* Astronomy

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RE: Sagittarius A*

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Title: Concerning the Distance to the Center of the Milky Way and its Structure
Authors: Daniel J. Majaess

The distance to the Galactic center inferred from OGLE RR Lyrae variables observed in the direction of the bulge is Ro=8.1±0.6 kpc. An accurate determination of Ro is hindered by countless effects that include an ambiguous extinction law, a bias for smaller values of Ro because of a preferential sampling of variable stars toward the near side of the bulge owing to extinction, and an uncertainty in characterizing how a mean distance to the group of variable stars relates to Ro. A VI-based period-reddening relation for RR Lyrae variables is derived to map extinction throughout the bulge. The reddening inferred from RR Lyrae variables in the Galactic bulge, LMC, SMC, and IC 1613 match that established from OGLE red clump giants and classical Cepheids. RR Lyrae variables obey a period-colour (VI) relation that is relatively insensitive to metallicity. Edge-on and face-on illustrations of the Milky Way are constructed by mapping the bulge RR Lyrae variables in tandem with catalogued red clump giants, globular clusters, planetary nebulae, classical Cepheids, young open clusters, HII regions, and molecular clouds. The sample of RR Lyrae variables do not trace a prominent Galactic bar or triaxial bulge oriented at phi~25 degrees.

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Astronomers have long known that the supermassive black hole at the center of the Milky Way Galaxy, known as Sagittarius A* (or Sgr A* for short), is a particularly poor eater. The fuel for this black hole comes from powerful winds blown off dozens of massive young stars that are concentrated nearby. These stars are located a relatively large distance away from Sgr A*, where the gravity of the black hole is weak, and so their high-velocity winds are difficult for the black hole to capture and swallow. Scientists have previously calculated that Sgr A* should consume only about 1% of the fuel carried in the winds.

However, it now appears that Sgr A* consumes even less than expected - ingesting only about one percent of that one percent. Why does it consume so little? The answer may be found in a new theoretical model developed using data from a very deep exposure made by NASA's Chandra X-ray Observatory.

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Galactic Center
In celebration of the International Year of Astronomy 2009, NASA's Great Observatories -- the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory -- have collaborated to produce an unprecedented image of the central region of our Milky Way galaxy.

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Credit: X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy

In this spectacular image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the center of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon.

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Chandra Captures New Vista Of Milky Way Center
A dramatic new vista of the center of the Milky Way galaxy from NASA's Chandra X-ray Observatory exposes new levels of the complexity and intrigue in the Galactic center. The mosaic of 88 Chandra pointings represents a freeze-frame of the spectacle of stellar evolution, from bright young stars to black holes, in a crowded, hostile environment dominated by a central, supermassive black hole.
Permeating the region is a diffuse haze of X-ray light from gas that has been heated to millions of degrees by winds from massive young stars - which appear to form more frequently here than elsewhere in the Galaxy - explosions of dying stars, and outflows powered by the supermassive black hole - known as Sagittarius A* (Sgr A*). Data from Chandra and other X-ray telescopes suggest that giant X-ray flares from this black hole occurred about 50 and about 300 years earlier.
The area around Sgr A* also contains several mysterious X-ray filaments. Some of these likely represent huge magnetic structures interacting with streams of very energetic electrons produced by rapidly spinning neutron stars or perhaps by a gigantic analogy of a solar flare.

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New telescopic device looks at black holes
A team led by University of Florida astronomy Professor Stephen Eikenberry says it used a university-designed and built camera-spectrometer affixed to the Gemini South telescope in Chile to take its "first light" images of the supermassive black hole located at the center of our galaxy. That black hole is thought to be as massive as 4 million suns put together, the scientists said.

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Title: Imaging an Event Horizon: submm-VLBI of a Super Massive Black Hole
Authors: Sheperd Doeleman (1), Eric Agol (2), Don Backer (3), Fred Baganoff (4) Geoffrey C. Bower (3), Avery Broderick (5), Andrew Fabian (6), Vincent Fish (1), Charles Gammie (7), Paul Ho (8), Mareki Honma (9), Thomas Krichbaum (10), Avi Loeb (11), Dan Marrone (12), Mark Reid (11), Alan E.E. Rogers (1), Irwin Shapiro (11), Peter Strittmatter (13), Remo Tilanus (14), Jonathan Weintroub (11), Alan Whitney (1), Melvyn Wright (3), Lucy Ziurys (13) ((1) MIT Haystack Observatory, (2) University of Washington, (3) UC Berkeley, (4) MIT, (5) CITA, (6) University of Cambridge, (7) U. Illinois Urbana-Champaign, (8) ASIAA, (9) NAOJ, (10) MPIfR, (11) Harvard Smithsonian CfA, (12) University of Chicago & NRAO, (13) University of Arizona, (14) Joint Astronomy Centre)

A long standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realising this goal would open a new window on the study of General Relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of an event horizon, and fundamental black hole physics (e.g., spin). Steady long-term progress on improving the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made it extremely likely that this goal will be achieved within the next decade. The most compelling evidence for this is the recent observation by 1.3mm VLBI of Schwarzschild radius scale structure in SgrA*, the compact source of radio, submm, NIR and xrays at the center of the Milky Way. SgrA* is thought to mark the position of a ~4 million solar mass black hole, and because of its proximity and estimated mass presents the largest apparent event horizon size of any black hole candidate in the Universe. Over the next decade, existing and planned mm/submm facilities will be combined into a high sensitivity, high angular resolution "Event Horizon Telescope" that will bring us as close to the edge of black hole as we will come for decades. This white paper describes the science case for mm/submm VLBI observations of both SgrA* and M87 (a radio loud AGN of a much more luminous class that SgrA*). We emphasise that while there is development and procurement involved, the technical path forward is clear, and the recent successful observations have removed much of the risk that would normally be associated with such an ambitious project.

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Title: Observing a Black Hole Event Horizon: (Sub)Millimetre VLBI of Sgr A*
Authors: Vincent L. Fish, Sheperd S. Doeleman (MIT Haystack Observatory)

Very strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects. Recent millimetre very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimetre VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimetre VLBI "Event Horizon Telescope" will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing. These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

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Title: The variability of Sagittarius A* at 3 millimetre
Authors: Juan Li, Zhi-Qiang Shen, Atsushi Miyazaki, Lei Huang, R.J. Sault, Makoto Miyoshi, Masato Tsuboi, Takahiro Tsutsumi

We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behaviour of Sgr A*. Sgr A* appeared to undergo a high and stable state in 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling can not be ignored, and a minimum mass-loss rate of 9.7*10^{-10}M_sun /yr is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*.

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Avery Broderick of the Canadian Institute of Theoretical Astrophysics and his colleagues have analysed previous infrared and radio observations of the galactic centre and put forward the strongest evidence yet that an object at our galaxy's centre does indeed have an event horizon.
The team reasoned that if the object were not a black hole, it should glow in the infrared. This is because the kinetic energy of matter hitting the object would be converted into heat. Given the rate that matter appears to be falling onto the central object, it should have a temperature of at least a few hundred Kelvin, they calculate. The resulting infrared glow would be 250 times as bright as the actual glow coming from the region containing the massive object and its disc of matter, when previously measured during quieter moments when the disc is not flaring up.

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