Revisiting The Spectral and Timing Properties of NGC 4151

TL;DR

This study reanalyzed X-ray data of NGC 4151, revealing that absorption effects, not relativistic reflection, explain observed spectral and timing features, and providing new insights into the structure of its broad line region.

Contribution

It demonstrates that absorption effects, rather than relativistic reflection, dominate the spectral and timing properties of NGC 4151, revising previous interpretations.

Findings

Absence of evidence for relativistic reflection in new data.

Energy-dependent lags are caused by absorption effects.

The Fe Kα line variations are delayed by about 3.3 days, indicating an origin in the inner broad line region.

Abstract

NGC 4151 is the brightest Seyfert 1 nucleus in X-rays. It was the first object to show short time delays in the Fe K band, which were attributed to relativistic reverberation, providing a new tool for probing regions at the black hole scale. Here, we report the results of a large XMM-Newton campaign in 2015 to study these short delays further. Analyzing high quality data that span time scales between hours and decades, we find that neutral and ionized absorption contribute significantly to the spectral shape. Accounting for their effects, we find no evidence for a relativistic reflection component, contrary to early work. Energy-dependent lags are significantly measured in the new data, but with an energy profile that does not resemble a broad iron line, in contrast to the old data. The complex lag-energy spectra, along with the lack of strong evidence for a relativistic spectral component, suggest that the energy-dependent lags are produced by absorption effects. The long term spectral variations provide new details on the variability of the narrow Fe K$\alpha$ line . We find that its variations are correlated with, and delayed with respect to, the primary X-ray continuum. We measure a delay of $\tau= 3.3^{+1.8}_{-0.7}$ days, implying an origin in the inner broad line region (BLR). The delay is half the H$\beta$ line delay, suggesting a geometry that differs slightly from the optical BLR.