The changing X-ray time lag in MCG-6-30-15

TL;DR

This study analyzes X-ray time lags in the galaxy MCG-6-30-15, revealing a transition from hard to soft lag at low frequencies and suggesting multiple variability mechanisms including coronal and geometrical changes.

Contribution

It provides the first detailed comparison of X-ray time lags over a decade, highlighting the changing lag behavior and proposing a dual-mechanism model for variability.

Findings

Hard lag increases with energy at high frequencies

Soft lag dominates at frequencies below ~1e-4 Hz

No clear iron K lag detected in either observation

Abstract

MCG-6-30-15 is one of the most observed Narrow Line Seyfert 1 galaxies in the X-ray band. In this paper we examine the X-ray time lags in this source using a total of 600 ks in observations (440 ks exposure) taken with the XMM-Newton telescope (300 ks in 2001 and 300 ks in 2013). Both the old and new observations show the usual hard lag that increases with energy, however, the hard lag turns over to a soft lag at frequencies below ~1e-4 Hz. The highest frequencies (~1e-3 Hz) in this source show a clear soft lag, as previously presented for the first 300 ks observation, but no clear iron K lag is detected in either the old or new observation. The soft lag is more significant in the old observation than the new. The observations are consistent with a reverberation interpretation, where the soft, reflected emission is delayed with respect to the hard powerlaw component. These spectral timing results suggest that two distinct variability mechanisms are important in this source: intrinsic coronal variations (which lead to correlated variability in the reprocessed emission), and geometrical changes in the corona. Variability due to geometrical changes does not result in correlated variability in the reflection, and therefore inhibits the clear detection of an iron K lag.