Daily fluctuations propagate damply through a black hole accretion disk

Abstract: Propagating fluctuations within accretion disks are known to induce multi-wavelength variability across diverse timescales. While these fluctuations have been widely invoked to explain rapid timing phenomena within the inner disk region in the frequency domain, observational signatures of outer-disk fluctuations propagating in the time domain remain sparse. Here, we present an analysis of observations by the Hard X-ray Modulation Telescope (HXMT) during the 2023 outburst of the newly discovered low-mass black hole X-ray binary Swift J1727.8-1613. Follow-up, high-cadence monitoring reveals intense variability in disk emission, attributable to fluctuations in the accretion rate. These disk fluctuations exhibit damped amplitudes and shortened flare periods. We interpret these features as observational evidence of fluctuations originating at and propagating from large radii, supported by fitting the disk light curves with a propagating fluctuation model. Furthermore, we propose that a plausible mechanism driving these fluctuations is the cyclical propagation of heating and cooling fronts in the context of the disk instability model. This work bridges theoretical predictions with time-domain observations, offering critical insights into the dynamic processes governing accretion disks.