A short-timescale negative optical continuum lag in SDSS J083717.88+191647

Continuum reverberation mapping (RM) is a powerful technique for constraining the accretion disk structure in active galactic nuclei (AGNs). In typical cases, the shorter-wavelength emission is used as the reference, and a positive time lag is observed since the inner, hotter regions of the accretion disk respond earlier than the cooler outer regions at longer wavelengths. However, we detected a short-timescale negative inter-band lag in SDSS J083717.88+191647 using RM techniques, where the -band emission lags behind the -band emission. The light curves from the Transient Facility reveal two distinct phases, a stabilizing phase and a declining phase, in which the time lags show opposite signs. Using with the g-band as the reference, we obtained time lags of 3.68^ g r Zwicky JAVELIN +1.94 _ -2.78 days during the stabilizing phase and -1.60^ +0.69 _ -0.54 days during the declining phase. Although negative continuum lags have been reported in a few previous studies, the present case is distinguished by its clear phase dependence and the accompanying color evolution. We attribute the observed lag reversal to a moving dust-cloud obscuration scenario in which the cloud crossing the line of sight preferentially obscures emission from the outer longer-wavelength regions of the disk, causing the r-band to decline earlier than the g-band and thus producing the observed negative inter-band lag. Our results indicate that AGN variability may be more complex than previously thought. Future high-cadence, multiband observations will be essential to test this dust-obscuration model and to further explore the interplay between the accretion disk emission and dust in AGNs.