2026ApJ..1000...84B

Recent reverberation delay measurements have moved beyond the 10 keV X-ray range, providing evidence for the Compton hump (aka the reflection hump) in the lag spectra. We report the relativistic reverberation of the reflection hump in the bright Seyfert 1 galaxy IC 4329A based on a long Nuclear Spectroscopic Telescope Array (NuSTAR) observation. We find a delayed response of the 20-30 keV X-ray band, with a lag time of ∼1825 s at frequencies of 0.5-1.5 × 10^–4 Hz. The lag amplitude drops to ∼195 s as the frequencies increase to (1.5-10) × 10^–4 Hz. Including IC 4329A, so far five sources have been explored for reflection hump reverberation. We perform reverberation modeling of the 3-50 keV lag-energy spectra using the general relativistic transfer function code, which provides independent timing-based measurements of the black hole mass ${M}_{{\rm{BH}}}=1.3{7}_{-0.36}^{+0.33}\times 1{0}^{8}\,{M}_{\odot }$ and the coronal height $h=2.4{5}_{-2.36}^{+1.92}\,{R}_{{\rm{g}}}$ (with uncertainties at 90% confidence). Within the uncertainties, the measured mass is found to be consistent with the previous finding. Furthermore, we undertake reflection spectroscopy to account for the hump feature and the associated relativistic effect using the time-averaged flux spectrum. Further sampling of the NuSTAR data (with a bin width of 0.2/0.4 keV below and above 10 keV) that reshapes the spectral resolution allows us to constrain the coronal temperature at $50.2{6}_{-4.03}^{+5.58}$ keV-consistent with the previous result from the combined Suzaku and NuSTAR data.