Measuring Black Hole Spin via the X-ray Continuum Fitting Method: Beyond the Thermal Dominant State

TL;DR

This study extends the X-ray continuum-fitting method for measuring black hole spins beyond the thermal dominant state by demonstrating its effectiveness on spectra with strong Comptonization, using self-consistent modeling of RXTE data.

Contribution

The paper introduces a self-consistent Comptonization model enabling spin measurements from spectra with strong power-law components, broadening the applicability of the continuum-fitting method.

Findings

Inner disk radius remains constant despite increased Comptonization.

The method yields consistent spin estimates across different spectral states.

Application to sources like Cyg X-1 is now feasible.

Abstract

All prior work on measuring the spins of stellar-mass black holes via the X-ray continuum-fitting method has relied on the use of weakly-Comptonized spectra obtained in the thermal dominant state. Using a self-consistent Comptonization model, we show that one can analyze spectra that exhibit strong power-law components and obtain values of the inner disk radius, and hence spin, that are consistent with those obtained in the thermal dominant state. Specifically, we analyze many RXTE spectra of two black hole transients, H1743-322 and XTE J1550-564, and we demonstrate that the radius of the inner edge of the accretion disk remains constant to within a few percent as the strength of the Comptonized component increases by an order of magnitude, i.e., as the fraction of the thermal seed photons that are scattered approaches 25%. We conclude that the continuum-fitting method can be applied to a much wider body of data than previously thought possible, and to sources that have never been observed to enter the thermal dominant state (e.g., Cyg X-1).