Relativistic reflection features in the X-ray spectra of black hole binaries and AGNs originate from illumination of the inner part of the accretion disk by a hot corona. 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 even test Einstein's theory of general relativity in the strong field regime. There are a few relativistic reflection models available today and developed by different groups. All these models present some differences and have a number of simplifications introducing systematic uncertainties. The question is whether different models provide different measurements of the properties of black holes and how to arrive at a common model for the whole X-ray astronomy community. In this paper, we start exploring this issue by analyzing a Suzaku observation of the stellar-mass black hole in GRS 1915+105 and simultaneous XMM-Newton and NuSTAR observations of the supermassive black hole in MCG-6-30-15. The relativistic reflection component of these sources is fitted with RELCONV$\times$REFLIONX, RELCONV$\times$XILLVER, and RELXILL. We discuss the differences and the impact on the study of accreting black holes.
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 currently one of the leading techniques for studying the inner part of accretion disks around black holes, measuring black hole spins, and even testing fundamental physics in strong gravitational fields. However, the accuracy of these measurements depends on the reflection models employed for the spectral analysis, which are sometimes questioned. In this work, we use a general relativistic magnetohydrodynamic (GRMHD) code to generate a thin accretion disk in Kerr spacetime and ray-tracing techniques to calculate its relativistically broadened reflection spectru…
We present new reflection models specifically tailored to model the X-ray radiation reprocessed in accretion disks around neutron stars, in which the primary continuum is characterized by a single temperature blackbody spectrum, emitted either at the surface of the star, or at the boundary layer. These models differ significantly from those with a standard power-law continuum, typically observed in most accreting black holes. We show comparisons with earlier reflection models, and test their performance in the NuSTAR observation of the neutron star 4U 1705-44. Simulations of upcoming missions …
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…
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…
Supermassive stars (SMSs) collapsing via the general-relativistic (GR) instability are invoked as the possible progenitors of supermassive black holes. Their mass and angular momentum at the onset of the instability are key in many respects, in particular regarding the possibility for observational signatures of direct collapse. Here, we study the stability of rotating, rapidly accreting SMSs against GR and derive the properties of these stars at death. On the basis of hylotropic structures, relevant for rapidly accreting SMSs, we define rotation profiles under the assumption of local angular …
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…
In a previous paper, we presented an extension of our reflection model RELXILL_NK to include the finite thickness of the accretion disk following the prescription in Taylor & Reynolds (2018). In this paper, we apply our model to fit the 2013 simultaneous observations by NuSTAR and XMM-Newton of the supermassive black hole in MCG-06-30-15 and the 2019 NuSTAR observation of the Galactic black hole in EXO 1846-031. The high-quality data of these spectra had previously led to precise black hole spin measurements and very stringent constraints on possible deviations from the Kerr metric. We find th…
Pulse profile modelling of accreting millisecond pulsars with disc occultation and its impact on parameter inference
Nanjing University（南京大学）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（南京大学现代天文与天体物理教育部重点实验室）；Anton Pannekoek Institute for Astronomy, University of Amsterdam（阿姆斯特丹大学安东·潘内克天文研究所）；Gravitation and Astroparticle Physics Amsterdam (GRAPPA), University of Amsterdam（阿姆斯特丹大学引力与天体粒子物理学阿姆斯特丹中心）；University of Helsinki（赫尔辛基大学）；University of Turku（图尔库大学）
We present a new release of the RELTRANS model to fit the complex cross-spectrum of accreting black holes as a function of energy. The model accounts for continuum lags and reverberation lags self-consistently in order to consider the widest possible range of X-ray variability timescales. We introduce a more self-consistent treatment of the reverberation lags, accounting for how the time variations of the illuminating flux change the ionisation level of the accretion disc. This process varies the shape of the reflection spectrum in time causing an additional source of lags besides the light cr…
Modeling reflection spectra of super-Eddington X-ray sources
Fudan University（复旦大学）；New Uzbekistan University（新乌兹别克斯坦大学）；Ulugh Beg Astronomical Institute（乌鲁格别克天文研究所）；Institut für Astronomie und Astrophysik, Eberhard-Karls Universität Tübingen（蒂宾根大学天文与天体物理研究所）；Institute for Advanced Studies, New Uzbekistan University（新乌兹别克斯坦大学高等研究院）；University of Warwick（华威大学）；Institute of Astronomy, University of Cambridge（剑桥大学天文学研究所）
Broadband Timing and Spectral Study of Accreting Millisecond X-ray Pulsar SAX J1808.4$-$3658 during Its 2022 Outburst
Inter-University Centre for Astronomy and Astrophysics (IUCAA)（大学间天文学与天体物理学中心）；Universitá degli Studi di Cagliari（卡利亚里大学）；Department of Applied Sciences, UPES（印度石油大学应用科学系）
A fraction of the electromagnetic radiation emitted from the surface of a geometrically thin and optically thick accretion disk of a black hole returns to the disk because of the strong light bending in the vicinity of the compact object (returning radiation). While such radiation clearly affects the observed spectrum of the source, it is often neglected in theoretical models. In the present paper, we study the impact of the returning radiation on relativistic reflection spectra. Assuming neutral material in the disk, we estimate the systematic uncertainties on the measurement of the propertie…
This work presents an investigation of the stochastic X-ray variability from the 164 Hz accreting millisecond pulsar IGR J17062-6143, based on regular observations collected with the Neutron Star Interior Composition Explorer between 2017 July and 2020 August. Over this period, the power density spectrum showed a stable morphology, with broad $\sim25\%$ rms band-limited noise below 16 Hz. Quasi-periodic oscillations (QPOs) were occasionally observed, with the most notably detections including a low-frequency QPO centered at 2.7 Hz and a sharp QPO centered at 115 Hz that may be a 2:3 resonance …
The upcoming IXPE (2-8 keV) and XL-Calibur (15-75 keV) missions will make it possible to measure the linear polarization of X-rays from mass accreting stellar mass black holes with unprecedented sensitivity, enabling the accurate measurement of percent-level and in some cases even sub-percent level polarization fractions. The measurements are expected to constrain the spins, inclinations, and the structure of the accretion flows of the observed black holes. The effects of Faraday rotation and birefringence of the Quantum Electrodynamics (QED) vacuum may impact the observable polarization fract…
The impact of superradiance on the spin evolution of variably accreting massive black holes
Center for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire（赫特福德大学物理、天文与数学系天体物理学研究中心）；Kavli Institute for Cosmology, University of Cambridge（剑桥大学卡弗里宇宙学研究所）；Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh（爱丁堡大学皇家天文台天文学研究所）
