On measuring the Hubble constant with X-ray reverberation mapping of active galactic nuclei

TL;DR

This paper proposes a novel X-ray reverberation mapping method to measure distances to active galactic nuclei, enabling an independent estimate of the Hubble constant that could resolve current cosmological tensions.

Contribution

The authors develop a new model, RTDIST, for X-ray reverberation mapping that directly estimates distances to AGNs, offering a novel approach to measure the Hubble constant.

Findings

Simulated application to 25 AGNs yields a Hubble constant estimate with 6 km/s/Mpc uncertainty.

The method is independent of traditional distance ladder and CMB techniques.

Potential to address the Hubble tension with future observational data.

Abstract

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 re-emitted with a characteristic `reflection' spectrum that includes a prominent $\sim 6.4$ 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 constrains 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 $\sim 25$ AGNs can be used to measure the Hubble constant with a $3 \sigma$ statistical uncertainty of $\sim 6~{\rm km}~{\rm s}^{-1}{\rm Mpc}^{-1}$. 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.