Narrow Fe-K$\alpha$ Reverberation Mapping Unveils the Deactivated Broad-Line Region in a Changing-Look Active Galactic Nucleus

Abstract

"Changing-look active galactic nuclei" (CLAGNs) are known to change their apparent types between types 1 and 2, usually accompanied by a drastic change in their luminosity on timescales of years. However, it is still unclear whether materials in broad-line regions (BLRs) in CLAGNs appear and disappear during the type-transition or remain at the same location while the line production is simply activated or deactivated. Here we present our X-ray-optical monitoring results of a CLAGN, NGC 3516, by Suzaku, Swift, and ground telescopes, with our primary focus on the narrow Fe-K$\alpha$ emission line, which is an effective probe of the BLR materials. We detected significant variations of the narrow Fe-K$\alpha$ line on a timescale of tens of days during the type-2 (faint) phase in 2013-2014, and conducted "narrow Fe-K$\alpha$ reverberation mapping," comparing its flux variation with those of the X-ray continuum from a corona and $B$-band continuum from an accretion disk. We derived, as a result, a time lag of $10.1_{-5.6}^{+5.8}$ days ($1\sigma$ errors) for the Fe-K$\alpha$ line behind the continuum, which is consistent with the location of the BLR determined in optical spectroscopic reverberation mapping during the type-1 (bright) phase. This finding shows that the BLR materials remained at the same location without emitting optical broad-lines during the type-2 phase. Considering the drastic decrease of the radiation during the type-transition, our result is possibly inconsistent with the hotly-discussed formation models of the BLR which propose that the radiative pressure from an accretion disk should be the main driving force.

15 pages, 7 figures, 3 tables, accepted for the publication in ApJ

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Cited by in corpus (4)

• Long-term optical spectral monitoring of a changing-look active galactic nucleus NGC 3516 -- II. Broad-line profile variability
• Updated Picture of the Active Galactic Nuclei with Dusty/Dust-free Gas Structures and Effects of the Radiation Pressure
• Changing Look AGN: A study of Optical/UV and the Highly Ionized Fe K$\alpha$ X-ray Line Flux Variations Using Photo-Ionization Simulations
• Unveiling BLR Structure in AGN with High Resolution X-Ray Spectra: An Analytic Approach to Wind Emission Line Profiles