Volume 18, July 2004 - In the News

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In other news, we welcome Robin Hernnstein to our editorial staff. As always, please feel free to contact any of us with GC newsworthy items or future article ideas. We also remind our readers to please submit their abstracts to GCNEWS. As you know, GCNEWS promises rapid delivery of your results to fellow GC researchers. While astro-ph is a valuable resource, GCNEWS directly targets those readers with an acknowledged interest in GC science.

In this Volume

In this volume, we report on a conference devoted to Sgr A* and its environment, and also provide an update of GC research presented at the summer AAS meeting in Denver.

We are especially excited to have a comprehensive, invited review article by Sheperd Doeleman and Geoff Bower. In their article, they demonstrate how far we have come in understanding the structure of Sgr A* since its discovery. Their article describes the methods used to model and constrain the size of the radio source from radio interferometric data and also highlights future observational prospects - they point out that we are poised on the edge of being able to use such techniques to image the shadow of Sgr A*.

A number of GC abstracts have come in this spring and summer (and as mentioned before, we could always use more!) and are listed at the end of the issue.

Happy 30th Sgr A*! - Cornelia Lang

The discovery of the compact radio source at the center of the Milky Way was made by Bruce Balick and Bob Brown in 1974 February. They had originally been searching for compact regions of star formation in the vicinity of the Galactic Center. The placement of the 45-foot telescope near Huntersville, West Virginia, as the 35 km outstation for the NRAO Green Bank radio link interferometer (consisting of three, 85-foot antennas at Green Bank), was crucial for resolving out the extended confusion from Sgr A West and providing spatial resolutions of 0.3'' (at 3.7 cm) and 0.7'' (at 11 cm) and detecting the very compact radio source, Sgr A*.

An opening review talk on massive objects at the centers of galaxies was given by Roger Blandford, followed by a series of historical talks given by the original observers, Bruce Balick and Bob Brown, and the Green Bank site director at that time, Dave Hogg. A number of fascinating letters from Balick to Brown were shown which provided additional insight into the details of the discovery (also summarized recently by Goss, Brown, & Lo 2003, Astron. Nachr., S1, 1). Roy Booth, K. Y. Lo, and Ron Ekers also presented accounts of early work on the size and structure of Sgr A* based on some of the first aperture synthesis studies at Jodrell Bank, Owens Valley Radio Observatory, and the VLA.

Following the historical talks on the discovery of Sgr A*, a series of talks on recent results were presented. Don Backer gave an overview of interstellar scattering properties toward Sgr A*, and Geoff Bower and Zhi-Qiang Shen reported on VLBA closure phase techniques used to constrain the size of this compact source. Results on the linear and circular polarization of Sgr A* were given by Geoff Bower. Andrea Ghez, Rainer Schödel, Jun Hui-Zhao and Fred Baganoff confirmed that variability of Sgr A* on a number of timescales has now been detected across the spectrum from radio to near-IR to X-rays. Progress reports by Ramesh Narayan, Sera Markoff, Eliot Quataert, and Fulvio Melia followed, describing the various models to explain the spectrum, polarization, and variability of Sgr A*. Perspectives on the current state and characteristics of the modeling were given by Heino Falcke. The stellar and interstellar environment surrounding Sgr A* was discussed in a series of talks on Friday morning. The scientific talks concluded at noon with a thoughtful review by Mark Morris.

Following the scientific program, a dedication ceremony was held at the 45-foot antenna, where NRAO director Fred Lo presented Bruce Balick and Bob Brown with commemorative framed posters (see picture). A plaque on the 45-foot antenna and an informational adjacent sign for visitors were unveiled. An informal scientific session was held on Friday evening to discuss prospects for future millimeter and sub-millimeter VLBI observations of Sgr A*. The next generation of telescopes will provide an excellent opportunity for imaging on the scale of the event horizon, allowing for detailed tests of accretion models and general relativity.

The scientific program, PDF and PPT versions of the talks, and photographs are available at http://www.aoc.nrao.edu/~gcnews/GCconfs/SgrAstar30/.

GC appearances at Denver AAS in June 2004 - Cornelia Lang

The enigmatic, magnetic ``non-thermal filaments'' (NTFs) in the GC made an appearance in a number of presentations and posters this summer:

• F. Yusef-Zadeh et al. presented a talk entitled ``A Radio Survey of the GC Region: Detection of Numerous Linear Filaments'' in which he presented a 20 cm survey of a 300 pc region around the GC and a several-degree region toward low Galactic longitudes along the Galactic plane. A number of NTFs were discovered in this image. (17.03)

• J. Hewitt et al. followed up on these results and presented high resolution images from this 20 cm survey, highlighting a number of the new filamentary features and their associations with compact, star forming regions. (59.03)

• T. Freismuth et al. presented recent work on the peculiar NTF known as the ``Snake.'' For the first time, the orientation of the intrinsic magnetic field in this object has been revealed to lie along the NTF, even in the region where the Snake undergoes substantial ``kinks.'' Large rotation measures (> 4000 rad m^-2) are also observed toward much of the Snake. (59.05)

The combination of Green Bank single dish data with Very Large Array interferometric data was a highlight of the poster by C. Law et al. (and also the previously mentioned 59.03 poster):

• Law et al. focused on understanding the large-scale radio continuum emission in the GC. In particular, they are interested in one of the Galactic center ``Lobes'' of extended emission oriented perpendicularly to the plane of the Galaxy. They presented the distribution of spectral index along and across the lobes to quantify the energetics of this feature. (59.02)

There were two press releases by NRAO on the combination of GBT and VLA datasets (59.02, 59.03). See http://www.nrao.edu/pr/2004/filaments/ and http://www.nrao.edu/pr/2004/GBTlobe/ for more details.

Several posters were focused on the interstellar medium in the GC region, on large and small scales:

• C. Martin et al. showed results from the submillimeter observatory, AST/RO, on several unusual molecular clumps in the GC region. He had fully-sampled maps in the lines of CO (7->6), CO (4->3) and C (³P₁ -> ³P₀) which suggest that these clouds are tidally shocked through interactions with the bar potential in the Milky Way. (59.06)

• I. Azcárate et al. investigated the departure from LTE in the GC region by observing nine positions around the GC in H159 alpha and H200 beta radio recombination lines. They employed the 30-m telescope of the Instituto Argentino de Radioastronomia. (59.01)

• Lang et al. presented their findings from a large-scale survey of HI absorption toward the central 250 pc of the GC in order to clarify the nature of the physical associations of the atomic, ionized and molecular components. In addition, line of sight distances to continuum features can be constrained as well as column densities estimated. Specific results on Sgr B2 and the Radio Arc were shown. (59.04)

Other GC highlights:

• S. Liu et al. demonstrated that nonthermal electrons accelerated by plasma wave turbulence in the central concentration of hot gas may be responsible for portions of the spectrum of Sgr A* and also its variability in the infrared and X-ray wavebands. (17.01)

• N. Weinberg et al. outlined the prospects for observing stellar dynamics in the neighborhood of Sgr A* using the proposed Thirty Meter Telescope (TMT). The TMT will allow astronomers to measure the mass of the black hole and the distance to the GC with accuracies better than about 0.1%. Some relativistic effects (such as prograde orbital procession) will also be detectable. (17.02)