X-ray Measurements of Accreting black holes with Polarimetric-Spectral-timing techniques

Introduction

Black hole (BH) X-ray binaries (XRBs) radiate a huge X-ray flux from the BH vicinity. The X-ray signal, radiated by a hot corona and a cooler disk, contains information on the BH mass and spin, and the strong gravitational field close to its horizon. However, the X-ray emitting region is unresolvable, necessitating indirect mapping techniques.

Methodology

I pioneered forward-modelling spectral-timing techniques that exploit rapid spectral variability and played a key role in the first studies of X-ray polarization. X-MAPS will take the novel and transformational step of combining these two powerful diagnostics into polarimetric-spectral-timing. This requires a huge increase in the computational intensity of my state-of-the-art models that my group will enable with machine learning.

Science Goals

1. Understand state transitions: We will constrain how the structure of the disk-corona system evolves as the spectral shape changes and large-scale transient jets are launched, informing on the jet launching mechanism and thus how supermassive BHs influence their host galaxies.

2. Measure BH mass: The current observational picture that BHs in gravitational wave (GW) sources are heavier than those in Galactic XRBs has deep implications for binary evolution theory. We will make BH mass measurements using the X-ray signal alone, enabling measurements for XRBs inaccessible to traditional optical techniques (~70% of the population), which are thought to harbour heavier BHs. This will enable us to test the importance of observational bias in the comparison of XRBs with GW sources.

3. Measure 3D BH spin orientation: I recently found strong evidence that quasi-periodic oscillations (QPOs) observed in the X-ray flux of XRBs are driven by relativistic precession of the corona around the BH spin axis: variation of X-ray polarization with QPO phase. We will reconstruct the precession cone and thus the BH spin vector, providing new insights into binary evolution theory.