The Close AGN Reference Survey (CARS) - What is causing Mrk1018's return to the shadows after 30 years?

TL;DR

This study investigates the cause of Mrk 1018's transition from type 1 to a more obscured state after 30 years, concluding that a decline in accretion disk luminosity, not absorption, is the primary factor.

Contribution

It combines multi-wavelength observations to demonstrate that accretion rate decline, rather than absorption, drives the changing-look behavior of Mrk 1018.

Findings

X-ray flux declined by a factor of ~8 from 2010 to 2016.

Optical-UV spectra fit a standard accretion disk model obeying L∼T^4.

Detection of a new narrow-line absorber possibly indicating outflows or a companion black hole.

Abstract

We recently discovered that the active galactic nucleus (AGN) of Mrk 1018 has changed optical type again after 30 years as a type 1 AGN. Here we combine Chandra, NuStar, Swift, Hubble Space Telescope and ground-based observations to explore the cause of this change. The 2-10keV flux declines by a factor of ~8 between 2010 and 2016. We show with our X-ray observation that this is not caused by varying neutral hydrogen absorption along the line-of-sight up to the Compton-thick level. The optical-UV spectral energy distributions are well fit with a standard geometrically thin optically thick accretion disc model that seems to obey the expected $L\sim T^4$ relation. It confirms that a decline in accretion disc luminosity is the primary origin for the type change. We detect a new narrow-line absorber in Lya blue-shifted by ~700km/s with respect to the systemic velocity of the galaxy. This new Lya absorber could be evidence for the onset of an outflow or a companion black hole with associated gas that could be related to the accretion rate change. However, the low column density of the absorber means that it is not the direct cause for Mrk 1018's changing-look nature.