Candidate Periodically Variable Quasars from the Dark Energy Survey and the Sloan Digital Sky Survey
Creators: name: Chen, Yu-Ching
name: Liu, Xin
name: Liao, Wei-Ting
name: Holgado, A. Miguel
name: Guo, Hengxiao
name: Gruendl, Robert A.
name: Morganson, Eric
name: Shen, Yue
name: Zhang, Kaiwen
name: Abbott, Tim M. C.
name: Aguena, Michel
name: Allam, Sahar
name: Avila, Santiago
name: Bertin, Emmanuel
name: Bhargava, Sunayana
name: Brooks, David
name: Burke, David L.
name: Rosell, Aurelio Carnero
name: Carollo, Daniela
name: Kind, Matias Carrasco
name: Carretero, Jorge
name: Costanzi, Matteo
name: da Costa, Luiz N.
name: Davis, Tamara M.
name: De Vicente, Juan
name: Desai, Shantanu
name: Diehl, H. Thomas
name: Doel, Peter
name: Everett, Spencer
name: Flaugher, Brenna
name: Friedel, Douglas
name: Frieman, Joshua
name: García-Bellido, Juan
name: Gaztanaga, Enrique
name: Glazebrook, Karl
name: Gruen, Daniel
name: Gutierrez, Gaston
name: Hinton, Samuel R.
name: Hollowood, Devon L.
name: James, David J.
name: Kim, Alex G.
name: Kuehn, Kyler
name: Kuropatkin, Nikolay
name: Lewis, Geraint F.
name: Lidman, Christopher
name: Lima, Marcos
name: Maia, Marcio A. G.
name: March, Marisa
name: Marshall, Jennifer L.
name: Menanteau, Felipe
name: Miquel, Ramon
name: Palmese, Antonella
name: Paz-Chinchón, Francisco
name: Plazas, Andrés A.
name: Sanchez, Eusebio
name: Schubnell, Michael
name: Serrano, Santiago
name: Sevilla-Noarbe, Ignacio
name: Smith, Mathew
name: Suchyta, Eric
name: Swanson, Molly E. C.
name: Tarle, Gregory
name: Tucker, Brad E.
name: Varga, Tamas Norbert
name: Walker, Alistair R.
Publisher: name: arXiv
Year: 2020
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Periodically variable quasars have been suggested as close binary supermassive black holes. We present a systematic search for periodic light curves in 625 spectroscopically confirmed quasars with a median redshift of 1.8 in a 4.6 deg$^2$ overlapping region of the Dark Energy Survey Supernova (DES-SN) fields and the Sloan Digital Sky Survey Stripe 82 (SDSS-S82). Our sample has a unique 20-year long multi-color ($griz$) light curve enabled by combining DES-SN Y6 observations with archival SDSS-S82 data. The deep imaging allows us to search for periodic light curves in less luminous quasars (down to $r{\sim}$23.5 mag) powered by less massive black holes (with masses $\gtrsim10^{8.5}M_{\odot}$) at high redshift for the first time. We find five candidates with significant (at $&gt;$99.74% single-frequency significance in at least two bands with a global p-value of $\sim$7$\times10^{-4}$--3$\times10^{-3}$ accounting for the look-elsewhere effect) periodicity with observed periods of $\sim$3--5 years (i.e., 1--2 years in rest frame) having $\sim$4--6 cycles spanned by the observations. If all five candidates are periodically variable quasars, this translates into a detection rate of ${\sim}0.8^{+0.5}_{-0.3}$% or ${\sim}1.1^{+0.7}_{-0.5}$ quasar per deg$^2$. Our detection rate is 4--80 times larger than those found by previous searches using shallower surveys over larger areas. This discrepancy is likely caused by differences in the quasar populations probed and the survey data qualities. We discuss implications on the future direct detection of low-frequency gravitational waves. Continued photometric monitoring will further assess the robustness and characteristics of these candidate periodic quasars to determine their physical origins.
description: 21 pages, 8 figures, MNRAS accepted