Compton hump reverberation lag in the bright Seyfert 1 galaxy IC 4329A with NuSTAR

Abstract

Recent reverberation delay measurements have moved beyond the 10 keV X-ray range, providing evidence for the Compton hump (a.k.a. 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 {\it NuSTAR} observation. We find a delayed response of the 20--30 keV X-ray band, with a lag time of $\sim1825$ s at frequencies $0.5-1.5 \times 10^{-4}$ Hz. The lag amplitude drops to $\sim195$ s as the frequencies increase to $(1.5-10)\times10^{-4}$ Hz. Including IC\,4329A, so far five sources have been explored for reflection hump reverberation. We perform reverberation modelling 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 $M_{\rm BH}=1.37_{-0.36}^{+0.33}\times10^8~M_{\odot}$ and the coronal height $h=2.45_{-2.36}^{+1.92}~R_{\rm g}$ (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 {\it 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 $50.26_{-4.03}^{+5.58}$ keV -- consistent with the previous result from the combined {\it Suzaku} and {\it NuSTAR} data.

Figures (8)

• Figure 1: Zooming the ratio of the NuSTAR spectrum to the simple power law model. The plot shows the presence of a strong narrow Fe K emission line at $\sim6.4$ keV and a clear Compton hump feature at $\sim$20--30 keV. The data is rebinned for visual clarity.
• Figure 2: High frequency lag-energy spectra of IC 4329A. From left to right the lag spectra are calculated for frequencies $(0.5-1.5)\times10^{-4}$ Hz, $(1.5-10)\times10^{-4}$) Hz, and $(0.5-12)\times10^{-4}$) Hz, respectively. The plots show the clear delayed response of the reflection hump at 20--30 keV band (indicated by the vertical shaded region), with lag amplitudes of $\sim1825$ s, $\sim195$ s, and $\sim205$ s from left to right, respectively. A broader frequency range is used in the right panel to increase the signal-to-noise.
• Figure 3: Low-frequency lag-energy spectra of IC 4329A, computed for frequencies below $0.5\times10^{-4}$ Hz and $2\times10^{-4}$ Hz. Two frequencies are shown for demonstrative purposes, indicating that the lag decreases as the frequencies are increased. In each spectrum, there appears a peak in the 20--30 keV band where the reflection hump dominates.
• Figure 4: Modelled lag spectra of IC 4329A. Different frequency ranges are shown in different colors and symbols. The spectra are fitted using the relativistic transfer function model kynreverb. The dashed lines represent the model lags whereas the error bars represent the observed lags over energy bins.
• Figure 5: NuSTAR spectra of IC 4329A fitted using relativistic reflection model relxillCp. Shown are the binned spectra from FPMA(blue) and FPMB (orange) fitted simultaneously. The default NuSTAR bin width of 0.04 keV is changed to 0.2/0.4 keV above and below 10 keV (see text).