Revealing the X-ray source in IRAS 13224-3809 through flux-dependent reverberation lags

TL;DR

This study analyzes flux-dependent X-ray reverberation lags in IRAS 13224-3809, revealing that the source's compactness varies with flux, affecting the lag characteristics and providing insights into the accretion disk and corona geometry.

Contribution

It demonstrates flux-dependent changes in reverberation lags, linking lag behavior to source compactness and light bending effects in IRAS 13224-3809.

Findings

Soft lag frequency and amplitude vary with flux state.

Low-flux intervals show higher frequency, smaller amplitude soft lags.

Lag signature at Fe K line energy is prominent during low-flux periods.

Abstract

IRAS 13224-3809 was observed in 2011 for 500 ks with the XMM-Newton observatory. We detect highly significant X-ray lags between soft (0.3 - 1 keV) and hard (1.2 - 5 keV) energies. The hard band lags the soft at low frequencies (i.e. hard lag), while the opposite (i.e. soft lag) is observed at high frequencies. In this paper, we study the lag during flaring and quiescent periods. We find that the frequency and absolute amplitude of the soft lag is different during high-flux and low-flux periods. During the low flux intervals, the soft lag is detected at higher frequencies and with smaller amplitude. Assuming that the soft lag is associated with the light travel time between primary and reprocessed emission, this behaviour suggests that the X-ray source is more compact during low-flux intervals, and irradiates smaller radii of the accretion disc (likely because of light bending effects). We continue with an investigation of the lag dependence on energy, and find that isolating the low-flux periods reveals a strong lag signature at the Fe K line energy, similar to results found using 1.3 Ms of data on another well known Narrow-Line Seyfert I galaxy, 1H0707-495.