XMM‐NewtonObservations of Broad Iron Kα Emission from Seyfert 1.9 Galaxy MCG −5‐23‐16

Abstract

XMM-Newton observations of the bright Seyfert 1.9 galaxy MCG -5-23-16 have revealed a broad Fe Kα emission line that is nearly symmetric in contrast to the broad and redshifted asymmetric Fe Kα line sometimes observed from Seyfert 1 galaxies. The Fe Kα line has two distinct components—a narrow unresolved component with an equivalent width of ~40 eV and a broad component with a FWHM of ~40,000 km s-1 and an equivalent width of ~120 eV. An absorption feature at ~7.1 keV has also been observed. The energies of the emission and absorption features are consistent with those arising from neutral iron. The broad component is consistent with a Fe Kα emission line expected from a relativistic accretion disk around a Schwarzschild or Kerr black hole. Alternatively, most of the flux in the broad component could also be modeled as reflection emission, which mimics emission line-like features because of the presence of iron K-shell edge at ~7.1 keV; however, the reflection fraction, R ~ 3, is much higher than that inferred from the BeppoSAX observations (R ~ 0.5). The disk inclination angle of ~47°, inferred from the disk-line fits, and the absorption column (NH ~ 1022 cm-2), inferred from the low-energy spectral curvature due to photoelectric absorption, suggest that our line of sight passes through the outer edge of a putative torus and are consistent with those expected for a Seyfert 1.9 galaxy falling within the unification scheme. The strength of the narrow iron Kα emission and the optical depth of the iron K absorption edge suggest their origin in the putative torus with NH ~ 1024 cm-2 in the inner regions and NH ~ 1022 cm-2 in the outer edges. The strength of the broad component of Fe Kα varied by a factor of ~2 between the two XMM-Newton observations taken ~6 months apart, while the narrow component of Fe Kα and the continuum flux did not appear to vary appreciably. There is evidence for a weakening in the strength of the broad iron Kα emission with the flattening of the observed continuum. This can perhaps be explained if the shape of the continuum is coupled with the ionization stage of the reflector.