GCNEWS
A Newsletter for Galactic Center Research
This Volume was edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel & Robin Herrnstein
email: gcnews@aoc.nrao.edu
GCNEWS
GC magnetic field: The fundamental question discussed with respect to the large-scale magnetic field in the GC is whether it traces a large scale field or local magnetic field density enhancements. Is there a pervasive field of milli-Gauss strength present or is the field predominantly weak (10-100 mu G), with localized filamentary maxima having B 1 mG? The double helix nebula identified on Spitzer images provides possible evidence for large scale magnetic fields that may be twisted by orbital motion of the ISM around the GC.
Molecular/atomic gas in the GC: One of the open questions is the nature of the heating mechanism of the GC molecular gas that is significantly warmer than the molecular gas in the rest of the Galaxy. What is the exact relation of the molecular gas to the magnetic field? How, and where do massive clusters such as Quintuplet and Arches form? What is the mechanism for the m = 1 offset? There is new evidence from mm observations for an interaction between the Sgr A East SN remnant shell and the 50 km s-1 molecular cloud. Additional evidence comes from observations of the IR H2-line.
The circumnuclear disk/ring (CND): There is still considerable debate concerning the mass of the CND and lower masses seem more likely now (about 104-105 M_o). The question of the density and mass of the clumps in the CND is also intimately related with the possibility of star formation in the CND. The CND may be the prototype of dusty tori in (LL)AGN. The clumpy nature is in contrast to the rather homogeneous rings assumed in the originally proposed AGN models.
Diffuse 8 keV emission: What is the origin of the 8 keV diffuse emission in the central 40 pc? A reasonable solution appears to be that the emission comes from unresolved cataclysmic variables. This is among other supported by the fact that the profile of the diffuse 8 keV emission is the same as that of the stellar cluster derived from IR observations.
X-ray line emission from GC molecular clouds: The Suzaku X-ray maps provide an excellent new source of data. They allow to use line ratios to derive T_e. Focus of the debate is whether the emission from the clouds is related to increased activity of Sgr A* a few hundred years ago.
Spitzer IRAC survey: A highlight of the conference was the presentation of the detailed, large scale IRAC survey images from the Spitzer space telescope. They will be an important mine in the years to come to address issues such as star formation, the large scale structure of the central bulge, or the properties of dusty clouds.
The formation and properties of the young stars in the central half-parsec of the Milky Way was an issue that received much attention at the conference. The young stars in the GC are found to be a few million years old and to be located in one, possibly two disks/rings. While the existence of the clockwise rotating disk appears to be well established, the evidence for the existence of the second (counter-clockwise rotating) disk is still debated. It may be possible to explain the observations with just one system of young stars. There is evidence that the IMF of the young stars is top-heavy. In combination with the observation that the density profile of the disks displays a steep fall-off from the center, the currently favored - but not undisputed - scenario is that the young stars formed in an accretion disk in possibly relatively short time (104-105 yr). Should this scenario be true this would lead, however, to the problem that if most of the material in the accretion disk is used for star formation then how do quasars get their fuel? Also it appears not clear yet whether the accretion disk scenario can explain the relatively high eccentricities of the orbits of some of the young stars.
The central topics in this session were above all the effects of mass segregation and the puzzle of the origin of the young, massive main sequence stars in the immediate environment of Sgr A* (the ``S-stars''). Detailed theoretical models predict significant effects of mass segregation in the GC cluster. It is expected that of the order 20,000 stellar mass BHs have accumulated in the central parsec over the lifetime of the galaxy. The radial density profiles of stars of different magnitudes provide indeed some observational evidence for mass segregation. The S-stars appear to have the properties of normal main sequence stars. Thus scenarios appear less likely in which the S-stars are actually exotic objects or older stars rejuvenated through some unknown mechanism. If the S-stars formed simultaneously with the other young stars in the central half-parsec, then there must exist a mechanism that scatters these stars efficiently into tight orbits around Sgr A*. Several mechanisms appear viable, but all are confronted with challenges in terms of producing the right numbers. A promising mechanism may be resonant relaxation of orbits around the BH.
A new feature of the 2006 workshop compared to earlier GC workshops was expanding the focus of attention to include nearby low-luminosity galactic nuclei. Results from high-resolution mm-observations of nearby LLAGN indicate that gas at distances greater than 100 pc does not appear to be transported further inward and feed the AGNs. The LLAGN in M31 and M81 are the next stepping stones away from Sgr A* toward higher luminosities and larger distances. They are the nearest logical links in the chain toward AGNs and quasars and are now in the focus of observations from X-rays to the radio regime. Coordinated multi-wavelength campaigns have proved to be highly useful tools for understanding Sgr A*. Such a campaign was directed to M81* in spring/summer 2005. First results were presented at the conference. Successive campaigns are in preparation.
Observations of long-term changes in the turnover frequency of Sgr A* at long wavelengths indicate a variable mass flux of the stellar winds captured by the MBH, which could modulate the opacity of the local absorbing medium.
The most recent constraints on the size of Sgr A* from VLBI at a wavelength of 3 mm are 15+/-4 Schwarzschild radii. In these measurements the extrapolation of the scattering law toward short wavelengths is of paramount importance.There is still some discussion which power-law is the exact one to describe the scattering law.
Both RIAF (radiatively inefficient accretion flow) and jet models as well as their combination are consistent with the size limits on Sgr A*.
Observations of the flux from Sgr A* at X-ray, IR and mm wavelengths are particularly directed toward observing its variability. Variability time scales provide information on the properties of the accretion flow and -at the shortest frequencies - possibly on the angular momentum of the black hole. At mm wavelengths the amplitude of the variability is about 10% to 20%, with significant power - but no confirmed periodicity - at time scales of hours. First observations of the variable polarization of the mm-flux from Sgr A* have provided some hints at circular motion of the plasma.
Polarized NIR radiation has been observed for the first time from Sgr A* in 2005, providing clear evidence for the non-thermal nature of the NIR flares. Also, in agreement with earlier observations, there are indications of a variability with a 20 min period in the NIR polarization measurements. At X-rays, a thorough analysis of a bright flare observed with XMM shows a periodicity at 22+/-2 min. However, quasi-periodic behavior is only observed in some - not all - NIR and X-ray flares. There exists still debate as to the significance of the quasi-periodic oscillations, their relation to QPOs observed in microquasars, and their interpretation.
Disagreement between observers has also been found concerning the exact relation between NIR and X-ray flares, i.e. whether all X-ray flares occur simultaneous to a NIR flare. Also, different observers have obtained differing slopes of the spectra of the NIR flares. The cause is so far unclear.