The models currently used for the analysis of the reflection spectra of black holes usually assume a disk with constant ionization and electron density. However, there is some debate on the impact of these assumptions on the estimate of the properties of the sources, in particular when the fits suggest very steep emissivity profiles in the inner part of the accretion disk. In this work, we re-analyze a selected set of high-quality NuSTAR and Suzaku data of Galactic black holes and we fit the reflection component with three different models: RELXILL_NK, in which the ionization parameter and the electron density are constant, RELXILLION_NK, where the electron density is still constant but the ionization profile is described by a power law, and RELXILLDGRAD_NK, where the electron density profile is described by a power law and the ionization profile is calculated self-consistently from the electron density and the emissivity. While RELXILLION_NK may provide slightly better fits, we do not find any substantial difference in the estimate of the properties of the sources among the three models. Our conclusion is that models with constant electron density and ionization parameter are probably sufficient, in most cases, to fit the currently available X-ray data of accreting black holes.
Very steep reflection emissivity profiles in the inner part of accretion disks are commonly found in the analysis of X-ray observations of black hole binaries and AGN, but there is some debate about their exact origin. While steep reflection emissivity profiles can be naturally produced by compact coronae close to black holes, the measured radial emissivity parameter can be further increased by the radial disk ionization profile when the theoretical model assumes a disk with constant ionization. In this paper, we implement the possibility of a radial disk ionization profile in the reflection m…
High Density Reflection Spectroscopy: II. the Density of the Inner Black Hole Accretion Disc in AGN
利用高密度盘反射谱分析了17个Seyfert 1星系的盘内密度，研究了不同黑洞质量尺度和吸积率下的盘内密度。使用存档中所有可用的XMM-牛顿观测。光学/紫外波段的OM观测被用来估计它们的吸积率。我们发现，在我们的样本中，65%的源显示出明显高于1 e 15 cm^{-3}的盘密度，这是在以前的反射光谱分析中假设的。最佳拟合盘密度与黑洞质量和质量吸积率呈反相关。高密度圆盘反射模型可以成功地解释软过量发射，并显着降低推断的铁丰度。我们还比较了我们的黑洞自旋和光盘倾角测量与以前的分析。
High Density Reflection Spectroscopy I. A case study of GX~339-4
利用高密度反射模型，利用NuSTAR和Swift对黑洞双星GX~339-4进行了宽带光谱分析。这些观测是在2013年和2015年爆发期间源处于低通量硬态（LF），以及2015年处于非常高通量软态（HF）时进行的。高密度反射模型可以解释它的LF谱，而不需要额外的低温热分量。这个模型使我们能够约束GX~339-4在不同通量状态下的盘面密度。LF状态下的圆盘密度为$\log(n_{\rm e}/$ cm $^{-3})\approx21$，比HF状态下的密度（$\log(n_{\rm e}/$ cm $^{-3})=18.93^{+0.12}_{-0.16}$）高100倍。用变密度反射模型（分别为$Z_{\rm Fe}=1.50^{+0.12}_{-0.04}Z_{\odot}$和$Z_{\rm Fe}=1.05^{+0.17}_{-0.15}Z_{\odot}$）模拟低频和高频宽带光谱，得到了接近太阳的铁丰度。
High-Density Effects in X-ray Reflection Models from Accretion Disks
目前的模型，从吸积盘周围的黑洞和其他紧凑的物体反射的X射线光谱通常计算假设磁盘大气的密度是恒定的几个汤姆逊深度从照射表面。这些模型的一个重要简化假设是，气体的电离结构完全由电离参数$ξ$的单个固定值指定，电离参数是入射通量与气体密度的比率。密度通常固定在$n_e=10^{15}$ cm $^{-3}$。受观测的启发，我们在计算反射光谱时考虑了更高的密度。通过计算$n_e \gtrsim 10^{17}$ cm $^{-3}$的模型光谱，我们发现高密度效应显著地改变了反射光谱。主要效果是在能量$\lesssim 2$ keV处提高热连续性。我们讨论了这些结果的影响，解释活动星系核和黑洞双星的观测。我们还讨论了目前可用的原子数据的质量所施加的模型的局限性。
X-ray reflection spectroscopy is a powerful tool for probing the strong gravity region of black holes and can be used for testing general relativity in the strong field regime. Simplifications of the available relativistic reflection models limit the capability of performing accurate measurements of the properties of black holes. In this paper, we present an extension of the model RELXILL_NK in which the accretion disk has a finite thickness rather than being infinitesimally thin. We employ the accretion disk geometry proposed by Taylor & Reynolds (2018) and we construct relativistic reflectio…
