Supermassive black holes inhabit the cores of every galaxy in our Universe. In many cases, material can reach very close to them forming a flat, thin accretion disc of hot gas producing one of the most energetic and bright sources in the sky -- known as Active Galactic Nuclei or AGN, observable all the way back to the formation of the first galaxies. As material spirals into the black hole it causes changes in the brightness of high-energy photons. As the light travels further away from the black hole, it echoes at larger distances in the surrounding disc. From the Earth, we observe correlated “echoes” of the inner parts of the disc (observed mainly in X-rays) at different wavelengths of the electromagnetic spectrum (from the ultraviolet to the near-infrared) shifted in time. The measurement of these inter-band delays is a fantastic technique --known as “echo mapping” or "reverberation mapping" -- to probe this otherwise inaccessible environment enabling astronomers to measure the size of the disc and the mass of supermassive black holes.
We have developed a new observational technique that measures these light echoes between wavebands by combining high-rate monitoring with the Las Cumbres Observatory optical telescope network and the orbiting NASA's Neil Geherel Swift UV/X-ray satellite. This improved technique known as "Intensive Disc-Reverberation Mapping" -- or IDRM -- has led to important advances in our picture of AGN central engines. Among these, we established for the first time that AGN interband continuum lags increase smoothly with wavelength throughout the optical/UV wavebands, consistent with τ ∝ λ4/3 as predicted by standard thin accretion disc theory, and discovery of excess lags especially around the Balmer jump, apparently due to diffuse continuum emission from the broad-line region. However, the disc sizes appear too large and there is no simple relation between optical and X-ray variations, causing severe problems for the standard reprocessing model and forcing development of new models of AGN central engines.
During the 2020 LCO Key Project, our team will employ IDRM of an additional six targets (and repeat/continued monitoring of two previously-observed ones), significantly expanding the sample of AGN with high-quality IDRM monitoring, with special emphasis on targets with extreme Eddington ratios. We aim to achieve the following science goals:
- i) measure interband lags and quantifying the contribution of the broad-line region to the delay spectrum so that we can estimate the size of the disc;
- ii) resolve the velocity field of the broad-line region to identify inflows and/or outflows;
- iii) decompose the AGN spectral energy distribution into variable (disc) and non-variable (galaxy) components to compare with theoretical expectations;
- iv) we will establish the AGN Variability Archive (AVA), a legacy open access database of AGN variability records that we will use and open to the community for testing black hole accretion models.