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Post Info TOPIC: Sagittarius A*


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RE: Sagittarius A*
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UCLA astronomers and colleagues have taken the first clear picture of the centre of our Milky Way galaxy, including the area surrounding the supermassive black hole, using a new laser virtual star at the W.M. Keck observatory in Hawaii.

"Everything is much clearer now. We used a laser to improve the telescope's vision a spectacular breakthrough that will help us understand the black hole's environment and physics. It's like getting Lasik surgery for the eyes, and will revolutionise what we can do in astronomy" - Andrea Ghez, UCLA professor of physics and astronomy, who headed the research team.


The centre of our Milky Way galaxy, as seen in the infrared using the Keck Laser Guide Star (left panel) and the Keck Natural Guide Star (right). The white cross marks the location of the supermassive black hole.
The Narrow-field image of the Galactic Centre at 3.8 microns (L prime) was obtained with the Keck Laser Guide Star System on July 26, 2004 versus an image using the Keck Natural Guide Star System. Strehl ratio in the LGS image was measured at 75%, double the previous performance at this wavelength. A Strehl ratio of 100% represents a perfect, fully-corrected image. The resolution is 82 milliarcseconds.
Credit: W.M. Keck Observatory/UCLA Galactic Centre Group


Astronomers are used to working with images that are blurred by the Earth's atmosphere. However, a laser virtual star, launched from the Keck telescope, can be used to correct the atmosphere's distortions and clear up the picture. This new technology, called Laser Guide Star adaptive optics, will lead to important advances for the study of planets in our solar system and outside of our solar system, as well as galaxies, black holes, and how the universe formed and evolved.

"We have worked for years on techniques for 'beating the distortions in the atmosphere' and producing high-resolution images. We are pleased to report the first Laser Guide Star adaptive optics observations of the centre of our galaxy" - Andrea Ghez

Ghez and her colleagues took "snapshots" of the centre of the galaxy, targeting the supermassive black hole 26,000 light years away, at different wavelengths. This approach allowed them to study the infrared light emanating from very hot material just outside the black hole's "event horizon," about to be pulled through.


The Laser Guide Star Adaptive Optics System at the W. M. Keck Observatory takes infrared images of the Galactic Center. The image above is taken in the L'-band (3.8 microns), is 10 arcseconds in size and has a resolution of 82 milliarcseconds. At 3.8 microns, the Strehl ratio was measured at 75%, double the previous performance at this wavelength. A Strehl ratio of 100% represents a perfect, fully-corrected image. At K' (2.1 microns) the Strehl ratio was measured at 35%, a modest improvement, and the resolution was 56 milliarcseconds.

"We are learning the conditions of the infalling material and whether this plays a role in the growth of the supermassive black hole. The infrared light varies dramatically from week to week, day to day and even within a single hour" - Andrea Ghez

The research, federally funded by the National Science Foundation, will be published Dec. 20 in the Astrophysical Journal Letters.
The research was conducted using the 10-meter Keck II Telescope, which is the world's first 10-meter telescope with a laser on it. Laser Guide Star allows astronomers to "generate an artificial bright star" exactly where they want it, which reveals the atmosphere's distortions.
Since 1995, Ghez has been using the W.M. Keck Observatory to study the galactic centre and the movement of 200 nearby stars.
Black holes are collapsed stars so dense that nothing can escape their gravitational pull, not even light. Black holes cannot be seen directly, but their influence on nearby stars is visible, and provides a signature. The supermassive black hole, with a mass more than 3 million times that of our sun, is in the constellation of Sagittarius. The galactic centre is located due south in the summer sky.
The black hole came into existence billions of years ago, perhaps as very massive stars collapsed at the end of their life cycles and coalesced into a single, supermassive object.

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Title: The X-ray Ridge Surrounding Sgr A* at the Galactic Centre
Authors:
Gabriel Rockefeller, Christopher L. Fryer, Frederick K. Baganoff, and
Fulvio Melia

Researchers present the first detailed simulation of the interaction between the
supernova explosion that produced Sgr A East and the wind-swept inner ~ 2-pc region at the Galactic centre.
The passage of the supernova ejecta through this medium produces an X-ray ridge ~ 9'' to 15'' to the NE of the supermassive blackhole Sagittarius A* (Sgr A*).
They show that the morphology and X-ray intensity of this feature match very well with recently obtained Chandra images, and infer a supernova remnant age of less than 2,000 years.
This young age - a factor 3-4 lower than previous estimates - arises from their inclusion of stellar wind effects in the initial (pre-explosion) conditions in the medium.
The supernova does not clear out the central ~ 0.2-pc region around Sgr~A* and does not significantly alter the accretion rate onto the central black hole upon passage through the Galactic centre.

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There is some evidence, though yet unconfirmed, that Sagittarius A*--the supermassive black hole at the Galactic centre--emits its radio waves modulated with a ~100-day period.
What is intriguing about this apparent quasi-periodicity is that, though the amplitude of the modulation increases with decreasing wavelength (from 3.6 to 1.3 cm), the quasi-period itself does not seem to depend on the frequency of the radiation. It is difficult to imagine how a binary companion, were that the cause of this modulation, could have escaped detection until now. Instead, it has been suggested that the spin-induced precession of a disk surrounding a slowly rotating black hole could have the right period to account for this behaviour.
A new study examines how Sagittarius A*'s light curve could be modulated by such a mechanism. It demonstrate that the partial occultation of a nonthermal halo by a compact, radio-opaque disk does indeed produce the observed frequency-dependent amplitude. This appears to be in line with other observational arguments suggesting that Sagittarius A*'s mm/sub-mm spectrum is produced by a ~10 Schwarzschild-radius disk, whereas its cm-waves originate from a nonthermal particle distribution in a halo extending out to over 20 Schwarzschild radii.
Interestingly, the model suggests that the observed period corresponds to half the precession period and that a non-axisymmetric disk could produce a second period roughly twice as long as the first.

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