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Hard X-ray lags in active galactic nuclei: Testing the distant reverberation hypothesis with NGC 6814

Accepted version
Peer-reviewed

Type

Article

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Authors

Walton, DJ 
Zoghbi, A 
Cackett, EM 
Uttley, P 
Harrison, FA 

Abstract

We present an X-ray spectral and temporal analysis of the variable active galaxy NGC 6814, observed with Suzaku during November 2011. Remarkably, the X-ray spectrum shows no evidence for the soft excess commonly observed amongst other active galaxies, despite its relatively low level of obscuration, and is dominated across the whole Suzaku bandpass by the intrinsic powerlaw-like continuum. Despite this, we clearly detect the presence of a low frequency hard lag of ~1600s between the 0.5-2.0 and 2.0-5.0 keV energy bands at greater than 6-sigma significance, similar to those reported in the literature for a variety of other AGN. At these energies, any additional emission from e.g. a very weak, undetected soft excess, or from distant reflection must contribute less than 3% of the observed countrates (at 90% confidence). Given the lack of any significant continuum emission component other than the powerlaw, we can rule out models that invoke distant reprocessing for the observed lag behavior, which must instead be associated with this continuum emission. These results are fully consistent with a propagating fluctuation origin for the low frequency hard lags, and with the interpretation of the high frequency soft lags - a common feature seen in the highest quality AGN data with strong soft excesses - as reverberation from the inner accretion disk.

Description

Keywords

black hole physics, galaxies: active, X-rays: individual (NGC 6814)

Journal Title

Astrophysical Journal Letters

Conference Name

Journal ISSN

2041-8205
2041-8213

Volume Title

777

Publisher

American Astronomical Society
Sponsorship
Science and Technology Facilities Council (ST/J001538/1)
Science and Technology Facilities Council (ST/K000985/1)
National Aeronautics and Space Administration (NNX12AE13G)