Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

A Decade-Long Increasing Mid-Infrared Luminosity in Galaxy NGC6447: a Turning-On Candidate of Active Galactic Nucleus

Xinyu Dai, Nate Adams, Natalie Kovacevic, Kaitlyn Parrinello, Marko Micic, Heechan Yuk, Zijun Gao, Lorelei Starling, Francesco Shankar

Abstract

It is widely expected that the obscured accretion stage can be the initial turning-on stage of active galactic nuclei from quiescent galaxies. We present mid-infrared light curves of NGC 6447 in 3.5$μ$m and 4.6 $μ$m bands observed by WISE/NEOWISE, which show an almost monotonic increasing trend of 1.2 mag over 14 years. The optical light curve from ASAS-SN during the same period is consistent with a constant showing no variability. The mid-infrared color evolution shows that the galaxy transitioned into an active galactic nucleus (AGN) in 2018. The SPHEREx spectrum reveals an increasing continuum resembling warm to hot dust emission from an AGN. NuSTAR detected an X-ray source with a 2-30 keV luminosity of $8.4\times10^{41}$ ergs/s at the lower boundary of AGN X-ray emission range, and a factor of >7 variability in one year compared to the Swift upper limit. NGC 6447 was classified as a quiescent galaxy in the literature. The multi-wavelength timing and spectral properties of NGC 6447 are consistent with the expected AGN turning on event, where the obscuring material around the AGN central engine is gradually dispersed, revealing the central engine. This example shows that long-term infrared variability can be a powerful tool to find similar sources. Based on the sample selection statistics, we estimate the duration of the episodes of AGN accretion (duty cycle) signified by the turning-on event as $10^4$-$10^6$ yr.

A Decade-Long Increasing Mid-Infrared Luminosity in Galaxy NGC6447: a Turning-On Candidate of Active Galactic Nucleus

Abstract

It is widely expected that the obscured accretion stage can be the initial turning-on stage of active galactic nuclei from quiescent galaxies. We present mid-infrared light curves of NGC 6447 in 3.5m and 4.6 m bands observed by WISE/NEOWISE, which show an almost monotonic increasing trend of 1.2 mag over 14 years. The optical light curve from ASAS-SN during the same period is consistent with a constant showing no variability. The mid-infrared color evolution shows that the galaxy transitioned into an active galactic nucleus (AGN) in 2018. The SPHEREx spectrum reveals an increasing continuum resembling warm to hot dust emission from an AGN. NuSTAR detected an X-ray source with a 2-30 keV luminosity of ergs/s at the lower boundary of AGN X-ray emission range, and a factor of >7 variability in one year compared to the Swift upper limit. NGC 6447 was classified as a quiescent galaxy in the literature. The multi-wavelength timing and spectral properties of NGC 6447 are consistent with the expected AGN turning on event, where the obscuring material around the AGN central engine is gradually dispersed, revealing the central engine. This example shows that long-term infrared variability can be a powerful tool to find similar sources. Based on the sample selection statistics, we estimate the duration of the episodes of AGN accretion (duty cycle) signified by the turning-on event as - yr.
Paper Structure (3 sections, 3 figures)

This paper contains 3 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Selection and Data Analysis
  3. Results and Discussion

Figures (3)

  • Figure 1: Light curves of NGC 6447 in MIR, optical, and X-ray bands. The top panel shows the WISE/NEOWISE W1 and W2 light curves over 15 years, where an almost monotonic brightening trend is shown in both W1 and W2 light curves. Second panel: the W1$-$W2 color evolution, where the W1$-$W2 color moved into the AGN regime in 2018 based on the classifications of 2018MNRAS.478.3056B. Third panel: ASAS-SN optical light curve in V and g bands, where V and g magnitudes are normalized. The optical light curve is consistent with a constant. Bottom Panel: 2--10 keV X-ray light curve from 3$\sigma$Swift upper limit and NuSTAR detections. Significant variability by a factor $>7.3$ is detected in the X-ray band. The 2-10 keV X-ray luminosity is $3.1\times10^{41}$ erg s$^{-1}$.
  • Figure 2: MIR color evolution of NGC 6447 based on the NEOWISE light curves, which evolves deeper into the AGN region specified by 2018MNRAS.478.3056B2022AJ....163..224H. Other MIR AGN selection criteria plotted are from 2011ApJ...735..112J2012ApJ...753...30S2012MNRAS.426.3271M. We assume that the W2$-$W3 color is the same as measured by WISE. A typical uncertainty is plotted at the bottom-left corner.
  • Figure 3: SPHEREx spectrum of NGC 6447, showing characteristics of AGN emission. The rising continuum represents warm and hot dust emission from the central engine. A few excess emission features are identified as Fe ii, Pa$\beta$, and Pa$\delta$ lines.