Accretion disc reverberation mapping in a high-redshift quasar

F. Pozo Nuñez, E. Bañados, S. Panda, J. Heidt

First listed 2025-07-31 | Last updated 2025-07-31

Abstract

Powered by supermassive black holes at their centers, quasars are among the most luminous objects in the Universe, serving as important probes of cosmic history and galaxy evolution. 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 $\sim2$ Gyrs 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. We mapped the disc and located its ultraviolet-emitting outermost region at $3.02^{+0.33}_{-0.57}$ light-days from the black hole ($\sim 500$ AU). Given a supermassive black hole 900 million times the mass of the Sun, these measurements validate accretion disc theory at an unprecedented redshift and pave the way for efficient black hole mass estimates, reducing decades-long spectroscopic reverberation campaigns to just a few years or less.

Short digest

Six months of medium-band photometric reverberation mapping on the MPG/ESO 2.2 m captured inter-band lags in the z=2.66 quasar QSO J0455-4216, isolating continuum windows with ~7% variability and day-scale micro-variations. GPCC-derived lags follow a wavelength-dependent delay spectrum consistent with an SS73 thin disc plus lamp-post, placing the ultraviolet-emitting outermost radius at 3.02(+0.33/−0.57) light-days (~500 AU). Transfer-function modeling shows the light curves are disc-dominated, with only minor diffuse BLR continuum allowed; large DCE fractions oversmooth the variability. This delivers the first direct disc-size measurement at this redshift for an ~9×10^8 M⊙ black hole, validating thin-disc expectations and enabling faster high‑z mass estimates.

Key figures to inspect

• Figure 1: Inspect the multi-band light curves to see ~7% campaign-long variability, 2–3% day-scale micro-variations, and visually trace a few‑day inter-band lag while confirming the field star’s stability.
• Figure 2: Read the GPCC lag posteriors versus wavelength and compare the peak lags to the SS73 lamp‑post curve to verify the disc-like delay spectrum and the model’s assumed MBH, luminosity, and redshift.
• Figure 3: Examine the accretion-disc transfer functions broadening with wavelength and how the joint GPCC latent signal convolved with these kernels reproduces sharp features across all bands—key evidence for disc reprocessing.
• Figure 4: Check the BLR/DCE transfer function (centered ~162 light-days for CIV) and the sequence of model curves; note how ≥50% DCE over-smooths the green-band light curve, constraining DCE to a minor (~few %) role.