Test whether the disk remains at the ISCO throughout the soft state

Establish, through simultaneous broadband spectroscopy and X-ray polarimetry over a wider span of the GRS 1739-278 outburst, whether the accretion disk remains at the innermost stable circular orbit throughout the soft state.

Background

The three NuSTAR observations indicate that the disk normalization and inferred physical inner radius remain nearly constant while the disk temperature declines, consistent with an inner disk edge near the ISCO. However, the conclusion is based on a short 12-day interval and limited spectral coverage, while the empirical curvature model does not provide a fully self-consistent treatment of reflection and returning radiation.

The authors therefore identify coordinated observations combining broadband spectroscopy and polarimetry across a larger portion of the outburst as necessary to test whether the apparent ISCO anchoring persists throughout the soft state rather than representing only the observed epochs.