Dr Yusef-Zadeh will use multiwavelength time-series observations to test two models to explain the flaring observed in the Milky Way's central radio source. According to the 'plasmon' model, blobs of radio-bright material move outwards from the central black hole, brightening at successively longer wavelengths as they become optically thin to their own synchrotron radiation. If the flares are instead caused by hot-spots in an accretion disk around the black hole, the source should brighten simultaneously at all wavelengths. Sub-millimeter observations would be taken in both the northern and southern sky with single-dish telescopes, and radio-telescope arrays would be used at centimeter wavelengths, to measure both total intensity and polarization over periods of several days. Data from two such campaigns in 2007 is now in hand, and another that includes observations with NASA's Chandra space telescope in the X-ray is scheduled for mid-2008. Future campaigns may include mid-infrared data from ESA's Herschel space telescope. Archival data from the Very Large Array will also be used. Theoretical work will include an extension of the plasmon model to predict polarization that arises from the anisotropic emitting region.
Dr. Yusef-Zadeh has developed an interactive web client with problem-based exercises for undergraduate and adult-education classes. He will develop further exercises for this system, based around this project. He will also continue to coordinate a program of interdisciplinary science lectures for the public, to teach at Adler Planetarium, and to involve students in public outreach. A postdoc will be trained through involvement in the research.
Using a large of number telescopes simultaneously spying on the black hole, we studied daily flaring activity of the supermassive black hole at the center of our galaxy. We used telescopes that operated at X-ray, infrared, radio, millimeter and submillimeter wavelengths, all at the same time looking for changes in the brightness of emission from the black hole. We discovered that flares show up first at infrared wavelengths and then at submillimeter, millimeter and centimeter wavelengths with a time delay that lasts for few hours. This time delay in the peak of flare emission suggested to us that the flare is opaque first and then become transparent with time. We modeled flare emission from the supermassive black hole in terms of adiabatic expansion of hot plasma. Adiabatic expansion results in rapid cooling as the gas expands, the density of electrons that radiate and the magnetic field that is couple to electrons decrease. What happens first is the expansion of the plasma increases the surface area of a blob of expanding gas. In this satge theflux ofradiation increases with time. However, the expansion decreases thedensity of electrons and the strength of the magnetic field until theblob of plasma becomes opttically thin. At this stage, the flux of the radiation starts decreasing until there is no flare emission. It turns out that the peak of the emission appears first at high frequencies followed by low frequency emission. The model predicts such a behavior in terms of frequency dependence of opacity. Another topic of interest is the formation of young stars near this supermassive black hole, a seemingly unlikely place for young stars to form. However, nature is quite clever in finding a way to open a stellar nursery in the most violent spot in the Milky Way Galaxy. Recent ideas on how stars are formed near the black hole will be discussed. We considered that a cloud of gas must have migrated toward the black hole and some of the material from the cloud gets captured before stars are formed.
