Black hole accretion and outflows: novel multi-wavelength perspectives

Black holes are unique natural laboratories. Recreating the high temperatures and stresses of infalling matter is impossible on Earth. This makes black holes ideal sites to study extreme cosmic environments. Yet astronomers have struggled to accurately measure even basic properties such as the sum total of energy output from black hole environments, or to understand the nature of powerful 'jets' of outflowing matter known to emerge from them.

This has not been for lack of trying. Massive black holes found in the nuclei of all large galaxies are often cocooned within obscuring layers of interstellar gas and dust which hide them from direct view. Additionally, bright young star clusters invariably clutter our view of black holes, making it impossible to isolate their emission.

Stellar-mass black holes pose separate challenges. These are the most compact astronomical objects known (with characteristic sizes of only ~30 km for a typical Galactic source). As matter accretes (infalls) on them, part of its energy emerges as strongly variable light and fast-outflowing matter, a manifestation of the chaotic environment from which it emerges. Analysis of the variability patterns can yield direct information on the physics of accretion and jets. But this requires the use of fast and sensitive cameras which have not been available until recently.

My work has opened up new avenues to explore these issues. By using the exquisite resolution provided by very large telescopes in Chile, I have shown that the infrared emission from dusty clouds in the immediate vicinities of massive black holes can be successfully isolated from surrounding stars. Furthermore, the infrared provides a perfect 'bolometric' (total) emission measure, giving us a new tool to probe these obscured black holes.

Secondly, in order to understand jet physics on short times, I have studied variable X-ray, infrared and visible light in Galactic black holes. Using a novel fast camera, I led a team to observe visible light fluctuations only a fraction (~1/20th) of a second long from stellar black holes. We were stunned to find remarkable patterns amidst the fluctuating noise in the form of optical flashes which are correlated with X-ray variations. The optical emission was widely thought to be a secondary response to primary X-ray outbursts, but the optical fluctuations that I discovered are much too speedy to fit this scenario. Instead, my findings point to fast and chaotic magnetic energy extraction in the outflowing jets of matter. Using NASA's latest infrared space telescope called WISE, I was next able to measure the magnetic field strength and physical dimensions of the inner jet, quantities which has long eluded direct measurement. Although radio telescopes have long studied emission from extended jets, using the optical and infrared instead allowed me to probe the physical conditions in the jet near its base. This is the first step for understanding the extreme physics of relativistic plasma acceleration in compact sources in detail.

So we now have two new direct handles to study accretion and outflows in black holes. But studies to date have only explored small and patchy collections of objects, so I am proposing to explore large and well-defined samples in the sky. I will carry out the first high resolution infrared survey of all X-ray bright massive black holes in the nearby Universe. And, I will systematically explore the jets in outbursting stellar-mass black holes in optical, infrared and X-rays.

Black holes span an incredible range of masses and sizes unlike any other cosmic source. The only way to gain full insight into their properties is to utilise all tools at hand. This is why I am proposing a multi-pronged and multi-wavelength approach. The advent of large infrared telescopes, as well as new cameras capable of fast imagery, makes this research very timely.