The closest look at 1H0707-495: X-ray reverberation lags with 1.3 Ms of data

Reverberation lags in active galactic nuclei (AGN) were first discovered in the NLS1 galaxy, 1H0707-495. We present a follow-up analysis using 1.3 Ms of data, which allows for the closest ever look at the reverberation signature of this remarkable source. We confirm previous findings of a hard lag of ∼100 s at frequencies ν ∼ [0.5-4] × 10^-4 Hz, and a soft lag of ∼30 s at higher frequencies, ν ∼ [0.6-3] × 10^-3 Hz. These two frequency domains clearly show different energy dependences in their lag spectra. We also find evidence for a signature from the broad Fe Kα line in the high-frequency lag spectrum. We use Monte Carlo simulations to show how the lag and coherence measurements respond to the addition of Poisson noise and to dilution by other components. With our better understanding of these effects on the lag, we show that the lag-energy spectra can be modelled with a scenario in which low-frequency hard lags are produced by a compact corona responding to accretion rate fluctuations propagating through an optically thick accretion disc, and high-frequency soft lags are produced by short light-travel delay associated with reflection of coronal power-law photons off the disc.