接塌缩形成黑洞的可能性。
X-rays are absorbed and scattered by dust grains when they travel through theinterstellar medium. The scattering within small angles results in an X-ray “halo”. Thehalo properties are significantly affected by the energy of radiation, the optical depth ofthe scattering, the grain size distributions and compositions, and the spatial distributionof dust along the line of sight (LOS). Therefore analyzing the X-ray halo properties is animportant tool to study the size distribution and spatial distribution of interstellar grains,which plays a central role in the astrophysical study of the interstellar medium, suchas the thermodynamics and chemistry of the gas and the dynamics of star formation.With excellent angular resolution, good energy resolution and broad energy band, theChandra ACIS is so far the best instrument for studying the X-ray halos. But the directimages of bright sources obtained with ACIS usually suffer from severe pileup whichprevents us from obtaining the halos in small angles.
We first improve the method proposed by Yao et al to resolve the X-ray dust scat-tering halos of point sources from the zeroth order data in CC-mode or the first or-der data in TE mode with Chandra HETG/ACIS. Using this method we re-analyzethe Cygnus X-1 data observed with Chandra. Then we studied the X-ray dust scat-tering halos around 17 bright X-ray point sources using Chandra data. All sourceswere observed with the HETG/ACIS in CC-mode or TE-mode. Using the inter-stellar grain models of WD01 model and MRN model to fit the halo profiles, weget the hydrogen column densities and the spatial distributions of the scattering dustgrains along the line of sights (LOS) to these sources. We find there is a good lin-ear correlation not only between the scattering hydrogen column density from WD01model and the one from MRN model, but also between NH derived from spectralfits and the one derived from the grain models WD01 and MRN (except for GX301-2 and Vela X-1): NH,WD01 = (0.720 ± 0.009) × N_(H,abs) + (0.051 ± 0.013) andN_(H,MRN) = (1.156 ± 0.016) × N_(H,abs) + (0.062 ± 0.024) in the units 10~(22) cm~(-2). Thenthe correlation between FHI and NH is obtained. Both WD01 model and MRN modelfits show that the scattering dust density very close to these sources is much higher thanthe normal interstellar medium and we consider it is the evidence of molecular cloudsaround these X-ray binaries. We also find that there is the linear correlation between
the effective distance through the galactic dust layer and hydrogen scattering columndensity NH excluding the one in x = 0.99 ? 1.0 but the correlation does not exist be-tween he effective distance and the NH in x = 0.99?1.0. It shows that the dust nearbythe X-ray sources is not the dust from galactic disk. Then we estimate the structure anddensity of the stellar wind around the special X-ray pulsars Vela X-1 and GX 301-2.Finally we discuss the possibility of probing the three dimensional structure ofthe interstellar using the X-ray halos of the transient sources, probing the spatial dis-tributions of interstellar dust medium nearby the point sources, even the structure ofthe stellar winds using higher angular resolution X-ray dust scattering halos and testingthe model that the black hole can be formed from the direct collapse of a massive starwithout supernova using the statistical distribution of the dust density nearby the X-raybinaries.
