Volume 21, July 2005 - In the News

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In this volume we have a short summary and links to the presentations for the recent GC Meeting at the KITP in Santa Barbara, as well as links to the talks from the ``High Energy Phenomena in the Galactic Center'' meeting which was held June 15-17 in Paris (see below). We also have some initial information about two GC-related workshops that will occur in the Fall.

It is our pleasure to host the last article in our series highlighting the recent results from high-energy observatories. Starting with INTEGRAL in Vol. 19 and moving on to Whipple/VERITAS in Vol. 20, we finish with an article by Jim Hinton and Felix Aharonian about H.E.S.S.. Beyond the actual measurements, which were also covered in the last volume, the authors discuss in more detail some of the theoretical models which may be tested with the new observations.

And of course, we have the latest abstracts.

KITP GC Meeting Summary Rainer Schödel

On April 14-16, 2005 the GC community plus some new friends came together at the Kavli Institute for Theoretical Physics (KITP) in Santa Barbara. Andrea Ghez, Sera Markoff, and Eliot Quataert were the organizers of the three-day workshop "The Paradoxes of Massive Black Holes: A Case Study in the Milky Way". The conference was focused mainly on two topics: understanding the emission of Sgr A* across all wavelengths, and, understanding the presence of young stars in the vicinity of Sgr A*.

Here, we provide a brief summary of the main topics and debates of the conference. The complete program as well as slides and audio files of all the talks can be found online at the KITP web site: http://online.itp.ucsb.edu/online/galactic_c05/

Besides the bright so-called He-stars in the central parsec that were identified as massive young stars more than a decade ago, the fainter stars in the immediate environment of Sgr A*--the overwhelming majority of the so-called S-stars--have now also been identified as young stars. With the help of integral-field-spectroscopy and adaptive optics they have been classified as B-type main sequence stars. The presence of these stars in immediate proximity to the supermassive black hole is not yet understood. Currently, there are largely two approaches for explaining the paradox of the young stars: the infall and dissolution of a massive cluster, or star formation in a formerly present accretion disk. In case of the first scenario, a question of considerable interest and debate in the conference was the question whether infalling clusters may contain intermediate mass black holes. Recent observations of the circumnuclear disk with millimeter interferometry indicate that gas densities in the the CND may be high enough to protect the clouds from tidal disruption as well as to support star formation in the CND or in a cloud core infalling from the CND into the central parsec. Clusters forming in the CND would have to migrate only a short distance into the central parsec. Hence, requirements on their densities and total masses could be less extreme than in the case of an infalling cluster that formed at distances 5-10 pc from Sgr A*. However even with higher densities, the strong tidal forces and most likely low star formation efficiency make it unclear as to whether a feature with the properties of the CND could form a single cluster with enough mass to account for the young stars in the central parsec.

Variable emission from Sgr A* can now be observed routinely at radio to submm-wavelengths, in the near-infrared, and in the X-ray regime. The first campaigns have recently been successful in simultaneously measuring the emission from Sgr A* in the different wavelength regimes. There appears to be a close correlation between emission at NIR/X-ray wavelengths. It has become clear that the extreme under-luminosity of Sgr A* is, on the one hand, due to a very low mass accretion rate. On the other hand, low mass accretion is not sufficient to explain the observations: The accretion flow in Sgr A* must be of the ADAF/RIAF type (Advection Dominated Accretion Flow/Radiatively Inefficient Accretion Flow), i.e. the gravitational energy released in the infalling plasma is advected beyond the event horizon. Also, from a comparison between the mass within the Bondi capture radius and the measured limits on the actually accreted mass there must be a considerable outflow of matter from the black hole. Here, one of the main debates is the existence of a jet and in as much it contributes to/dominates the emission and mass outflow from Sgr A*. While the direct observability of a jet with millimeter interferometry may still lie some years in the future, other methods, such as measuring position shifts of Sgr A* with frequency, may be able to provide some constraints in the near future.

As for the flaring of Sgr A*, i.e. the outbursts that are observed with a frequency of up to several times per day at X-ray/NIR wavelengths, there is general agreement that only small modifications in the properties of the radiating electron population are needed for their explanation. Synchrotron emission and synchrotron self-Compton emission then can produce the observed emission and rapid variability. It is still not clear what exactly causes the flares, e.g. magnetic reconnection as in solar flares, and whether flares may represent tentative phase transitions toward a different accretion/emission state of Sgr A*, in which it moves closer to the characteristics observed in brighter accreting black holes.

Paris GC Meeting Notes

The talks are not yet online, but Andrea Goldwurm from the SOC will be posting them under the scientific program link.

Upcoming GC Meetings

On September 1, there will be a one day ``mini-symposium'' on the GC as part of a GLAST collaboration meeting at Stanford/SLAC. The website is not yet up, but will be posted on the main website: http://www-glast.slac.stanford.edu/. The meeting is being organized by the GLAST Science Working Group, including Chuck Dermer, Steve Ritz and Peter Michelson.

There will also be a series of two one-week Galactic Center meetings at the International Space Science Institute (ISSI, Bern, Switzerland), with preliminary plans for the first to be held in November-December 2005 and the second in May-June 2006. The first meeting will focus on the inner few pc, and the second will extend to 300 pc. These are not open meetings, but rather will bring together about a dozen experts for a week of focused discussion. The meetings were successfully proposed to the ISSI by Felix Aharonian, Anne Decourchelle and Bob Warwick. More information will be posted at http://www.issi.unibe.ch/.

In April 2006 there will be a larger-scale GC meeting in Bad Honnef, which will be open to the community and fall along the lines of previous meetings in Tucson and Hawaii. Please see the announcement at the end of the Newsletter!