Correlated continuum and ionized iron emission and absorption in the black hole X-ray binary V4641 Sgr

We report on NuSTAR observations of the black hole X-ray binary V4641 Sgr during its 2021 outburst. NuSTAR observed the source 5 times during roughly a 1 month window, capturing its complex soft state spectrum. V4641 Sgr is known to exhibit a scattered continuum spectrum, accompanied by highly ionized emission lines. NuSTAR, with its 3-80 keV broadband X-ray coverage, captured the 6-7 keV and ~ 8 keV iron emission features. The spectrum also displays a curious absorption edge feature at ~9.5 keV. We will present our detailed analysis of the NuSTAR spectra. We confirm the previously reported edge feature, and show that it is consistently present across all 5 observations. This edge feature was previously claimed to be Nickel in the reprocessing gas, however we argue that it is likely associated with ionized iron. Furthermore, the emission line features are tightly correlated with the continuum emission in terms of observed flux, suggesting a close connection between the scattered and line emission.