QSO J0455-4216 medium-band filt. light curves

The size of the accretion disc surrounding the black hole is a critical parameter for understanding quasar physics and their potential use as standard candles in cosmology. However, direct measurements of accretion disc sizes have so far been confined to the local universe (z<0.2), limiting our understanding of quasars during the peak of cosmic activity.

Here we report the first direct measurement of the accretion disc size in the quasar QSO J0455-4216 at z=2.66, when the Universe was only ~2Gyrs old. Medium-band filters mounted on the MPG/ESO 2.2-metre telescope at La Silla Observatory were used to isolate continuum emission regions during a six-month monitoring campaign. The light curves exhibit pronounced variability features and enabled the detection of inter-band time delays from different parts of the disc.