1,688 research outputs found
Evolution of Massive Black Hole Binaries
We present the result of large-scale N-body simulations of the
stellar-dynamical evolution of a massive black-hole binary at the center of a
spherical galaxy. We focus on the dependence of the hardening rate on the
relaxation timescale of the parent galaxy. A simple theoretical argument
predicts that a binary black hole creates the ``loss cone'' around it. Once the
loss cone is formed, the hardening rate is determined by the rate at which
field stars diffuse into the loss cone. Therefore the hardening timescale
becomes proportional to the relaxation timescale. Recent N-body simulations,
however, have failed to confirm this theory and various explanations have been
proposed. By performing simulations with sufficiently large N (up to )
for sufficiently long time, we found that the hardening rate does depend on N.
Our result is consistent with the simple theoretical prediction that the
hardening timescale is proportional to the relaxation timescale. This
dependence implies that most massive black hole binaries are unlikely to merge
within the Hubble time through interaction with field stars and gravitational
wave radiation alone.Comment: Reviced version accepted for publication in ApJ. Scheduled to appear
in the February 10, 2004 issu
Measurement of the radial mode spectrum of photons through a phase-retrieval method
We propose and demonstrate a simple and easy-to-implement
projective-measurement protocol to determine the radial index 'p' of a
Laguerre-Gaussian (LGlp) mode. Our method entails converting any specified
high-order LG0p mode into a near-Gaussian distribution that matches the
fundamental mode of a single-mode fiber (SMF) through the use of two
phase-screens (unitary transformations) obtained by applying a phase-retrieval
algorithm. The unitary transformations preserve the orthogonality of modes and
guarantee that our protocol can, in principle, be free of crosstalk. We measure
the coupling efficiency of the transformed radial modes to the SMF for
different pairs of phase-screens. Because of the universality of
phase-retrieval methods, we believe that our protocol provides an efficient way
of fully characterizing the radial spatial profile of an optical field
The fine structure line deficit in S 140
We try to understand the gas heating and cooling in the S 140 star forming
region by spatially and spectrally resolving the distribution of the main
cooling lines with GREAT/SOFIA. We mapped the fine structure lines of [OI] (63
{\mu}m) and [CII] (158 {\mu}m) and the rotational transitions of CO 13-12 and
16-15 with GREAT/SOFIA and analyzed the spatial and velocity structure to
assign the emission to individual heating sources. We measure the optical depth
of the [CII] line and perform radiative transfer computations for all observed
transitions. By comparing the line intensities with the far-infrared continuum
we can assess the total cooling budget and measure the gas heating efficiency.
The main emission of fine structure lines in S 140 stems from a 8.3'' region
close to the infrared source IRS 2 that is not prominent at any other
wavelength. It can be explained by a photon-dominated region (PDR) structure
around the embedded cluster if we assume that the [OI] line intensity is
reduced by a factor seven due to self-absorption. The external cloud interface
forms a second PDR at an inclination of 80-85 degrees illuminated by an UV
field of 60 times the standard interstellar radiation field. The main radiation
source in the cloud, IRS 1, is not prominent at all in the fine structure
lines. We measure line-to-continuum cooling ratios below 10^(-4), i.e. values
lower than in any other Galactic source, rather matching the far-IR line
deficit seen in ULIRGs. In particular the low intensity of the [CII] line can
only be modeled by an extreme excitation gradient in the gas around IRS 1. We
found no explanation why IRS 1 shows no associated fine-structure line peak,
while IRS 2 does. The inner part of S 140 mimics the far-IR line deficit in
ULIRGs thereby providing a template that may lead to a future model.Comment: Accepted for publication in Astronomy & Astrophysic
The integrin-binding defective FGF2 mutants potently suppress FGF2 signalling and angiogenesis.
We recently found that integrin αvβ3 binds to fibroblast growth factor (FGF)-αvβ31 (FGF1), and that the integrin-binding defective FGF1 mutant (Arg-50 to glutamic acid, R50E) is defective in signalling and antagonistic to FGF1 signalling. R50E suppressed angiogenesis and tumour growth, suggesting that R50E has potential as a therapeutic. However, FGF1 is unstable, and we had to express R50E in cancer cells for xenograft study, since injected R50E may rapidly disappear from circulation. We studied if we can develop antagonist of more stable FGF2. FGF2 is widely involved in important biological processes such as stem cell proliferation and angiogenesis. Previous studies found that FGF2 bound to αvβ3 and antagonists to αvβ3 suppressed FGF2-induced angiogenesis. However, it is unclear how FGF2 interacts with integrins. Here, we describe that substituting Lys-119/Arg-120 and Lys-125 residues in the predicted integrin-binding interface of FGF2 to glutamic acid (the K119E/R120E and K125E mutations) effectively reduced integrin binding to FGF2. These FGF2 mutants were defective in signalling functions (ERK1/2 activation and DNA synthesis) in NIH3T3 cells. Notably they suppressed, FGF2 signalling induced by WT FGF2 in endothelial cells, suggesting that the FGF2 mutants are antagonists. The FGF2 mutants effectively suppressed tube formation in vitro, sprouting in aorta ring assays ex vivo and angiogenesis in vivo The positions of amino acids critical for integrin binding are different between FGF1 and FGF2, suggesting that they do not interact with integrins in the same manner. The newly developed FGF2 mutants have potential as anti-angiogenic agents and useful tools for studying the role of integrins in FGF2 signalling
Spatial Determination of Magnetic Avalanche Ignition Points
Using time-resolved measurements of local magnetization in the molecular
magnet Mn12-ac, we report studies of the propagation of magnetic avalanches
(fast magnetization reversals) that originate from points inside the crystals
rather than at the edges. The curved nature of the fronts produced by
avalanches is reflected in the time-of-arrival at micro-Hall sensors placed at
the surface of the sample. Assuming that the avalanche interface is a spherical
bubble that grows with a radius proportional to time, we are able to locate the
approximate ignition point of each avalanche in a two-dimensional cross-section
of the crystal. For the samples used in these studies, avalanches in a given
crystal are found to originate in a small region with a radius of roughly 150
microns.Comment: 3 pages, 4 figure
Gamma Irradiation on Growth and Development of Amorphophallus Muelleri Blume.
