X-ray reverberation in NLS1

📝 Abstract

Reverberation from scattering material around the black hole in active galactic nuclei is expected to produce a characteristic signature in a Fourier analysis of the time delays between directly-viewed continuum emission and the scattered light. Narrow-line Seyfert 1 galaxies (NLS1) are highly variable at X-ray energies, and are ideal candidates for the detection of X-ray reverberation. We show new analysis of a small sample of NLS1 that clearly shows the expected time-delay signature, providing strong evidence for the existence of a high covering fraction of scattering and absorbing material a few tens to hundreds of gravitational radii from the black hole. We also show that an alternative interpretation of time delays in the NLS1 1H0707-495, as arising about one gravitational radius from the black hole, is strongly disfavoured in an analysis of the energy-dependence of the time delays.

💡 Analysis

Reverberation from scattering material around the black hole in active galactic nuclei is expected to produce a characteristic signature in a Fourier analysis of the time delays between directly-viewed continuum emission and the scattered light. Narrow-line Seyfert 1 galaxies (NLS1) are highly variable at X-ray energies, and are ideal candidates for the detection of X-ray reverberation. We show new analysis of a small sample of NLS1 that clearly shows the expected time-delay signature, providing strong evidence for the existence of a high covering fraction of scattering and absorbing material a few tens to hundreds of gravitational radii from the black hole. We also show that an alternative interpretation of time delays in the NLS1 1H0707-495, as arising about one gravitational radius from the black hole, is strongly disfavoured in an analysis of the energy-dependence of the time delays.

📄 Content

arXiv:1106.3648v1 [astro-ph.HE] 18 Jun 2011 X-ray reverberation in NLS1 Lance Miller∗a and T. Jane Turnerb aDept. of Physics, Oxford University, U.K. E-mail: L.Miller@physics.ox.ac.uk bDept. of Physics, University of Maryland Baltimore County, U.S.A. E-mail: tjturner@umbc.edu Reverberation from scattering material around the black hole in active galactic nuclei is expected to produce a characteristic signature in a Fourier analysis of the time delays between directly- viewed continuum emission and the scattered light. Narrow-line Seyfert 1 galaxies (NLS1) are highly variable at X-ray energies, and are ideal candidates for the detection of X-ray reverberation. We show new analysis of a small sample of NLS1 that clearly shows the expected time-delay signature, providing strong evidence for the existence of a high covering fraction of scattering and absorbing material a few tens to hundreds of gravitational radii from the black hole. We also show that an alternative interpretation of time delays in the NLS1 1H 0707–495, as arising about one gravitational radius from the black hole, is strongly disfavoured in an analysis of the energy-dependence of the time delays. Narrow-Line Seyfert 1 Galaxies and their place in the Universe - NLS1, April 04-06, 2011 Milan Italy ∗Speaker. c ⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ X-ray reverberation in NLS1 Lance Miller

1. Introduction The measurement of reverberation at optical wavelengths has become a powerful technique for estimating the size of the broad-line region in active galactic nuclei (AGN), and combining with line velocity widths allows estimates of black hole mass in AGN to be made (e.g. [1]). Here we describe the characteristic signature we expect from reverberation and show evidence that this signature has been detected at X-ray energies, implying the existence of large global covering factors of material a few light hours from the black hole, corresponding to a few tens to hundreds of gravitational radii (rg ≡GM/c2).
2. Reverberation signatures in Fourier space 2.1 The analysis of data Reverberation signatures are measured by cross-correlating two time series and searching for time delays between them. In AGN at optical wavelengths, one of the time series is the flux measured in a continuum bandpass, the other the flux measured in an emission line. A measured time delay between these two gives us the light travel time between the continuum source and the line-emission region, averaged over the distribution of circumnuclear material. A natural approach to measuring cross-correlation time delays is to work in Fourier space, and we shall see below that reverberation creates a characteristic signature in the Fourier phases as a function of the frequency of variation of the source. However, we cannot simply take the Fourier transforms of the two light curves to make such an analysis, for several reasons:
3. Observed time series are discretely sampled, with non-uniform coverage and large gaps be- tween multiple observations. If Fourier transformed, the window function of the observations would completely dominate the signal.
4. Observed time series have measurement noise, which adds a floor to the powerspectrum and which may bias the measured time delay if uncorrected.
5. Any one set of data is just a particular realisation of the source’s variations: it is a snapshot in time. In order to correctly estimate the uncertainties in our measurements, we must allow for the expected statistical variations in the source. To tackle these issues, we have developed a maximum-likelihood approach, which finds the pow- erspectra and cross-powerspectra that best fit the time-domain data, taking full account of the sam- pling and noise, and with estimated measurement uncertainties that fully account for the “cosmic variance” of item (3) above. The method is based on the “direct likelihood” approach used for analysis of small cosmic microwave background datasets [2]. More details are given by Miller et al. [3, 4]. 2.2 The reverberation signature In the following work, we shall consider the “lag spectrum”: the time lags between two time series as a function of the frequency of the source variation. These time lags τ are obtained from the phases φ of the Fourier transform of the cross-correlation function, τ = φ/ω, where ω is the angular frequency. 2 X-ray reverberation in NLS1 Lance Miller 10−4 10−3 0.01 −200 0 200 400 600 τ /s ν /Hz 0.01 0.1 0.02 0.05 0.2 −2 0 2 4 τ /day ν /day−1 Figure 1: Left panel: The lag spectrum expected from a simple partial-covering thin shell of diameter 2000 light-seconds, showing the rapid transition in time delay at ν ≃5 × 10−4 Hz, when the time period of the Fourier mode equals the light travel time across the shell (the low-frequency time delay of 500 s has been diluted in this realisation by adding direct light into the scattered-light time series). Right

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