The University of Southampton
University of Southampton Institutional Repository

Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry

Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry
Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry
Context. Theoretical modelling of time-lags between variations in the Fe Kα emission and the X-ray continuum might shed light on the physics and geometry of the X-ray emitting region in active galaxies (AGN) and X-ray binaries. We here present the results from a systematic analysis of time-lags between variations in two energy bands (5−7 vs. 2−4 keV) for seven X-ray bright and variable AGN.

Aims. We estimate time-lags as accurately as possible and fit them with theoretical models in the context of the lamp-post geometry. We also constrain the geometry of the X-ray emitting region in AGN.

Methods. We used all available archival XMM-Newton data for the sources in our sample and extracted light curves in the 5−7 and 2−4 keV energy bands. We used these light curves and applied a thoroughly tested (through extensive numerical simulations) recipe to estimate time-lags that have minimal bias, approximately follow a Gaussian distribution, and have known errors. Using traditional χ2 minimisation techniques, we then fitted the observed time-lags with two different models: a phenomenological model where the time-lags have a power-law dependence on frequency, and a physical model, using the reverberation time-lags expected in the lamp-post geometry. The latter were computed assuming a point-like primary X-ray source above a black hole surrounded by a neutral and prograde accretion disc with solar iron abundance. We took all relativistic effects into account for various X-ray source heights, inclination angles, and black hole spin values.

Results. Given the available data, time-lags between the two energy bands can only be reliably measured at frequencies between ~5 × 10-5 Hz and ~10-3 Hz. The power-law and reverberation time-lag models can both fit the data well in terms of formal statistical characteristics. When fitting the observed time-lags to the lamp-post reverberation scenario, we can only constrain the height of the X-ray source. The data require, or are consistent with, a small (≲ 10 gravitational radii) X-ray source height.

Conclusions. In principle, the 5−7 keV band, which contains most of the Fe Kα line emission, could be an ideal band for studying reverberation effects, as it is expected to be dominated by the X-ray reflection component. We here carried out the best possible analysis with XMM-Newton data. Time-lags can be reliably estimated over a relatively narrow frequency range, and their errors are rather large. Nevertheless, our results are consistent with the hypothesis of X-ray reflection from the inner accretion disc.
0004-6361
Epitropakis, A.
ba1b8bff-b304-4f31-883b-b8e3667e3d73
Papadakis, I. E.
2dbbcb1a-a6a1-4593-b885-2fa598ca8c85
Dovčiak, M.
e3d584fc-50d8-41f4-b227-1bec1b5b400b
Pecháček, T.
d6c5ab02-f64c-44a4-be16-d81061743511
Emmanoulopoulos, D.
ee2db4c6-3379-4604-8711-f779fb74f538
Karas, V.
652ebd3f-247a-418c-861d-741e4eb88fa3
Mchardy, I.M.
4f215137-9cc4-4a08-982e-772a0b24c17e
Epitropakis, A.
ba1b8bff-b304-4f31-883b-b8e3667e3d73
Papadakis, I. E.
2dbbcb1a-a6a1-4593-b885-2fa598ca8c85
Dovčiak, M.
e3d584fc-50d8-41f4-b227-1bec1b5b400b
Pecháček, T.
d6c5ab02-f64c-44a4-be16-d81061743511
Emmanoulopoulos, D.
ee2db4c6-3379-4604-8711-f779fb74f538
Karas, V.
652ebd3f-247a-418c-861d-741e4eb88fa3
Mchardy, I.M.
4f215137-9cc4-4a08-982e-772a0b24c17e

Epitropakis, A., Papadakis, I. E., Dovčiak, M., Pecháček, T., Emmanoulopoulos, D., Karas, V. and Mchardy, I.M. (2016) Theoretical modelling of the AGN iron-line vs continuum time-lags in the lamp-post geometry. Astronomy & Astrophysics, 594, [A71]. (doi:10.1051/0004-6361/201527748).

Record type: Article

Abstract

Context. Theoretical modelling of time-lags between variations in the Fe Kα emission and the X-ray continuum might shed light on the physics and geometry of the X-ray emitting region in active galaxies (AGN) and X-ray binaries. We here present the results from a systematic analysis of time-lags between variations in two energy bands (5−7 vs. 2−4 keV) for seven X-ray bright and variable AGN.

Aims. We estimate time-lags as accurately as possible and fit them with theoretical models in the context of the lamp-post geometry. We also constrain the geometry of the X-ray emitting region in AGN.

Methods. We used all available archival XMM-Newton data for the sources in our sample and extracted light curves in the 5−7 and 2−4 keV energy bands. We used these light curves and applied a thoroughly tested (through extensive numerical simulations) recipe to estimate time-lags that have minimal bias, approximately follow a Gaussian distribution, and have known errors. Using traditional χ2 minimisation techniques, we then fitted the observed time-lags with two different models: a phenomenological model where the time-lags have a power-law dependence on frequency, and a physical model, using the reverberation time-lags expected in the lamp-post geometry. The latter were computed assuming a point-like primary X-ray source above a black hole surrounded by a neutral and prograde accretion disc with solar iron abundance. We took all relativistic effects into account for various X-ray source heights, inclination angles, and black hole spin values.

Results. Given the available data, time-lags between the two energy bands can only be reliably measured at frequencies between ~5 × 10-5 Hz and ~10-3 Hz. The power-law and reverberation time-lag models can both fit the data well in terms of formal statistical characteristics. When fitting the observed time-lags to the lamp-post reverberation scenario, we can only constrain the height of the X-ray source. The data require, or are consistent with, a small (≲ 10 gravitational radii) X-ray source height.

Conclusions. In principle, the 5−7 keV band, which contains most of the Fe Kα line emission, could be an ideal band for studying reverberation effects, as it is expected to be dominated by the X-ray reflection component. We here carried out the best possible analysis with XMM-Newton data. Time-lags can be reliably estimated over a relatively narrow frequency range, and their errors are rather large. Nevertheless, our results are consistent with the hypothesis of X-ray reflection from the inner accretion disc.

Text
1607.02625v2 - Accepted Manuscript
Download (462kB)

More information

Accepted/In Press date: 7 July 2016
e-pub ahead of print date: 13 October 2016
Published date: October 2016
Organisations: Astronomy Group, Physics & Astronomy

Identifiers

Local EPrints ID: 411799
URI: http://eprints.soton.ac.uk/id/eprint/411799
ISSN: 0004-6361
PURE UUID: 2b4365ca-24d5-4d59-9c99-dfb81278c080
ORCID for D. Emmanoulopoulos: ORCID iD orcid.org/0000-0002-3335-2446

Catalogue record

Date deposited: 26 Jun 2017 16:30
Last modified: 10 Dec 2019 01:41

Export record

Altmetrics

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×