A systematic search for changing-look quasars in SDSS

Abstract

We present a systematic search for changing-look quasars based on repeat photometry from SDSS and Pan-STARRS1, along with repeat spectra from SDSS and SDSS-III BOSS. Objects with large, |∆g| > 1 mag photometric variations in their light curves are selected as candidates to look for changes in broad emission line (BEL) features. Out of a sample of 1011 objects that satisfy our selection criteria and have more than one epoch of spectroscopy, we find 10 examples of quasars that have variable and/or "changing-look" BEL features. Four of our objects have emerging BELs; five have disappearing BELs, and one object shows tentative evidence for having both emerging and disappearing BELs. With redshifts in the range 0.20 < z < 0.63, this sample includes the highest-redshift changing-look quasars discovered to date. We highlight the quasar J102152.34+464515.6 at z = 0.204. Here, not only have the Balmer emission lines strongly diminished in prominence, including Hβ all but disappearing, but the blue continuum f ν ∝ ν 1/3 typical of an AGN is also significantly diminished in the second epoch of spectroscopy. Using our selection criteria, we estimate that > 15% of strongly variable luminous quasars display changing-look BEL features on rest-frame timescales of 8 to 10 years. Plausible timescales for variable dust extinction are factors of 2-10 too long to explain the dimming and brightening in these sources, and simple dust reddening models cannot reproduce the BEL changes. On the other hand, an advancement such as disk reprocessing is needed if the observed variations are due to accretion rate changes.

iPTF Discovery of the Rapid “Turn-on” of a Luminous Quasar

“…The continued intrinsic variability during the high state of iPTF 16bco, as also seen in the study of changing-look quasars by MacLeod et al (2016), may also provide insight as to the nature of what caused the "changing look" of the quasar. The optical variability amplitudes of these sources in their type 1 quasar states of ∼0.5-1.0 mag are on the high-amplitude tail of what is typically observed on these timescales for quasars of a similar luminosity range (MacLeod et al 2012), although we note that in both of our studies the changing-look quasars were selected by their optical variability.…”

“…Figure 6 shows the redshift and [O III] luminosity of all the changing-look quasars in the literature that pass our [O III] luminosity cut (thus, we exclude SDSS J0126-0839 and SDSS J2336+0017 from Ruan et al 2016), color-coded by whether they show appearing broad lines or disappearing broad lines. We also do not include three changing-look quasars from the MacLeod et al (2016) sample that do not have good coverage of the broad Hα line in its high state (appearing SDSS J214613 at z=0.62, disappearing SDSS J022562 at z=0.63, and both appearing and disappearing SDSS J022556 at z=0.50). Given that all the other appearing changing-look quasars are from MacLeod et al (2016), there appears to be a bias toward finding appearing broad lines in higher-redshift galaxies.…”

“…We also do not include three changing-look quasars from the MacLeod et al (2016) sample that do not have good coverage of the broad Hα line in its high state (appearing SDSS J214613 at z=0.62, disappearing SDSS J022562 at z=0.63, and both appearing and disappearing SDSS J022556 at z=0.50). Given that all the other appearing changing-look quasars are from MacLeod et al (2016), there appears to be a bias toward finding appearing broad lines in higher-redshift galaxies. This is likely due to the fact that the BOSS spectra extend to longer wavelengths than the SDSS spectra, and thus finding an appearing broad Hα line in the BOSS spectrum is more likely at higher redshift than finding a disappearing broad Hα line in the SDSS spectrum.…”

“…Since then, there was a systematic search by Ruan et al (2016) of SDSS quasars with multiple epochs of spectra for which the spectroscopic pipeline classification changed between a "QSO" and a "GALAXY," or vice versa, which recovered SDSS J0159+0033 and revealed two more cases of changing-look quasars (at z=0.198 and z=0.243) with a dimming in their continuum and disappearance of the broad Hβ line on a timescale of 5-7 yr in the rest frame. An archival search of SDSS quasars with multiple epochs of spectra and large amplitudes of variability (D > g 1 mag) by MacLeod et al (2016) also yielded SDSS J0159 +0033 and nine more changing-look quasars at -= z 0.2 0.6, five of which show the appearance of broad Hβ between the SDSS and BOSS spectral epochs. New epochs of spectra from the Time-Domain Spectroscopic Survey (TDSS; Morganson et al 2015) have also revealed one new case of a transition from a type 1 to type 1.9 quasar at z=0.246 (Runnoe et al 2016).…”

The Optical Variability of SDSS Quasars From Multi-Epoch Spectroscopy. Iii. A Sudden Uv Cutoff in Quasar SDSS J2317+0005

“…Subsequently, several more changing-look quasars were uncovered from repeated Sloan Digital Sky Survey (SDSS) observations (Ruan et al 2015;Runnoe et al 2016). Now many researchers (e.g., MacLeod et al 2016) are carefully examining all the SDSS galaxies and quasars systematically, or searching the Time Domain Spectroscopies Survey (Morganson et al 2015;Ruan et al 2016)to find more changing-look AGN.…”