因此我们首先改进了Yao等人提出的从HETG/ACIS的CC模式的0阶数据或者用TE模式中的1阶数据提取小角度处的晕的径向分布的方法,并重新分析了Chandra观测的Cygnus X-1数据。然后我们用这种方法得到了17个X射线点源的晕的形状,再利用星际尘埃模型WD01和MRN去拟合晕的强度分布,得到了氢的散射柱密度以及星际尘埃的空间分布,从WD01模型和MRN模型中拟合出来的氢的散射柱密度有很好的线性相关性,并且它们与能谱拟合中得到的氢的吸收柱密度也有很好的线性关系,它们的关系如下:NH,WD01 =(0.720±0.009)×N_(H,abs) + (0.051±0.013)和N_(H,MRN) = (1.156±0.016)×N_(H,abs) +(0.062±0.024),单位为10~(22) cm~(-2)。利用1.5角秒到60角秒之间的晕我们得到了晕的相对强度(FHI)与NH的统计关系。WD01模型和MRN模型的拟合结果都显示在点源附近有相对较高密度的星际尘埃存在,我们认为这是在X射线双星附近存在分子云的有力证据。我们发现扣除点源附近(x = 0.99 ? 1.0)的氢后的氢的散射柱密度与点源的X射线光子所经过的有效散射距离有一个很好的线性关系,而点源附近的氢的散射柱密度与有效散射距离则没有明显的相关性,说明点源附近的星际尘埃与银河系的尘埃盘没有必然联系。我们还估算了两颗特殊的X射线脉冲星Vela X-1和GX 301-2附近的星风结构以及密度等。
最后我们提出用X射线暂现源的晕来研究星际尘埃的立体结构,用更高角分辨率的X射线散射晕来研究点源附近星际尘埃的分布,甚至是星风的结构,以及利用点源附近的尘埃柱密度的统计分布来验证大质量恒星不经过超新星爆发而直接塌缩形成黑洞的可能性。
X-rays are absorbed and scattered by dust grains when they travel through theinterstellar medium. The scattering within small angles results in an X-ray “halo”. Thehalo properties are significantly affected by the energy of radiation, the optical depth ofthe scattering, the grain size distributions and compositions, and the spatial distributionof dust along the line of sight (LOS). Therefore analyzing the X-ray halo properties is animportant tool to study the size distribution and spatial distribution of interstellar grains,which plays a central role in the astrophysical study of the interstellar medium, suchas the thermodynamics and chemistry of the gas and the dynamics of star formation.With excellent angular resolution, good energy resolution and broad energy band, theChandra ACIS is so far the best instrument for studying the X-ray halos. But the directimages of bright sources obtained with ACIS usually suffer from severe pileup whichprevents us from obtaining the halos in small angles.
We first improve the method proposed by Yao et al to resolve the X-ray dust scat-tering halos of point sources from the zeroth order data in CC-mode or the first or-der data in TE mode with Chandra HETG/ACIS. Using this method we re-analyzethe Cygnus X-1 data observed with Chandra. Then we studied the X-ray dust scat-tering halos around 17 bright X-ray point sources using Chandra data. All sourceswere observed with the HETG/ACIS in CC-mode or TE-mode. Using the inter-stellar grain models of WD01 model and MRN model to fit the halo profiles, weget the hydrogen column densities and the spatial distributions of the scattering dustgrains along the line of sights (LOS) to these sources. We find there is a good lin-ear correlation not only between the scattering hydrogen column density from WD01model and the one from MRN model, but also between NH derived from spectralfits and the one derived from the grain models WD01 and MRN (except for GX301-2 and Vela X-1): NH,WD01 = (0.720 ± 0.009) × N_(H,abs) + (0.051 ± 0.013) andN_(H,MRN) = (1.156 ± 0.016) × N_(H,abs) + (0.062 ± 0.024) in the units 10~(22) cm~(-2). Thenthe correlation between FHI and NH is obtained. Both WD01 model and MRN modelfits show that the scattering dust density very close to these sources is much higher thanthe normal interstellar medium and we consider it is the evidence of molecular cloudsaround these X-ray binaries. We also find that there is the linear correlation between
the effective distance through the galactic dust layer and hydrogen scattering columndensity NH excluding the one in x = 0.99 ? 1.0 but the correlation does not exist be-tween he effective distance and the NH in x = 0.99?1.0. It shows that the dust nearbythe X-ray sources is not the dust from galactic disk. Then we estimate the structure anddensity of the stellar wind around the special X-ray pulsars Vela X-1 and GX 301-2.Finally we discuss the possibility of probing the three dimensional structure ofthe interstellar using the X-ray halos of the transient sources, probing the spatial dis-tributions of interstellar dust medium nearby the point sources, even the structure ofthe stellar winds using higher angular resolution X-ray dust scattering halos and testingthe model that the black hole can be formed from the direct collapse of a massive starwithout supernova using the statistical distribution of the dust density nearby the X-raybinaries.
引文
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