X-ray time delays from the Seyfert 2 galaxy IRAS 18325-5926

TL;DR

This study detects and analyzes energy-dependent X-ray time delays in the Seyfert 2 galaxy IRAS 18325-5926, revealing that hard X-ray variations lag behind softer ones, with implications for understanding accretion processes.

Contribution

First detection of energy-dependent X-ray time lags in IRAS 18325-5926, challenging simple reflection models and supporting propagating fluctuations or scattering as explanations.

Findings

Hard X-ray lags up to ~3ks increase with energy separation.

Lag-energy spectrum follows a log(E) shape, unlike reflection predictions.

Both propagating fluctuations and scattering models fit the data well.

Abstract

Using new XMM-Newton observations we detect hard X-ray time lags in the rapid variability of the Compton-thin Seyfert 2 galaxy IRAS 18325-5926. The higher-energy X-ray variations lag behind correlated lower-energy variations by up to ~3ks and the magnitude of the lag increases clearly with energy separation between the energy bands. We find that the lag-energy spectrum has a relatively simple log(E) shape. This is quite different in both shape and magnitude from the lags predicted by simple reflection models, but very similar to the hard X-ray lags often seen in black hole X-ray binaries. We apply several spectral models to the lag-energy spectrum and rule out simple reflection as an origin for the hard lags. We find that both propagating fluctuations embedded in the accretion flow and electron scattering from material embedded in or behind a cold absorbing medium offer equally good fits to the observed low-frequency hard X-ray lags and are both consistent with the time-averaged spectrum. Such models will likely look very different outside of XMM-Newton's observable bandpass, paving the way for future studies with NuSTAR.