3,683 research outputs found
The Afterglow of Massive Black Hole Coalescence
The final merger of a pair of massive black holes in a galactic nucleus is
compelled by gravitational radiation. Gravitational waves from the mergers of
black holes of masses (10^5-10^7)(1+z)^{-1} Msun at redshifts of 1-20 will be
readily detectable by the Laser Interferometer Space Antenna (LISA), but an
electromagnetic afterglow would be helpful in pinpointing the source and its
redshift. Long before the merger, the binary "hollows out" any surrounding gas
and shrinks slowly compared to the viscous timescale of a circumbinary disk.
The inner gas disk is truncated at the radius where gravitational torque from
the binary balances the viscous torque, and accretion onto the black holes is
diminished. Initially, the inner truncation radius is able to follow the
shrinking binary inward. But eventually the gravitational radiation timescale
becomes shorter than the viscous timescale in the disk, leading to a merged
black hole surrounded by a hollow disk of gas. We show that the subsequent
viscous evolution of the hollow, radiation-pressure dominated disk will create
a ~10^{43.5}(M/10^6Msun) ergs s^{-1} X-ray source on a timescale
\~7(1+z)(M/10^6Msun)^{1.32} yr. This justifies follow-up monitoring of
gravitational wave events with next-generation X-ray observatories. Analysis of
the detailed light curve of these afterglows will yield new insights into the
subtle physics of accretion onto massive black holes.Comment: 4 pages, 2 figures, Astrophys. J. Lett., in pres
Black Hole Feedback On The First Galaxies
We study how the first galaxies were assembled under feedback from the accretion onto a central black hole (BH) that is left behind by the first generation of metal-free stars through self-consistent, cosmological simulations. X-ray radiation from the accretion of gas onto BH remnants of Population III (Pop III) stars, or from high-mass X-ray binaries (HMXBs), again involving Pop III stars, influences the mode of second generation star formation. We track the evolution of the black hole accretion rate and the associated X-ray feedback starting with the death of the Pop III progenitor star inside a minihalo and following the subsequent evolution of the black hole as the minihalo grows to become an atomically cooling galaxy. We find that X-ray photoionization heating from a stellar-mass BH is able to quench further star formation in the host halo at all times before the halo enters the atomic cooling phase. X-ray radiation from a HMXB, assuming a luminosity close to the Eddington value, exerts an even stronger, and more diverse, feedback on star formation. It photoheats the gas inside the host halo, but also promotes the formation of molecular hydrogen and cooling of gas in the intergalactic medium and in nearby minihalos, leading to a net increase in the number of stars formed at early times. Our simulations further show that the radiative feedback from the first BHs may strongly suppress early BH growth, thus constraining models for the formation of supermassive BHs.Astronom
Experimental and Calculated Stark Widths Within the Kr I Spectrum
On the basis of the precisely recorded 20 neutral krypton (Kr I) line shapes (in the 5s−5p and 5s−6p transitions), we have obtained the basic plasma parameters, i.e., electron temperature (T) and electron density (N) using our line deconvolution procedure in a plasma created in a linear, low-pressure, pulsed arc discharge operated in krypton. The mentioned plasma parameters have also been measured using independent experimental diagnostics techniques. Agreement has been found among the two sets of the obtained parameters. This recommends our deconvolution procedure for plasma diagnostical purposes, especially in astrophysics where direct measurements of the main plasma parameters (T and N) are not possible. On the basis of the observed asymmetry of the Stark broadened line profile, we have obtained not only its ion broadening parameter (A) which is caused by influence of the ion-microfield over the line broadening mechanism but also the influence of the ion-dynamic effect (D) over the line shape. The separate electron (We) and ion (Wi) contributions to the total Stark width, which have not been measured so far, have also been obtained. Stark widths are calculated using the semiclassical perturbation formalism for electrons, protons, and helium ions as perturbers
The Merging History of Massive Black Holes
We investigate a hierarchical structure formation scenario describing the
evolution of a Super Massive Black Holes (SMBHs) population. The seeds of the
local SMBHs are assumed to be 'pregalactic' black holes, remnants of the first
POPIII stars. As these pregalactic holes become incorporated through a series
of mergers into larger and larger halos, they sink to the center owing to
dynamical friction, accrete a fraction of the gas in the merger remnant to
become supermassive, form a binary system, and eventually coalesce. A simple
model in which the damage done to a stellar cusps by decaying BH pairs is
cumulative is able to reproduce the observed scaling relation between galaxy
luminosity and core size. An accretion model connecting quasar activity with
major mergers and the observed BH mass-velocity dispersion correlation
reproduces remarkably well the observed luminosity function of
optically-selected quasars in the redshift range 1<z<5. We finally asses the
potential observability of the gravitational wave background generated by the
cosmic evolution of SMBH binaries by the planned space-born interferometer
LISA.Comment: 4 pages, 2 figures, Contribute to "Multiwavelength Cosmology",
Mykonos, Greece, June 17-20, 200
Lignani biljne vrste Achillea lingulata
Five lignans with a 2.6-diaryl-3.7-dioabicyclo[3,3.0]octane skeleton epieudesmin. kobusin, pinoresinol. fargesin and sesartemin, were isolated from the aerial parts and roots, of Achillea lingulata. Their structures were identified by comparison of their H-1-NMR and MS data to those in the literature. Fargesin and pinoresinol have not been isolated previously from any species of the genus Achillea.Iz nadzemnog dela i korena biljne vrste Achillea lingulata izolovano je pet lignana 2,6-diaril-3.7-dioksabicikloŠ3.3.0Ćoktanskog tipa. To su epieudesmin, kobusin, pinorezinol, fargezin i sezartemin. Izolovani lignani su identifikovani na osnovu identičnosti wihovih NMR i masenih spektara sa spektrima iz literature. Fargezin i pinorezinol nisu bili do sada izolovani iz biljnih vrsta roda Achillea
Reparatory and Manufacturing Hard-Facing of Working Parts Made of Stainless Steels in Confectionary Industry
In this paper, for the sake of improving the reparatory hard-facing technology is especially analyzed reparatory hard-facing of tools for manufacturing compressed products in confectionary industry. Those products are being made of a mixture consisting of several powdery components, which is compressed under high pressure. In that way the connection between particles is realized, thus achieving certain hardness and strength of the confectionary product. The considered tool is made of high-alloyed stainless steel. The tool contains 30 identical working places. Besides the production process wear, on those tools, from time to time, appear mechanical damage on some of the products' shape punches, as cracks at the edges, where the products' final shapes are formed. Those damages are small, size wise, but they cause strong effect on the products' final shape. The aggravating circumstance is that the shape punch is extremely loaded in pressure, thus after the reparatory hard-facing, the additional heat treatment is necessary. Mechanical properties in the heat affected zone (HAZ) are being leveled by annealing and what also partially reduces the residual internal stresses
Ionospheric storm forecasting technique by artificial neural network
In this work we further refine and improve the neural network based ionospheric characteristic's foF2 predictor,
which is actually a neural network autoregressive model with additional input signals (NNARX). Our analysis
is focused on choice of X parts of NNARX model in order to capture middle and long term dependencies. Daily
distribution of prediction error suggests need for structural changes of the neural network model, as well as
adaptation of running average lengths used for determination of X inputs. Generalisation properties of proposed
neural predictor are improved by carefully designed pruning procedure with additional regularisation term in
criterion function. