Extreme variability in an active galactic nucleus: Gaia16aax

G Cannizzaro,S Mattila,E Kankare,J E Pringle,Ł Wyrzykowski,T Wevers,J Harmanen,F Onori,M Fraser,P G Jonker,K E S Ford,C J Nixon,B Mckernan,P C Hewett,Z Kostrzewa-Rutkowska

doi:10.1093/mnras/staa186

Abstract

ABSTRACT We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass ${\sim }6\times 10^8\, \mathrm{M}_\odot$. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the H α and H β emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star ${\ge } 1 \, \mathrm{M}_{\odot }$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.

Highlights

Lying at the center of galaxies, supermassive black holes (SMBHs) have an enormous impact on the region surrounding them
Tidal Disruption Event (TDE) have been invoked as a possible explanation for numerous objects in the large samples described in Graham et al (2017); Lawrence et al (2016); Rumbaugh et al (2018)
Assuming that the MIR emission comes from a dusty torus surrounding the Broad Line Region (BLR), this delay would imply a distance of this torus from the central engine of 0.12 ± 0.08 pc

Summary

Introduction

Lying at the center of galaxies, supermassive black holes (SMBHs) have an enormous impact on the region surrounding them. Lawrence et al (2016) investigated the nature of a sample large amplitude nuclear transients discovered during the Pan-STARSS 3π survey The majority of these objects are classified as hypervariable AGNs. In some cases, AGNs will show slow and significant optical variability of more than 1 magnitude over several years, accompanied by spectral variability (e.g Matt et al 2003; LaMassa et al 2015; MacLeod et al 2016). A TDE happens when a star passes so close to a SMBH that the tidal forces of the SMBH are stronger than the star’s self-gravity This leads to the (partial) disruption of the star (Hills 1975; Rees 1988; Evans & Kochanek 1989). TDEs have been invoked as a possible explanation for numerous objects in the large samples described in Graham et al (2017); Lawrence et al (2016); Rumbaugh et al (2018)

Results

Discussion

Conclusion