2019ApJ...878..115K

We used Optical Gravitational Lensing Experiment (OGLE-IV) deep (stacked) images (V <= 23 mag, I <= 23 mag at 3σ) of the Magellanic System, encompassing an area of ∼670 deg², to perform a search for high-z quasar candidates. We combined the optical OGLE data with the mid-IR Wide-field Infrared Survey Explorer 3.4/4.6/12 µm data, and devised a multicolor selection procedure. We identified 33 promising candidates and then spectroscopically observed the two most variable sources. We report the discovery of two high-z quasars, OGLE J015531-752807 at a redshift z = 5.09 and OGLE J005907-645016 at a redshift of z = 4.98. The variability amplitude of both quasars at the rest-frame wavelength ∼1300 Å is much larger (∼0.4 mag) than other quasars (∼0.15 mag) at the same rest-frame wavelength but lower redshifts (2 < z < 5). A larger sample of such sources with at least decade-long light curves would be necessary to determine whether increased variability is typical in the population of high-z quasars, or whether we are finding extreme outliers. The Large Synoptic Survey Telescope will provide such light curves for sources 3-4 mag fainter than OGLE.