* Astronomy

Members Login
Username 
 
Password 
    Remember Me  
Post Info TOPIC: Sagittarius A*


L

Posts: 131433
Date:
RE: Sagittarius A*
Permalink  
 


Title: High-resolution Millimeter-VLBI Imaging of Sgr A*
Authors: Zhi-Qiang Shen

We present the highest resolution VLBI imaging observations of Sgr A* made at both 7 and 3.5 mm. These data reveal wavelength-dependent intrinsic sizes with an intrinsic emitting region at 3.5 mm of about 1 AU (at a distance of 8 kpc to the Galactic Centre). When combined with the lower limit on the mass of Sgr A*, these size measurements provide strong evidence that Sgr A* is a super-massive black hole. We also detected a structural variation which results in an intrinsically symmetrical structure that increases in its intrinsic size by more than 25% at 7 mm.

Read more (17kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Stellar Orbits in the Central Parsec.

GalCentre2
Image created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes

A 2.2 micron image of the stellar orbits in the central parsec. Images taken from the years 1995 through 2006 are used to track specific stars orbiting the proposed black hole at the center of the Galaxy. These orbits, and a simple application of Kepler's Laws, provide the best evidence yet for a supermassive black hole, which has a mass of 4 million times the mass of the Sun. Especially important are the stars S0-2, which has an orbital period of only 15.56 years, and S0-16, which comes a mere 90 astronomical units from the black hole.

See more

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: MHD Simulations of Accretion onto Sgr A*: Quiescent Fluctuations, Outbursts, and Quasi-Periodicity
Authors: Chi-kwan Chan, Siming Liu, Christopher L. Fryer, Dimitrios Psaltis, Feryal Ozel, Gabriel Rockefeller, Fulvio Melia

High resolution observations of Sgr A* have revealed a wide variety of phenomena, ranging from intense rapid flares to quasi-periodic oscillations, making this object an ideal system to study the properties of low luminosity accreting black holes. In this paper, we use a pseudo-spectral algorithm to construct and evolve a three-dimensional magnetohydrodynamic model of the accretion disk in Sgr A*. Assuming a hybrid thermal-nonthermal emission scheme, we show that the MHD turbulence can by itself only produce factor of two fluctuations in luminosity. These amplitudes in variation cannot explain the magnitude of flares observed in this system. However, we also demonstrate that density perturbations in the disk do produce outbursts qualitatively similar to those observed by XMM-Newton in X-rays and ground-based facilities in the near infrared. Quasi-periodic oscillations emerge naturally in the simulated lightcurves. We attribute these to non-axisymmetric density perturbations that emerge as the disk evolves back toward its quiescent state.

Read more (585kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: K-band polarimetry of an Sgr A* flare with a clear sub-flare structure
Authors: L. Meyer, R. Schoedel, A. Eckart, V. Karas, M. Dovciak, W. J. Duschl

Context: The supermassive black hole at the Galactic centre, SgrA*, shows frequent radiation outbursts, often called 'flares'. In the near-infrared some of these flares were reported as showing intrinsic quasi-periodicities. The flux peaks associated with the quasi-periodic behaviour were found to be highly polarized. Aims: The aim of this work is to present new evidence to support previous findings of the properties of the polarized radiation from SgrA* and to again provide strong support for the quasi-periodicity of ~18±3 min reported earlier.
Methods: Observations were carried out at the European Southern Observatory's Very Large Telescope on Paranal, Chile. We used the NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the polarimetric lightcurves with a hot-spot model, we addressed the question of whether the data are consistent with this model. To fit the observed data we used a general relativistic ray-tracing code in combination with a simple hot-spot/ring model.
Results: We report on new polarisation measurements of a K-band flare from the supermassive black hole at the Galactic centre. The data provide very strong support for a quasi-periodicity of 15.5 ±2 min. The mean polarisation of the flare is consistent with the direction of the electric field vector that was reported in previous observations. The data can be modelled successfully with a combined blob/ring model. The inclination i of the blob orbit must be i > 20 deg on a 3sigma level, and the dimensionless spin parameter of the black hole is derived to be a > 0.5.

click here to read more. (85kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The Intrinsic Size of Sagittarius A* from 0.35 cm to 6 cm
Authors: Geoffrey C. Bower, W.M. Goss, Heino Falcke, Donald C. Backer, Yoram Lithwick

Researchers present new high-resolution observations of Sagittarius A* at wavelengths of 17.4 to 23.8 cm with the Very Large Array in A configuration with the Pie Town Very Long Baseline Array antenna. They use the measured sizes to calibrate the interstellar scattering law and find that the major axis size of the scattering law is smaller by ~6% than previous estimates.
Using the new scattering law, the researchers are able to determine the intrinsic size of Sgr A* at wavelengths from 0.35 cm to 6 cm using existing results from the VLBA. The new law increases the intrinsic size at 0.7 cm by ~20% and <5% at 0.35 cm. The intrinsic size is 13^+7_-3 Schwarzschild radii at 0.35 cm and is proportional to lambda^gamma, where gamma is in the range 1.3 to 1.7.