X-ray reflection spectroscopy is a promising technique for testing general relativity in the strong field regime, as it can be used to test the Kerr black hole hypothesis. In this context, the parametrically deformed black hole metrics proposed by Konoplya, Rezzolla \& Zhidenko (Phys. Rev. D93, 064015, 2016) form an important class of non-Kerr black holes. We implement this class of black hole metrics in \textsc{relxill\_nk}, which is a framework we have developed for testing for non-Kerr black holes using X-ray reflection spectroscopy. We perform a qualitative analysis of the effect of the le…
Relativistic reflection features are commonly observed in the X-ray spectra of accreting black holes. In the presence of high quality data and with the correct astrophysical model, X-ray reflection spectroscopy can be quite a powerful tool to probe the strong gravity region, study the morphology of the accreting matter, measure black hole spins, and possibly test Einstein's theory of general relativity in the strong field regime. In the last decade, there has been significant progress in the development of the analysis of these features, thanks to more sophisticated astrophysical models and ne…
X-ray reflection spectroscopy is a powerful tool to probe the strong gravity region around black holes, but the available relativistic reflection models have a number of simplifications that lead to systematic uncertainties (not fully under control) in the measurement of the properties of a source. In Paper I, we considered the case of an optically thin plunging region and we studied the impact of the radiation produced by the other side of the disk or circling the black hole one or more times. In the present paper, we discuss the case of an optically thick plunging region and we study the imp…
Measuring black hole spins with x-ray reflection spectroscopy: A GRMHD outlook
Fudan University（复旦大学）；New Uzbekistan University（新乌兹别克斯坦大学）；Ulugh Beg Astronomical Institute（乌鲁伯格天文学研究所）；Eberhard-Karls Universität Tübingen（蒂宾根大学）；Tsung-Dao Lee Institute, Shanghai Jiao-Tong University（上海交通大学李政道研究所）；Shanghai Jiao-Tong University（上海交通大学）；Goethe Universität（法兰克福歌德大学）
Einstein-Maxwell dilaton-axion gravity is a string-inspired model arising from the low energy effective action of heterotic string theory and an important candidate as alternative to General Relativity. Recently, some authors have explored its astrophysical implications in the spectra of accreting black holes and inferred the constraint $r_2 < 0.1$, where $r_2 \ge 0$ is the black hole dilaton charge and General Relativity is recovered for $r_2 = 0$. In the present paper, we study the impact of a non-vanishing black hole dilaton charge on the reflection spectrum of the disk. From the analysis o…
The Novikov-Thorne model is the standard framework for the description of geometrically thin and optically thick accretion disks around black holes and is widely used to study the electromagnetic spectra of accreting black holes. One of the assumptions of the model is that the particles of the gas move on nearly-geodesic circular orbits on the equatorial plane. In this work, we propose to test the Keplerian velocity of the particles in the accretion disk using X-ray reflection spectroscopy. We present a modified version of RELXILL in which we introduce a phenomenological parameter, $α$, to qua…
Low-energy cosmic-rays (CRs) control the thermo-chemical state and the coupling between gas and magnetic fields in dense molecular clouds, the sites of star-formation. However, current estimates of the low-energy CR spectrum ($E \lesssim 1$ GeV) and the associated CR ionization rate are highly uncertain. We apply, for the first time, a new method for constraining the CR ionization rate and the CR spectral shape using H$_2$ rovibrational lines from cold molecular clouds. Using the MMIRS instrument on the MMT, we obtained deep near-infrared (NIR) spectra in six positions within four dense cores,…
