On measuring the Hubble constant with X-ray reverberation mapping of active galactic nuclei
We show that X-ray reverberation mapping can be used to measure the distance to type 1 active galactic nuclei (AGNs). This is because X-ray photons originally emitted from the 'corona' close to the black hole irradiate the accretion disc and are reemitted with a characteristic 'reflection' spectrum that includes a prominent ∼6.4 keV iron emission line. The shape of the reflection spectrum depends on the irradiating flux, and the light-crossing delay between continuum photons observed directly from the corona and the reflected photons constrain the size of the disc. Simultaneously modelling the X-ray spectrum and the time delays between photons of different energies therefore constrains the intrinsic reflected luminosity, and the distance follows from the observed reflected flux. Alternatively, the distance can be measured from the X-ray spectrum alone if the black hole mass is known. We develop a new model of our RELTRANS X-ray reverberation mapping package, called RTDIST, that has distance as a model parameter. We simulate a synthetic observation that we fit with our new model, and find that this technique applied to a sample of ∼25 AGNs can be used to measure the Hubble constant with a 3σ statistical uncertainty of ∼6km s⁻¹ Mpc⁻¹. Since the technique is completely independent of the traditional distance ladder and the cosmic microwave background radiation, it has the potential to address the current tension between them. We discuss sources of modelling uncertainty, and how they can be addressed in the near future.
Additional Information© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2021 October 8. Received 2021 September 15; in original form 2021 July 9. Published: 18 October 2021. AI acknowledges support from the Royal Society. MvdK acknowledges support from an NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek) Spinoza grant. AI thanks K. Madsen for discussions about NuSTAR flux calibration and R. Davies for discussions about galaxy peculiar velocity. We thank the anonymous referee for insightful comments. Data Availability: The RELTRANS package can be downloaded from https://adingram.bitbucket.io/reltrans.html. The updates described in this paper will be included in a future public release.
Published - stab2950.pdf
Accepted Version - 2110.15651.pdf