The Spin of the Supermassive Black Hole in NGC 3783

TL;DR

This study uses the Suzaku Spin Survey approach to constrain the spin of the supermassive black hole in NGC 3783 by modeling relativistic reflection from the inner accretion disk. The authors construct a physically self-consistent broadband model that combines three-zone warm absorption guided by Chandra/HETG data, distant neutral reflection, a soft excess, and blurred ionized disk reflection via reflionx+relconv. They find a rapidly spinning black hole with $a \ge 0.98$, an inner disk with very low ionization ($\xi < 9$ erg cm s$^{-1}$), and a steep inner emissivity profile ($q_1 \approx 5.2$) that breaks at $r_{\rm br} \approx 5.4\,r_g$, with high iron abundance ($A_{\rm Fe} \approx 2.8$–$4.6$ solar); results are robust to variations in soft-band modeling and cross-normalization, though some parameter degeneracies persist. These findings demonstrate the feasibility and reliability of spin constraints in complex AGN spectra and inform potential selection biases in flux-limited SMBH spin studies.

Abstract

The Suzaku AGN Spin Survey is designed to determine the supermassive black hole spin in six nearby active galactic nuclei (AGN) via deep Suzaku stares, thereby giving us our first glimpse of the local black hole spin distribution. Here, we present an analysis of the first target to be studied under the auspices of this Key Project, the Seyfert galaxy NGC 3783. Despite complexity in the spectrum arising from a multi-component warm absorber, we detect and study relativistic reflection from the inner accretion disk. Assuming that the X-ray reflection is from the surface of a flat disk around a Kerr black hole, and that no X-ray reflection occurs within the general relativistic radius of marginal stability, we determine a lower limit on the black hole spin of a > 0.88 (99% confidence). We examine the robustness of this result to the assumption of the analysis, and present a brief discussion of spin-related selection biases that might affect flux-limited samples of AGN.

Figures (6)

• Figure 1: Co-added and background subtracted XIS light curves in the soft ($0.3-1 {\rm keV}$) and hard ($2-10 {\rm keV}$) bands, together with the background subtracted PIN ($16-45 {\rm keV}$) light curve. These light curves are shown with $5000 {\rm s}$ bins.
• Figure 2: Left panel: Simple power-law fit to the $3.5-45 {\rm keV}$ XIS-FI+PIN spectrum. Middle panel: Zoom-in on the 4--8 keV region of the simple power-law fit. Note the probable "Compton shoulder" on the immediate low-energy side of the strong 6.4 keV emission line. Right panel: Residuals remaining when the broad iron line component is removed from a simple phenomenological fit to the $3.5-45 {\rm keV}$ data (see §\ref{['sec:hard']}). In all panels the black points correspond to XIS 0+3 data, red to XIS 1 data and blue to PIN data. The green line represents a data-to-model ratio of unity.
• Figure 3: Folded Chandra/HETG spectrum and best fitting model as a function of observed energy. As described in the text (§\ref{['sec:hetg']}), the model is fitted simultaneously to the $\pm 1$ MEG (0.5--7 keV) and HEG (1--7.5 keV) data. However, for clarity, we only show here the $-$1-order MEG data (black; first three panels) and the $-$1-order HEG data (blue; bottom panel).
• Figure 4: Results of fitting the XIS+PIN spectrum with a model that includes the warm absorbers, distant reflection and scattering/leaked soft component but not the relativistic ionized accretion disk. While the fitting is performed on the 0.7--45 keV spectrum, we show for clarity only the residuals above 3 keV. Left: Strong residuals indicative of a broad iron line and Compton reflection hump are clearly visible. This motivates the inclusion of a relativistic disk component into the spectral model. The XIS 0+3 data are shown in black and XIS 1 data are in red, while the HXD/PIN data are in blue. The solid green line represents a data-to-model ratio of unity. Right: Zoom-in on the Fe K line region.
• Figure 5: Global modeling of the $0.7-45 {\rm keV}$ XIS-FI+PIN data. The left panel shows the resulting residuals from fitting the model (including the relativistic accretion disk) discussed in §\ref{['sec:global']}. Data point colors are as in Fig. 2. The right panel shows the best-fitting model color coded as follows: (a) green line, continuum power-law emission; (b) dark blue line, cold and ionized iron line emission from distant matter; (c) red line, soft excess modeled as blackbody; (d) magenta line, significant emission that scatters around or leaks through the warm absorber; (e) light blue line, relativistically-smeared disk reflection, (f) thick black line, total summed model spectrum. Warm absorption affects all components apart from (d).