Black Holes and Accretion: Observational Frontiers

Black holes (BHs) are objects so dense that beyond a certain point even light cannot escape their gravitational attraction, and are prevalent throughout the universe. Supermassive black holes (SMBHs; weighing over a million suns) reside at the centre of every major galaxy, and large numbers of smaller stellar-remnant black holes (StMBHs; tens of solar masses), the evolutionary end-point for the most massive stars, are scattered throughout them.

There is much that we still do not understand about these enigmatic objects. Critically, we do not know how SMBHs grew to be so massive. We now know some SMBHs were already grown when the universe was still in its infancy (~1 billion years old), but we do not know how they were able to grow so quickly. We also see that BHs can launch extreme jets of material traveling close to the speed of light that can significantly influence their surroundings (feedback), but we do not understand how these jets are launched.

These are some of the most important issues in contemporary astrophysics research, and can be addressed by studying how matter falls onto the 'local' SMBHs/StMBHs in our own and neighbouring galaxies (accretion). Observations at X-ray wavelengths in particular have the power to probe the nature and geometry of the infalling material in the immediate vicinity of the black hole. With the recent launch of the NuSTAR observatory, and both the Astro-H and ASTROSAT observatories also soon to be launched, we are entering a period of unprecedented observational capabilities in the X-ray regime. My proposal aims to leverage these new facilities to improve our understanding of this complex accretion process, and in turn of SMBH growth and the physics of jet launching.

Astonishingly, black holes themselves are fully defined by two fundamental properties: mass and spin (rotation). BH spin is a quantity of key importance, as a variety of jet launching models predict that the energy associated with BH rotation may be harnessed to launch these extreme outflows. Measuring BH spin is therefore a vital step in testing these models, and is one of the primary goals of my research.

In terms of understanding SMBH growth, I will take a dual approach. First, spin measurements are again vital, as SMBH spin tells us about the history of how these objects grew: growth via random mergers (chaotic growth) should spin SMBHs down, while growth via periods of prolonged accretion (ordered growth) should spin them up. In addition to measuring spin for SMBHs in the local universe I will begin testing models for how SMBH growth should vary with cosmic time by extending these measurements to much more distant SMBHs with gravitationally lensed sources. These are rare cases in which a foreground galaxy is aligned with our line-of-sight to the SMBH, which acts as a natural lens and significantly magnifies the light received, allowing far more sensitive studies than would otherwise be possible!

Second, I will also study how accretion operates at the most extreme rates, necessary to rapidly grow the SMBHs observed in the early universe given the short growth time available. Accretion at these rates is poorly understood, so further observational constraints are vital. We now that a rare population of 'ultraluminous' X-ray sources are StMBHs exhibiting such accretion. These are therefore ideal targets for further study in order to understand this exotic regime.

In combination, this research will bring significant advances in our understanding of accretion, spin, jets and the formation of the most massive black holes.