High‐Resolution X‐Ray Spectroscopy of the Fe K Complex in IC 4329A

Abstract

We report the detection of complex Fe K line emission from a Chandra High Energy Transmission Grating Spectrometer (HETGS) observation of the Seyfert 1 galaxy IC 4329A. The line emission is double-peaked, one peak centered at ~6.3 keV and the other at ~6.9 keV in the source rest frame. When modeled by Gaussians, the lower energy peak is resolved by the high-energy grating at greater than 99% confidence, while the higher energy peak is resolved at only less than 90% confidence. The best-fitting widths are ~21,000 and ~4000 km s-1 FWHM for the ~6.3 and ~6.9 keV peaks, respectively. If the peaks correspond to two distinct emission lines, then the peak energies are redshifted with respect to the expected line energies of Fe I Kα and Fe XXVI Lyα by at least 650 and 950 km s-1, respectively. Alternatively, the Fe K line profile may be due to a single line from a relativistic accretion disk. In that case, the inclination angle of the disk is required to be 24 degrees, the outer radius is constrained to several tens of gravitational radii, and the radial line emissivity is flatter than r-0.7. Another possibility is that both peaks are due to distinct lines but that each one is relativistically broadened by a disk. In that case, the lower energy peak could correspond to emission from Fe in a low ionization state and the high-energy peak to Fe XXVI Lyα emission. Then the inclination angle is even less, restricted to a few degrees. However, the radial emissivity law is allowed to be steeper (~r-2.5), and the outer radius does not have to be fine-tuned. Yet another scenario is that the lower energy peak originates in a disk but the higher energy peak originates in more distant matter. The disk inclination angle is then intermediate between the last two cases, but the emissivity is again required to be flat. We cannot rule out Fe XXV He-like absorption modifying the observed line profile. However, the data and the inferred emission-line parameters are insensitive to the presence of a Compton reflection continuum. Including Compton reflection does, however, allow a steeper radial emissivity law for the relativistic line. Future missions, such as Astro-E2, will be able to break a lot of the degeneracy in the physically distinct models that can all account for the Chandra data. Since IC 4329A is one of the brightest Seyfert 1 galaxies, it should be a good astrophysical laboratory for studying the ionization structure of accretion disks around supermassive black holes.