2024ApJ...973...92S

IGR J17091-3624, in addition to GRS 1915 + 105, is the only black hole X-ray binary that displays “heartbeat”-like variability, characterized by structured flares with high amplitudes. In this study, we conduct a detailed phase-resolved analysis of the recently identified “heartbeat”-like Class X variability in IGR J17091-3624 during its 2022 outburst, utilizing data from NICER and NuSTAR observations. A shortage in the high-energy (>20 keV) X-ray flux is detected at peak phases of the soft-X-ray flare at a ∼15σ confidence level from the phase-folded light curves. Furthermore, our phase-resolved spectral analysis reveals variations in the spectral shape, particularly showing significant synchronous variations in the disk temperature and flux with the count rate. These findings imply that the flare is primarily driven by instabilities within the accretion disk, consistent with previous studies of the well-known Class ρ variability in GRS 1915+105. However, we also observe a positive correlation between the disk temperature and flux over the flare cycle, which differs from the loop relation between the two parameters found in the Class ρ variability. This could suggest differences in the underlying physical processes between the two variability classes. Variations in the Comptonization component during flares are also observed: the electron temperature and covering fraction show anticorrelations with the disk flux, revealing potential interactions between the accretion disk and the corona during these flares.