A light bending model for the X-ray temporal and spectral properties of accreting black holes

By G. Miniutti, A. C. Fabian. 2003

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Some of the X-ray temporal and spectral properties of accreting black holes represent a challenge for current theoretical models. In particular, uncorrelated variability between direct continuum and reflection components (including the iron line, if present) has been reported in many cases. Here, we explore a light bending model in which we assume a primary source of X-rays located close to a central, maximally rotating Kerr black hole and illuminating both the observer at infinity and the accretion disc. We show that, due to strong light bending, the observed flux can vary by more than one order of magnitude as the height of the primary source above the accretion disc varies, even if its intrinsic luminosity is constant. We identify three different regimes in which the reflection-dominated component (and the iron line) is correlated, anticorrelated or almost independent with respect to the direct continuum. These regimes correspond to low, high and intermediate flux states of the X-ray source. As a general rule, the reflection component varies with much smaller amplitude than the continuum. X-ray observations of the Seyfert galaxy MCG-6-30-15 and of the Galactic black hole candidate XTE Jl 650-500 reveal that a series of predictions of our model is actually observed; the consistent behaviour of the iron line flux and equivalent width with respect to the direct continuum, as well as the increase of the relative strength of disc reflection as the flux drops, all match very well our predictions. The iron line profile is predicted to be narrower in high flux states and broader in (reflection-dominated) low flux states, in fairly good agreement with observations of the best-studied case of MCG-6-30-15. Observations of some other narrow-line Seyfert 1 galaxies (e.g. NGC 4051) also seem to support our model, which may explain what are otherwise puzzling characteristics of some sources. We also show that beaming along the equatorial plane can enhance the re-emission of narrow reflection features from distant material during low flux states providing a possible contribution to the observed X-ray Baldwin effect.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/21cf90be2b0f2cde80da4c4f05e3b011d9ced385

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Citations in the corpus

An absorption origin for the X-ray spectral variability of MCG-6-30-15 (2008, background): “…continuum coupled with distortions of photon geodesics near the black hole, rather than by rest-frame intensity variations, so that the reflected emission appears more constant than the primary continuum seen by the observer (Fabian & Vaughan, 2003; Miniutti et al., 2003; Miniutti & Fabian, 2004).”
The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes (2014, background): “This is consistent with a central corona collapsing to small scale heights, such that most of the hard X-ray emission is focused onto the disk rather than emitted isotropically ([243, 244, 245, 246]).”
Suzaku observations of 'bare' active galactic nuclei (2012, neutral): “…user on 14 February 2018 Downloaded from https://academic.oup.com/mnras/article-abstract/428/4/2901/992867 by Saint Mary's University user on 14 February 2018 Figure 1 – continued source is also located in a region of extreme gravity (Miniutti & Fabian 2004), or of relativistic time dilation.”
Suzaku Observations of the Hard X-ray Variability of MCG-6-30-15: the Effects of Strong Gravity Around a Kerr Black Hole (2006, background): “We point out that this is exactly the behaviour predicted by the light bending model (Miniutti & Fabian 2004).”
Constraining Black Hole Spin via X-Ray Spectroscopy (2006, background): “An alternative mechanism consists of the strong gravitational focusing of a high-latitude source above a rapidly rotating black hole (Martocchia & Matt 1996; Miniutti & Fabian 2004).”
Simultaneous X-ray/optical/UV snapshots of active galactic nuclei from XMM–Newton: spectral energy distributions for the reverberation mapped sample (2008, background): “It is possible to model the spectrum with partial covering or strong reflection due to light bending (Miniutti & Fabian 2004) for both of these objects, again making the intrinsic X-ray luminosity difficult to determine.”
Broad Iron-Kα Emission Lines as a Diagnostic of Black Hole Spin (2007, methodology): “Observational support for just such a geometry has emerged from detailed studies of iron line and reflection bump variability by XMM-Newton (Fabian & Vaughan 2003; Miniutti & Fabian 2004) and Suzaku (Miniutti et al. 2007).”
The role of the reflection fraction in constraining black hole spin (2014, methodology): “In the case of the lamp post model, as already noticed previously (Miniutti & Fabian 2004), when the source height decreases, strong gravitational beaming reduces the number of photons that reach infinity whilefAD increases, thus enhancing the reflection features (see Fig.”
An Explanation for the soft X-ray excess in AGN (2005, background): “If the source of the illuminating continuum is above the disc, the light from it will be bent and impact on the disc, with little escaping to be observed (Miniutti & Fabian 2004).”
Broad iron L line and X-ray reverberation in 1H0707-495 (2009, background): “The decreasing strength of the features at 1 and 7 keV is similar to that seen in MCG-6-30-15 (Vaughan et al. 2003) and is in line with the predictions of the light bending model (Miniutti & Fabian 2004).”
STELLAR-MASS BLACK HOLE SPIN CONSTRAINTS FROM DISK REFLECTION AND CONTINUUM MODELING (2009, methodology): “Other results have found steeper emissivity indices, p rhaps suggesting that the hard component is anisotropic (Miniutti & Fabian 2004).”
CAIXA: a catalogue of AGN in the XMM-Newton archive - II. Multiwavelength correlations (2009, background): “In one scenario, the soft excess arises from the enhancement of reflection from the inner regions of the accretion disc due to light-bending effects, together with a strong suppression of the primary emission (e.g. Miniutti & Fabian 2004).”
Global optical/infrared-X-ray correlations in X-ray binaries: quantifying disc and jet contributions (2006, background): “For example, the X-ray emitting region in BHXBs may have a larger scale height than in NSXBs and will therefore illuminate the disc more readily (e.g. Miniutti & Fabian 2004).”
Irradiation of an accretion disc by a jet: general properties and implications for spin measurements of black holes (2013, background): “Therefore less photons are left over to contribute to the continuum component, which is directly emitted towards the observer (Miniutti & Fabian 2004).”
Going with the Flow: Can the Base of Jets Subsume the Role of Compact Accretion Disk Coronae? (2005, background): “This value is actually a lower limit because it does not include the effects of light bending, which will serve to increase the reflection fraction, possibly quite significantly (Miniutti & Fabian 2004).”
Relativistic X-Ray Lines from the Inner Accretion Disks Around Black Holes (2007, background): “To the extent that variability can serve as a check on spin measurements from direct line fitting, current variability studies suggest that direct line fitting is yielding accurate spin constraints (see, e.g., Miniutti & Fabian 2004).”
BAT AGN Spectroscopic Survey. V. X-Ray Properties of the Swift/BAT 70-month AGN Catalog (2017, background): “This might lend support to the idea that this enhanced fraction of reprocessed flux with respect to the primary X-ray emission is caused by relativistic reflection, as predicted by the light-bending scenario (e.g., Miniutti & Fabian 2004).”