The Nature and Cause of Spectral Variability in LMC X-1

TL;DR

This study investigates the spectral variability of the black-hole X-ray binary LMC X-1, revealing rapid disk fluctuations likely caused by coronal occultation rather than viscous propagation, challenging previous models.

Contribution

It introduces a new model attributing rapid spectral changes to coronal occultation, explaining short-term variability in LMC X-1's accretion disk.

Findings

Rapid disk temperature changes occur within half an hour.

Variations are better explained by coronal occultation than viscous propagation.

The model aligns with observed inverse flux-temperature correlation.

Abstract

We present the results of a long-term observation campaign of the extragalactic wind-accreting black-hole X-ray binary LMC X-1, using the Proportional Counter Array on the Rossi X-Ray Timing Explorer (RXTE). The observations show that LMC X-1's accretion disk exhibits an anomalous temperature-luminosity relation. We use deep archival RXTE observations to show that large movements across the temperature-luminosity space occupied by the system can take place on time scales as short as half an hour. These changes cannot be adequately explained by perturbations that propagate from the outer disk on a viscous timescale. We propose instead that the apparent disk variations reflect rapid fluctuations within the Compton up-scattering coronal material, which occults the inner parts of the disk. The expected relationship between the observed disk luminosity and apparent disk temperature derived from the variable occultation model is quantitatively shown to be in good agreement with the observations. Two other observations support this picture: an inverse correlation between the flux in the power-law spectral component and the fitted inner disk temperature, and a near-constant total photon flux, suggesting that the inner disk is not ejected when a lower temperature is observed.