Iles-iles (Amorphophallus muelleri Blume) produces apomictic seeds lead to low genetic variation. In order to induce genetic variation, germinated seeds were exposed to Gamma irradiation (Co-60) at doses of 10 to 100 Gy. Seed irradiation was conducted at Center for the Application of Isotope and Irradiation Technology -National Nuclear Energy Agency (CAIRT), Indonesia. Morphology and yield of M1 generation were observed. Results showed that irradiation at a dose of 10 Gy close to LD50 with survival rate 56%. Gamma irradiation at a dose of 10 Gy delayed seeds germination. Germination rates gradually increased and reached maximum at 4 weeks after planting (WAP) for control plants, and 14 WAP of irradiated plants. At 16 WAP, germination rate of 10 Gy irradiated plants was 56% and 84% for those of control plants. Irradiation induced chimera as indicated by short petiole, variegated and abnornal shape of leaflets. Some irradiated plants entered dormancy at 8-10 weeks later than control ones. Prolong vegetative periode lead the plants to produce heavier corms. This study revealed the possibility to induce variation of A. muelleri by using gamma irradition. Keywords: Amorphophallus muelleri, gamma irradiation (Co-60), morphological variation, mutation breedin
Disulfide bridge formation between SecY and a translocating polypeptide localizes the translocation pore to the center of SecY
During their biosynthesis, many proteins pass through the membrane via a hydrophilic channel formed by the heterotrimeric Sec61/SecY complex. Whether this channel forms at the interface of multiple copies of Sec61/SecY or is intrinsic to a monomeric complex, as suggested by the recently solved X-ray structure of the Methanococcus jannaschii SecY complex, is a matter of contention. By introducing a single cysteine at various positions in Escherichia coli SecY and testing its ability to form a disulfide bond with a single cysteine in a translocating chain, we provide evidence that translocating polypeptides pass through the center of the SecY complex. The strongest cross-links were observed with residues that would form a constriction in an hourglass-shaped pore. This suggests that the channel makes only limited contact with a translocating polypeptide, thus minimizing the energy required for translocation
Measuring the Broad-band X-Ray Spectrum from 400 eV to 40 keV in the Southwest Part of the Supernova Remnant RX J1713.7-3946
We report on results from Suzaku broadband X-ray observations of the
southwest part of the Galactic supernova remnant (SNR) RX J1713.7-3946 with an
energy coverage of 0.4-40 keV. The X-ray spectrum, presumably of synchrotron
origin, is known to be completely lineless, making this SNR ideally suited for
a detailed study of the X-ray spectral shape formed through efficient particle
acceleration at high speed shocks. With a sensitive hard X-ray measurement from
the HXD PIN on board Suzaku, we determine the hard X-ray spectrum in the 12--40
keV range to be described by a power law with photon index Gamma = 3.2+/- 0.2,
significantly steeper than the soft X-ray index of Gamma = 2.4+/- 0.05 measured
previously with ASCA and other missions. We find that a simple power law fails
to describe the full spectral range of 0.4-40 keV and instead a power-law with
an exponential cutoff with hard index Gamma = 1.50+/- 0.09 and high-energy
cutoff epsilon_c = 1.2+/- 0.3 keV formally provides an excellent fit over the
full bandpass. If we use the so-called SRCUT model, as an alternative model, it
gives the best-fit rolloff energy of epsilon_{roll} = 0.95+/- 0.04 keV.
Together with the TeV gamma-ray spectrum ranging from 0.3 to 100 TeV obtained
recently by HESS observations, our Suzaku observations of RX J1713.7-3946
provide stringent constraints on the highest energy particles accelerated in a
supernova shock.Comment: 11 pages, 11 figures, accepted for publication in Publications of the
Astronomical Society of Japan (PASJ
Evolution of Massive Blackhole Triples I -- Equal-mass binary-single systems
We present the result of -body simulations of dynamical evolution of
triple massive blackhole (BH) systems in galactic nuclei. We found that in most
cases two of the three BHs merge through gravitational wave (GW) radiation in
the timescale much shorter than the Hubble time, before ejecting one BH through
a slingshot. In order for a binary BH to merge before ejecting out the third
one, it has to become highly eccentric since the gravitational wave timescale
would be much longer than the Hubble time unless the eccentricity is very high.
We found that two mechanisms drive the increase of the eccentricity of the
binary. One is the strong binary-single BH interaction resulting in the
thermalization of the eccentricity. The second is the Kozai mechanism which
drives the cyclic change of the inclination and eccentricity of the inner
binary of a stable hierarchical triple system. Our result implies that many of
supermassive blackholes are binaries.Comment: 20 pages, 12 figure
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