Variability as a Probe of Supermassive Black Hole Accretion and Outflow

超大质量黑洞（SMBH）位于每个大质量星系的中心，包括人马座A* (Sgr A*)，这是我们银河系中约400万太阳质量的黑洞。这些质量的集合，密度如此之大，以至于光无法逃脱它们的引力，对宿主星系的形成和结构产生了深远的影响，尽管它们的结构比我们的太阳系还小。SMBHs通常通过吸积盘流入的气体和尘埃生长。尽管吸积盘的核心是黑暗的物质，但它有时会伴随着喷流或风，使整个宿主星系都黯然失色。由于这种动态结构的每一部分都随时间而变化，因此对吸积的SMBHs进行战略性的“时域”监测可以探测这种独特的强重力状态下控制物质的物理。

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.