Extreme X-ray Reflection in the Nucleus of the Seyfert Galaxy NGC 5033

TL;DR

NGC 5033 exhibits extreme X-ray reflection features, including a very strong Fe Kα line, suggesting a high reflection covering factor and variable flux, with implications for understanding Seyfert galaxy classifications and the X-ray Baldwin effect.

Contribution

This study provides new optical and X-ray observations of NGC 5033, revealing a broad Hβ line and unusually strong Fe Kα emission, highlighting complex reflection and variability phenomena in this Seyfert galaxy.

Findings

Broad Hβ line confirms Seyfert-1.5 classification.

Fe Kα line EW is exceptionally high for unobscured AGN.

Persistent strong Fe Kα line suggests high reflection covering factor.

Abstract

NGC 5033 is an intriguing Seyfert galaxy because its sub-classification may change with time, and because optical and sub-mm observations find that the massive black hole does not sit at the dynamical center of the galaxy, pointing to a past merger. We obtained a new optical spectrum of NGC 5033 using the 200'' Hale telescope at Palomar that clearly reveals a broad H$\beta$ line (FWHM$=5400\pm 300~{\rm km}~{\rm s}^{-1}$). This signals a clear view of the optical broad line region (BLR) and requires Seyfert-1.5 designation. Some spectra obtained in the past suggest a Seyfert-1.9 classification, potentially signaling a variable or "changing-look" geometry. Our analysis of a 2019 Chandra spectrum of the massive black hole reveals very little obscuration, also suggesting a clean view of the central engine. However, the narrow Fe~K$\alpha$ emission line is measured to have an equivalent with of EW$=460^{+100}_{-90}$~eV. This value is extremely high compared to typical values in unobscured AGN. Indeed, the line is persistently strong in NGC 5033: the line equivalent width in a 2002 XMM-Newton snapshot is EW$=250^{+40}_{-40}$~eV, similar to the EW$=290^{+100}_{-100}$~eV equivalent width measured using ASCA in 1999. These results can likely be explained through a combination of an unusually high covering factor for reflection, and fluxes that are seen out of phase owing to light travel times. We examine the possibility that NGC 5033 may strengthen evidence for the X-ray Baldwin effect.