Kavli Affiliate: Erin Kara
| Summary:
Using XRISM/Resolve $439 , rm ks$ time-averaged spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 3783, we investigate the nature of the Fe K$α$ emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. Even the narrow component of the line is resolved with evident Fe K$α_1$ (6.404 keV) and K$α_2$ (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with a neutral gas with negligible bulk velocity. The narrow and intermediate-width components have a full-width at half maximum (FWHM) of 350 $pm$ 50 km/s and $3510 pm 470 , rm km/s$, respectively, suggesting that they arise in the outer disk/torus and/or BLR. We detect a $10%$ excess flux around 4 $-$ 7 keV that is not well described by a symmetric Gaussian line, but is consistent with a relativistically broadened emission line. In this paper, we take the simplest approach to model the asymmetric line as a single emission line (assuming either neutral, He-like or H-like iron) convolved with a relativistic disk line model. As expected, the inferred inclination angle is highly sensitive to the assumed ionization state, and ranges between $i=17-44^ circ$. This model also constrains the black hole spin via the extent of the red wing: the required gravitational redshift in the fitted disk-line profile disfavors a non-spinning (Schwarzschild) black hole. The derived inner radius is close to the radius of the innermost stable circular orbit $r_rm ISCO$ and strongly correlated with the black hole spin. To better constrain the spin, we fix the inner radius at $r_rm ISCO$ and derive a lower limit on the spin of $a ge 0.29$ at the 3 $σ$ confidence level. A Compton shoulder is detected in our data as well as a $2-3 , σ$ detection of the Cr K$α$ and Ni K$α$ lines.
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