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Mapping accreting black holes using X-ray variability

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Stellar-mass black holes accreting gas from a binary partner (X-ray binaries) and supermassive black holes accreting gas from their host galaxy (active galactic nuclei) can emit a huge X-ray flux from the vicinity of the black hole event horizon. This can be exploited to probe the strong field regime of General Relativity and measure the properties of the black hole: its mass and angular momentum. For all but two objects in the Universe, the vicinity of the accreting BH is far too small to directly image, necessitating the use of mapping techniques that exploit rapid X-ray variability. I will talk about two such techniques that utilise the relativistically broadened iron emission line routinely observed in the X-ray spectrum that results from continuum photons reflecting from the accretion disk. First I will talk about reverberation mapping, which exploits the light-crossing delay between directly observed and reflected photons. I will present our mass measurement of the black hole in Cygnus X-1 – the first X-ray reverberation mass measurement of a stellar-mass black hole. I will then talk about phase-resolved spectroscopy of quasi-periodic oscillations (QPOs) in X-ray binary systems. I will present our discovery of a ~4 s quasi-periodic variation of the iron line centroid energy in H 1743 -322, and our detailed `tomographic’ modelling of the QPO phase dependent reflection spectrum of MAXI J1535 -571. Both analyses provide strong evidence that the QPO results from nodal precession of the inner accretion flow, although it is unclear whether or not this precession is specifically relativistic Lense-Thirring precession.

This talk is part of the Institute of Astronomy Colloquia series.

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