Read more (59kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: Sub-milliarcsecond Imaging of SgrA* and M87
Authors: T.P. Krichbaum, D.A. Graham, M. Bremer, W. Alef, A. Witzel, J.A. Zensus, A. Eckart

Researchers present and discuss new result from mm-VLBI observations of M87 and SgrA*, using the Global mm-VLBI array (GMVA).
New 3mm-VLBI images of the inner jet of M87 are presented, showing details with a spatial resolution down to 15 Schwarzschild radii. This resolution corresponds to a similar spatial resolution (in terms of R_s) obtained for Sgr A*.
They discuss existing and new size determinations for this source provided by mm-VLBI at wavelengths of 3, 2, and 1.4 mm. With respect to the morphological difference between Sgr A* and M87 (the latter exhibits a long jet), a comparison of the sub-milliarcsecond structures seen with mm-VLBI may lead to a better understanding of the astro-physical processes acting in the vicinity of super-massive black holes.

Read more (172kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

Title: The Rotation Measure and 3.5mm Polarization of Sgr A*
Authors: Jean-Pierre Macquart, Geoffrey C. Bower, Melvyn C.H. Wright, Donald C. Backer, Heino Falcke

Researchers report the detection of variable linear polarisation from Sgr A* at a wavelength of 3.5mm, the longest wavelength yet at which a detection has been made. The mean polarisation is 2.1 ±0.1% at a position angle of 16 ±2 deg with rms scatters of 0.4% and 9 deg over the five epochs.
They also detect polarisation variability on a timescale of days. Combined with previous detections over the range 150-400GHz (750-2000 microns), the average polarisation position angles are all found to be consistent with a rotation measure of -4.4 ±0.3 x 10^5 rad/m^2. This implies that the Faraday rotation occurs external to the polarised source at all wavelengths. This implies an accretion rate ~0.2 - 4 x 10^-8 solar masses per year for the accretion density profiles expected of ADAF, jet and CDAF models and assuming that the region at which electrons in the accretion flow become relativistic is within 10 solar radii. The inferred accretion rate is inconsistent with ADAF/Bondi accretion. The stability of the mean polarisation position angle between disparate polarisation observations over the frequency range limits fluctuations in the accretion rate to less than 5%. The flat frequency dependence of the inter-day polarisation position angle variations also makes them difficult to attribute to rotation measure fluctuations, and suggests that both the magnitude and position angle variations are intrinsic to the emission.

Read more (34kb, PDF)

__________________


L

Posts: 131433
Date:
Permalink  
 

In the most comprehensive study of Sagittarius A* (Sgr A*), the enigmatic supermassive black hole in the centre of the Milky Way Galaxy, astronomers -- using nine ground and space-based telescopes including the Hubble Space Telescope and the XMM-Newton X-ray Observatory -- have discovered that Sgr A* produces rapid flares close to the innermost region of the black hole in many different wavelengths and that these emissions go up and down together.

This insight into the frequent bursts of radiation observed shooting off the black hole like firecrackers -- similar to solar flares -- will help scientists better understand the dynamics of Sgr A* and the source of its flares.
Farhad Yusef-Zadeh, professor of physics and astronomy at North-western University, who led a team of 11 astronomers from around the world in the study of Sgr A*, presented the team’s results at a press conference January 10 at the American Astronomical Society meeting in Washington, D.C.

"We observed that the less energetic infrared flares occur simultaneously with the more energetic X-ray flares as well the submillimeter flares. From this, we infer that the particles that are accelerated near the black hole give rise to X-ray, infrared and submillimeter emission. In addition, not all of the material that approaches the black hole gets sucked in. Some of the material may be ejected from the vicinity of the central black hole or event horizon. Our observations hint that these flares have enough energy to escape from the closest confines of the supermassive black hole’s sphere of influence" - Farhad Yusef-Zadeh.


