Variability as a Probe of Supermassive Black Hole Accretion and Outflow

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A supermassive black hole (SMBH) resides at the heart of every massive galaxy, including Sagittarius A* (Sgr A*), the ~4 million solar-mass black hole in our Milky Way. These collections of mass, so dense that 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 Solar System. SMBHs grow most often via gas and dust inflowing through an accretion disk. 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 "time-domain" monitoring of accreting SMBHs can probe the physics governing matter in this unique strong-gravity regime. Sgr A* is a bright radio and submm source, but is faint at high energies due to its extremely low accretion rate and efficiency. Frequent flares punctuate Sgr A*'s quiescence; in the near NIR flares occur approximately hourly, while X-ray flares rise about once per day and typically last about an hour. The NIR flares are associated with synchrotron emission, but the X-ray flare mechanism remains uncertain (despite X­-ray fluxes reaching 10 to 400 × quiescence). Viable physical models range from the tidal disruption of asteroids to magnetic reconnection. These vastly different mechanisms motivate observers to place tighter constraints on the timing and multiwavelength properties of the outbursts. X-ray flares may also help us relate Sgr A* to weakly accreting black holes across the mass spectrum. I am PI of four approved Chandra X-ray Observatory programs, several joint with simultaneous Very Large Array and XMM Newton observations, to monitor Sgr A* and the cold, dusty object G2. In Haggard et al. (2015) we will report the two largest X-ray flares ever observed from Sgr A*; in which we resolve large and small scale intra-­flare variability, a probe of the flares' origin within the accretion flow. With NSERC Discovery Grant support, I will build a team to analyze and interpret proprietary and archival observations of Sgr A*. My group will (1) generate Chandra X-ray and VLA radio light curves (and X-ray spectra) for Sgr A* and G2; (2) perform detailed studies of Sgr A*'s X-ray flares, including inter- and intra-flare variability; (3) cross-correlate and interpret the multiwavelength data (incorporating accretion theory and IR/radio data, in particular); and (4) analyze Galactic Center X-ray binaries, stellar populations, and extended emission. Through international collaborations we also have Fermi, Swift, XMM, Hubble, Spitzer, and VLBA observations of the Galactic Center, which will enhance these studies. Based on our findings, we will actively pursue joint observatory programs to obtain any additional simultaneous data required to identify the flare mechanism and to link it to LLAGN and XRB variability.