In 2017 my research group at McGill University, including postdoctoral fellows Melania Nynkaand John J. Ruan, led observations with the Chandra X-ray Observatorythat contributed to the discovery of the first electromagnetic counterpart to a gravitational wave source, the neutron star merger GW170817. Ours was one of the only (if not the only) Canadian team to lead a publication among the more than 80 manuscripts published in the “first wave”of discovery results on 16 October 2017. Unlike electromagnetic counterparts at opticalor infrared wavelengths that probed the initial explosion and faded within the first several week(s), the counterpart in X-ray and radio observations revealed the afterglow emission froma collimated outflow and continued to shine nearly 600 days after the merger. This long-termemission arises from a relativistic outflow seen from the side (or “off-axis”) and has completelyaltered our understanding of the class of explosive transients to which GW170817 belongs, i.e., short gamma-ray bursts.
In the next several years, astronomers are poised for more than one breakthrough using SgrA* and M87* as laboratories to test gravitational and plasma physics theories. These arise from multi-wavelength coordination with the Event Horizon Telescope (EHT; submillimeter) and the GRAVITY experiment (infrared), which will image these supermassive black holes’ accretion flows at scales only a few times their Schwarzschild radii. In late 2018 GRAVITY already released the first resolved orbits of hot spots extremely close to Sgr A*’s event horizon, proving for the first time that flares are a horizon-scale phenomenon. As a member of the EHT multi-wavelength coordination group I am leading approved X-ray observations with Chandra in 2017, 2018 and again in 2020. My team also tracks energetic flares from Sgr A*. Tune in for more exciting black hole discoveries!
Supermasive Black Holes
A supermassive black hole (SMBH) lurks at the heart of every massive galaxy, including our Milky Way. These monsters, agglomerations of mass so dense that even light cannot escape their gravitational pull, have a profound impact on the formation and structure of their host galaxies, despite being packed into structures smaller than our own Solar System. SMBHs grow in many ways, but most dramatically via gas and dust inflowing through a flat, variable accretion disk — during growth spurts, accreting SMBHs are called quasars or active galactic nuclei (AGN). Despite the dark mass at its core, the accretion disk, and sometimes an associated jet or wind, can outshine the entire host galaxy. Since every part of this dynamic structure varies with time, strategic monitoring of a large number of AGN promises new insight. Complementing our work on Sgr A*, my group is also a world-leader in observational studies of variable quasars and their connections to accretion physics.
Supermassive black holes frequently grow by collecting material via an accretion disk, and the same can be true of their lower-mass siblings, X-ray binaries (XRBs). These binary systems contain a compact stellar remnant (white dwarf, neutron star, or stellar-mass black hole) that is stealing material from its stellar companion. My team studies XRBs in dense stellar environments, where dynamical interactions between stars play an important role. Some of these systems appear in the Galactic Center and have offered us new insights into that violent and rapidly evolving environment – e.g., our studies of the Galactic Center magnetar have overhauled our understanding of star formation near Sgr A*. We have also contributed important works to our understanding of XRBs in the globular cluster Omega Centauri, one of the most massive stellar sub-systems in the Milky Way.