332 research outputs found
High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
Over a two year period (1995-1997), we have conducted a diffraction-limited
imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar
cluster using the Keck 10-m telescope. The K band images obtained reveal a
large population of faint stars. We use an unbiased approach for identifying
and selecting stars to be included in this proper motion study, which results
in a sample of 90 stars with brightness ranging from K=9-17 and velocities as
large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997;
Genzel et al. 1997), the source confusion is reduced by a factor of 9, the
number of stars with proper motion measurement in the central 25 arcsec^2 of
our galaxy is doubled, and the accuracy of the velocity measurements in the
central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar
surface density and the velocity dispersion are consistent with the position of
the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr
A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a
function of distance from Sgr A*, the velocity dispersion displays a falloff
well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo
confined to a volume of at most 10^-6 pc^3, consistent with earlier results.
Although uncertainties in the measurements mathematically allow for the matter
to be distributed over this volume as a cluster, no realistic cluster is
physically tenable. Thus, independent of the presence of Sgr A*, the large
inferred central density of at least 10^12 Mo/pc^3, which exceeds the
volume-averaged mass densities found at the center of any other galaxy, leads
us to the conclusion that our Galaxy harbors a massive central black hole.Comment: 19 pages, 8 figures, accepted for publications in the Astrophysical
Journa
Односторонний и двусторонний эффект памяти формы в [([3)12]]-монокристаллах сплава Ni[49]Fe[18]Ga[27]Co[6]
This study was performed to assess the role of additional myocardial perfusion imaging during high dose dobutamine/atropine stress magnetic resonance (DSMR-wall motion) for the evaluation of patients with intermediate (50-70%) coronary artery stenosis. Routine DSMR-wall motion was combined with perfusion imaging (DSMR-perfusion) in 174 consecutive patients with chest pain syndromes who were scheduled for a clinically indicated coronary angiography. When defining CAD as the presence of a ≥ 50% stenosis, the addition of perfusion imaging improved sensitivity (90 vs. 79%, P < 0.001) with a non-significant reduction in specificity (85 vs. 90%, P = 0.13) and an improvement in overall diagnostic accuracy (88 vs. 84%, P = 0.008). Adding perfusion imaging improved sensitivity in patients with intermediate stenosis (87 vs. 72%, P = 0.03), but not in patients with severe (≥70%) stenosis (93 vs. 84%, P = 0.06). In patients with severe stenosis specificity of DSMR-perfusion versus DSMR-wall motion decreased (61 vs 70%, P = 0.001) resulting in a lower overall accuracy (71 vs 74%, P = 0.03). Using a cutoff of ≥50% for the definition of CAD, sensitivity of DSMR-perfusion compared to DSMR-wall motion was significantly higher in patients with single vessel (88 vs. 77%, P = 0.03) and multi vessel disease (93 vs. 79%, P = 0.03), whereas no significant differences were found using a cutoff of ≥70% stenosis for the definition of CAD. The addition of perfusion imaging during DSMR-wall motion improved the sensitivity in patients with intermediate coronary artery stenosis. Overall diagnostic accuracy increased only when defining CAD as ≥50% stenosis. In patients with ≥70% stenosis DSMR-wall motion alone had higher accuracy due to more false-positive cases with DSMR-perfusion
Sixteen years of X-ray monitoring of Sagittarius A*: Evidence for a decay of the faint flaring rate from 2013 August, 13 months before a rise in the bright flaring rate
Recently, in a study the X-ray flaring activity of Sgr A* with Chandra and
XMM-Newton public observations from 1999 to 2014 and 2014 Swift data, it has
been argued that the "bright and very bright" flaring rate raised from 2014
Aug. 31. Thanks to 482ks of observations performed in 2015 with Chandra,
XMM-Newton and Swift, we test the significance of this rise of flaring rate and
determine the threshold of unabsorbed flare flux or fluence leading to any
flaring-rate change. The mean unabsorbed fluxes of the 107 flares detected in
the 1999-2015 observations are consistently computed from the extracted spectra
and calibration files, assuming the same spectral parameters. We construct the
observed flare fluxes and durations distribution for the XMM-Newton and Chandra
flares and correct it from the detection biases to estimate the intrinsic
distribution from which we determine the average flare detection efficiency for
each observation. We apply the BB algorithm on the flare arrival times
corrected from the corresponding efficiency. We confirm a constant overall
flaring rate in 1999-2015 and a rise in the flaring rate for the most
luminous/energetic flares from 2014 Aug. 31 (4 months after the passage of the
DSO/G2 close to Sgr A*). We also identify a decay of the flaring rate for the
less luminous and less energetic flares from 2013 Aug. and Nov., respectively
(10 and 7 months before the pericenter of the DSO/G2). The decay of the faint
flaring rate is difficult to explain by the tidal disruption of the DSO/G2,
whose stellar nature is now well established, since it occurred well before its
pericenter. Moreover, a mass transfer from the DSO/G2 to Sgr A* is not required
to produce the rise in the bright flaring rate since the energy saved by the
decay of the number of faint flares during a long time period may be later
released by several bright flares during a shorter time period. (abridged)Comment: Accepted in A&A in 2017 April 2
Randall-Sundrum model with and bulk brane viscosity
We study the effect of the inclusion of bulk brane viscosity on brane world
(BW) cosmology in the framework of the Eckart's theory, we focus in the
Randall-Sundrum model with negative tension on the brane.Comment: 4 pages. Submitted to PL
GRAVITY: getting to the event horizon of Sgr A*
We present the second-generation VLTI instrument GRAVITY, which currently is
in the preliminary design phase. GRAVITY is specifically designed to observe
highly relativistic motions of matter close to the event horizon of Sgr A*, the
massive black hole at center of the Milky Way. We have identified the key
design features needed to achieve this goal and present the resulting
instrument concept. It includes an integrated optics, 4-telescope, dual feed
beam combiner operated in a cryogenic vessel; near infrared wavefront sensing
adaptive optics; fringe tracking on secondary sources within the field of view
of the VLTI and a novel metrology concept. Simulations show that the planned
design matches the scientific needs; in particular that 10 microarcsecond
astrometry is feasible for a source with a magnitude of K=15 like Sgr A*, given
the availability of suitable phase reference sources.Comment: 13 pages, 11 figures, to appear in the conference proceedings of SPIE
Astronomical Instrumentation, 23-28 June 2008, Marseille, Franc
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