Discovery of 21 New Changing-look AGNs in the Northern Sky

Abstract

The rare case of changing-look (CL) AGNs, with the appearance or disappearance of broad Balmer emission lines within a few years, challenges our understanding of the AGN unified model. We present a sample of 21 new CL AGNs at 0.08 < z < 0.58, which doubles the number of such objects known to date. These new CL AGNs were discovered by several ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and SDSS, and (3) photometric variability and new spectroscopic observations. We use the photometric data from surveys, including the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient Survey, and the Palomar Transient Factory. The estimated upper limits of transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2 mag in W 1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than 3σ confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W 1 − W 2 become redder when the objects become brighter in the W 1 band, possibly due to a stronger hot dust contribution in the W 2 band when the AGN activity becomes stronger. The physical mechanism of type transition is important for understanding the evolution of AGNs.

A Mid-IR Selected Changing-look Quasar and Physical Scenarios for Abrupt AGN Fading

“…In addition to observing such rare phenomena as periodic quasars (e.g., Graham et al 2015aGraham et al , 2015bLiu et al 2015), flaring quasars (e.g., Lawrence et al 2016;Graham et al 2017;Kankare et al 2017), extreme broad absorption line variability (e.g., Rafiee et al 2016;Stern et al 2017), and tidal disruption events (e.g., Arcavi et al 2014;Blagorodnova et al 2017), this work has also identified a new class of "changing-look quasars" in which the strong UV continuum and broad hydrogen emission lines associated with unobscured quasars either appear or disappear on timescales of years (e.g., LaMassa et al 2015;Macleod et al 2016;Ruan et al 2016aRuan et al , 2016bRunnoe et al 2016;Gezari et al 2017;Yang et al 2018). The physical processes responsible for these changing-look quasars are still debated, but physical changes in the accretion disk structure appear to be the more likely cause rather than changes in obscuration.…”

“…We use this source as a touchstone to discuss physical models of abrupt quasar fading, a subject that has received considerable attention recently thanks to the growing ranks of wide-area ground-based optical surveys (e.g., LaMassa et al 2015;Macleod et al 2016;Ruan et al 2016aRuan et al , 2016bRunnoe et al 2016;Gezari et al 2017;Sheng et al 2017;Yang et al 2018;Assef et al 2018;Ross et al 2018). In particular, we emphasize the unique value of multiepoch mid-IR photometry to test and exclude models that attempt to ascribe large changes in quasar luminosities to obscuration by an intervening cloud.…”

SDSS J0159 as an outlier in the MBH–σ space: further clues to support a central tidal disruption event?

“…The nature of changing-look AGNs (CLAGNs) is not totally clear so far. More and more evidence strongly indicates that dust obscuration should be not the main reasons, but variations of accretion flows should play the key roles as well discussed in LaMassa et al (2015) and more recently discussed in Yang et al (2018). Once variations of accretion flows were accepted to CLAGNs, there should be two scenarios.…”

X-Ray Spectral Shape Variation in Changing-look Seyfert Galaxy SDSS J155258+273728

“…Disappearing or appearing broad Balmer emission lines have been known for many years in a number of local lowluminosity AGNs (e.g., Tohline & Osterbrock 1976;Cohen et al 1986;Denney et al 2014;Shappee et al 2014). With large-scale time-domain survey recent studies have identified a growing number of such cases in quasars with associated large, order-of-magnitude variations in the optical continuum on month to year timescales (LaMassa et al 2015;MacLeod et al 2016;Ruan et al 2016;Runco et al 2016;Runnoe et al 2016;Stern et al 2018;Wang et al 2018;Yang et al 2018;MacLeod et al 2019;Trakhtenbrot et al 2019;Sheng et al 2020). Their optical classification was caught to change between type 1.8-2 (narrow-line) to type 1 (broad-line) AGNs (or vice versa), and even from low-ionization nuclear emission-line region galaxies to broad-line quasars (Gezari et al 2017;Frederick et al 2019).…”