2025ApJ...994..151G

The development and implementation of self-consistent spectrotemporal models of X-ray reverberation off of the inner accretion disk have produced several recent X-ray-reverberation-based black hole (BH) mass measurements. We analyze the 2016 simultaneous NuSTAR + Swift observation of the Seyfert 1.5 galaxy 4U 1344-60, presenting the first temporal evidence of X-ray reverberation from the inner disk in this source by using Gaussian process regression to interpolate over gaps in the light curves. We find no such temporal signature in the 2011 Suzaku data, consistent with a previous analysis of that flux spectrum. By performing a joint spectrotemporal fit to the flux and lag spectra of the 2016 data, we show that the lag spectrum requires a boosted reverberation signal and prefers an enhanced coronal height compared to the flux spectrum. These inconsistencies may point toward a coronal and/or inner disk geometry significantly different than the assumed lamppost and razor-thin ones. The best model description prefers radial gradients in disk density and ionization, yielding a BH mass of ${M}_{\,\rm{BH}\,}=\left({2}_{-1}^{+2}\right)\times 1{0}^{7}\,{M}_{\odot }$, which is comparable to the range of BH mass estimates in the literature (i.e., ${M}_{\,\rm{BH}\,}\approx \left(1-8\right)\times 1{0}^{7}\,{M}_{\odot }$). The lack of inner disk reverberation during the 2011 epoch and its emergence during the 2016 epoch may be due to a reduction in the beaming of a vertically extended X-ray corona or the inward migration of the inner disk radius if the disk is “slim” instead of razor-thin.