Low-Eddington ratio, changing-look active galactic nuclei: The case of NGC 4614

Abstract

Active galactic nuclei (AGNs) are known to be variable sources across the entire electromagnetic spectrum, in particular at optical/ultraviolet and X-ray energies. Over the past few decades, a growing number of AGNs have been observed to change type, from type 1 to type 2 or vice versa, within a few years or even months. These galaxies have been commonly referred to as changing-look AGNs (CLAGNs). Here we report on a newly discovered CLAGN, NGC 4614, which transitioned from a type 1.9 to a type 2 state. NGC 4614 is a nearly face-on barred galaxy at redshift z = 0.016 and is classified as a low-luminosity AGN. Its central black hole has a mass of about 1.6 × 10^{7 M_{⊙ and an Eddington ratio of around 1 percent. We recently acquired optical spectra of NGC 4614 at the Telescopio Nazionale Galileo, and the data clearly suggest that the broad Hα component has strongly dimmed, if not disappeared. A very recent Swift observation confirmed our current optical data, with the AGN weakened by almost a factor of 10 with respect to previous X-ray observations. NGC 4614 had also been observed by Swift/XRT six times in 2011, and the source was clearly detected in all those observations. By fitting the stack of the 2011 Swift observations, we obtain a photon index of Γ = 1.3 ± 0.3 and an equivalent hydrogen column density of NH = 1.2 ± 0.3 × 1022 cm−2, indicating that NGC 4614 can be moderately absorbed in X-rays. Although a significant change in the foreground gas absorption that may have obscured the broad-line region cannot be entirely ruled out, the most likely explanation is that NGC 4614 is experiencing a change in its accretion state that reduces the radiative efficiency of the X-ray corona.}}