These images show the flare activity from A* (Sgr A*). Both show Sgr A* when it was quiescent or quiet (left), when it was active (middle) and the difference between them (right), which clearly shows Sgr A* is the only source that is flaring.
Credit Northwestern University


Yusef-Zadeh and his team observed Sgr A* during two four-day periods in 2004, one in March and one in September. (2004 marked the 30th anniversary of the discovery of Sgr A*, which has a mass equivalent to 3.6 million Suns and is located in the Sagittarius constellation.) The campaign captured data across a wide spectrum, including radio, millimetre, submillimeter, infrared, X-ray and soft gamma ray wavelengths.
The astronomers also determined that the real engine of the flare activity is in the infrared wavelength. Using observations from Hubble’s Near-Infrared Camera and Multi-Object Spectrometer, they found infrared activity 40 percent of the time, more than was observed at any other wavelength.

"This is not something we expected. Other black holes in other galaxies don’t show this flare activity. We believe it is the dynamics of the captured material -- very close to the event horizon of the black hole -- that produces the flares. And the flares are fluctuating at low levels, like flickers. The flare radiation results from fast-moving materials in the innermost region of the black hole. It’s a way of life for Sgr A*, this frequent low level of activity" - Farhad Yusef-Zadeh.

Because flares are variable and not constant, the study required a large number of telescopes devoted to studying flare activity simultaneously. The space-based telescopes used in this observation campaign were the Hubble Space Telescope, the XMM-Newton X-ray Observatory and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The ground telescopes used were Very Large Array (VLA) of the National Radio Astronomy Observatory; Caltech Submillimeter Observatory (CSO); Submillimeter Telescope (SMT); Nobeyama Array (NMA); Berkeley Illinois Maryland Array (BIMA); and Australian Telescope Compact Array (ATCA).

In addition to Yusef-Zadeh, the Sgr A* study was conducted by Howard A. Bushouse, Space Telescope Science Institute; Mark Wardle, Macquarie University, Australia; Douglas Roberts, Northwestern University and Adler Planetarium and Astronomy Museum; Craig Heinke, Northwestern University; C. Darren Dowell, Jet Propulsion Laboratory, California Institute of Technology; Geoffrey Bower, University of California, Berkeley; Stuart Shapiro, University of Illinois at Urbana-Champaign; Andrea Goldwurm and Guillaume Belanger, Service d’Astrophysique, France; and Baltasar Vila-Vilaro, National Astronomical Observatory of Japan.

Source

__________________


L

Posts: 131433
Date:
G359.95-0.04
Permalink  
 


G359.95-0.04: Pulsar Candidate Near Sgr A*
Authors: Q. D. Wang (UMass/IAS), F. J. Lu (UMass/IHEP), E. V. Gotthelf (Columbia U.)

Researchers report the discovery of a prominent nonthermal X-ray feature located near the Galactic centre that thhey identify as an energetic pulsar wind nebula.
This feature, G359.95-0.04, lies 1 light year north of Sgr A* (in projection), is comet-like in shape, and has a power law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging the rare class of ram-pressure confined pulsar wind nebulae.
The luminosity of the nebula at the distance of \sgra, consistent with the inferred X-ray absorptions, is 1 10^34 ergs s^-1 in the 2--10 keV energy band.
The cometary tail extends back to a region centred at the massive stellar complex IRS 13 and surrounded by enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic centre. They also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complex IRS 13 and Supernova remnant (SNR) Sgr A East.


Tri-colour image: the radio continuum image (red), the near-IR image (green), and a 2.5-9 keV image (blue), Sagittarius A*--the Galactic centre--appears as a pink dot.

Read More (PDF)

__________________


L

Posts: 131433
Date:
RE: Sagittarius A*
Permalink  
 



The orbits of stars within the central 1.0 X 1.0 arcseconds of our Galaxy.
In the background, the central portion of a diffraction-limited image taken in 2004 is displayed.
While every star in this image has been seen to move over the past 9 years, estimates of orbital parameters are only possible for the seven stars that have had significant curvature detected.
The annual average positions for these seven stars are plotted as coloured dots, which have increasing colour saturation with time. Also plotted are the best fitting simultaneous orbital solutions. These orbits provide the best evidence yet for a supermassive black hole, which has a mass of 3.7 million times the mass of the Sun.

__________________
«First  <  14 5 6 7  >  Last»  | Page of 7  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard