S. König

Simultaneous NIR/sub-mm observation of flare emission from Sagittarius A*

Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4×10 6 M⊙ ) black hole at the Galactic center. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory’s Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare model ing. Results. The observations reveal strong flare activity in the 0.87 mm ( 345 GHz) sub-mm domain and in the 3.8µ/2.2µm NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5±0.5 hours. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of vexp∼ 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of �=0.8 to 1.8, corresponding to particle spectral indices ∼2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz. Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than vexp or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.

Multiwavelength VLBI observations of Sagittarius A

Context. The compact radio, NIR, and X-ray source Sagittarius A* (Sgr A*), associated with the super massive black hole at the center of the Galaxy, has been studied with Very Long Baseline Interferometry (VLBI) observations performed on 10 consecutive days and at mm-wavelength.

An evolving hot spot orbiting around Sgr A

Here we report on recent near-infrared observations of the Sgr A* counterpart associated with the super-massive 4 x 106M⊙ black hole at the Galactic Center. We find that the May 2007 flare shows the highest sub-flare contrast observed until now, as well as evidence for variations in the profile of consecutive sub-flares. We modeled the flare profile variations according to the elongation and change of the shape of a spot due to differential rotation within the accretion disk.

Coordinated mm/sub-mm observations of Sagittarius A* in May 2007

At the center of the Milky Way, with a distance of ~8 kpc, the compact source Sagittarius A* (SgrA*) can be associated with a super massive black hole of ~4x10^6 solar masses. SgrA* shows strong variability from the radio to the X-ray wavelength domains. Here we report on simultaneous NIR/sub-millimeter/X-ray observations from May 2007 that involved the NACO adaptive optics (AO) instrument at the European Southern Observatorys Very Large Telescope, the Australian Telescope Compact Array (ATCA), the US mm-array CARMA, the IRAM 30m mm-telescope, and other telescopes. We concentrate on the time series of mm/sub-mm data from CARMA, ATCA, and the MAMBO bolometer at the IRAM 30m telescope.

High-frequency VLBI observations of Sgr A* during a multi-frequency campaign in May 2007

In May 2007 the compact radio source Sgr A* was observed in a global multi-frequency monitoring campaign, from radio to X-ray bands. Here we present and discuss first and preliminary results from polarization sensitive VLBA observations, which took place during May 14-25, 2007. Here, Sgr A* was observed in dual polarization on 10 consecutive days at 22, 43, and 86 GHz. We describe the VLBI experiments, our data analysis, monitoring program and show preliminary images obtained at the various frequencies. We discuss the data with special regard also to the short term variability.

LUMINOUS ACCRETION ONTO THE DARK MASS AT THE CENTER OF THE MILKY WAY

We report on the results of two successful, simultaneous observations of Sagittarius A* at the center of the Milky Way. The observations were carried out in 2004 and 2008 using telescopes operating from the mm-radio domain to the X-ray domain, and detected strong flux density variations in all wavelength bands. Modeling suggests that a combination of a synchrotron self Compton process and an adiabatic expansion of source components are at work. The luminous flare emission of Sagittarius A* also supports the presence of an accreting super massive black hole at that position. We also discuss the potential of NIR interferometry for further detailed investigations of the accretion process in SgrA*.

Inner structure and global kinematics: arp 220 revisited

With an infrared luminosity of 1012.2L⊙, Arp 220 is one of the prototypical Ultraluminous Infrared Galaxies. Its high IR luminosity, the presence of tidal tails detected in the optical, the double infrared and radio nuclei, and the perturbed optical ionized gas kinematics measured at large scales support the fact that Arp 220 is the late phase of an on-going merger. This, along with the most accepted evolutionary theories for ULIRGs, indicate that Arp 220 may evolve into an intermediate mass elliptical galaxy. Nonetheless, there are several questions that still remain open, such as the exact location and number of nuclei, their relation with the X-ray emission, and the kinematic behavior of the galaxy at different scales. Here we give an overview of what we know about Arp 220 up-to-date, regarding the NIR and radio regime, and report on new insights in kinematics and the structure on larger spatial scales of the merger making use of IRAM PdBI observations.