Abstract
We present an analysis of the X-ray time lags for the highly variable Seyfert 1 galaxy NGC 4051, based on a series of XMM–Newton observations taken in 2009. We investigate the Fourier frequency dependent time lags in the light curves between the 0.3–1.0 keV and 2.0–5.0 keV energy bands as a function of source flux, including simultaneous modelling of the resulting lag-frequency spectra. We find the shape of the lag-frequency spectra to vary significantly and systematically with source flux. We model the lag-frequency spectra using simple transfer functions, and find that two time lag components are required, one in each energy band. The simplest acceptable fits have only the relative contribution of the lagged component in the hard band varying with flux level, which can be associated with changes in the energy spectrum. We discuss the interpretation of these results in terms of the currently popular models for X-ray time lags.
Highlights
Active galactic nuclei (AGN) are thought to be powered via accretion of gas on to a supermassive black hole (SMBH), with MBH ∼ 106–109 M, through an optically thick, geometrically thin disc (Shakura & Sunyaev 1973)
We have studied the frequency-dependent time delays in the Seyfert galaxy NGC 4051 using a series of XMM–Newton observations made in 2009
We found the lag-frequency spectrum varies significantly and systematically with source flux
Summary
Active galactic nuclei (AGN) are thought to be powered via accretion of gas on to a supermassive black hole (SMBH), with MBH ∼ 106–109 M , through an optically thick, geometrically thin disc (Shakura & Sunyaev 1973). Kotov, Churazov & Gilfanov 2001; King et al 2004; Zdziarski 2005; Arevalo & Uttley 2006; Ingram & Done 2010; Kelly, Sobolewska & Siemiginowska 2011), where the local mass accretion rate through the inner regions of the disc powers an extended X-ray emitting corona of hot electrons This model reproduces many of the currently known spectral variability patterns in AGN and XRBs (Arevalo & Uttley 2006). The soft lag is thought to represent the ‘reverberation’ signal as the primary X-ray Another model for the time lags has the X-rays scattering off a shell of circumnuclear material located at tens to thousands of rg from the central source of X-ray emission (Miller et al 2010a,b, hereafter M10a and M10b, respectively). The results of simulations used to assess the reliability of the recovered lag-frequency spectrum are described in Appendix A
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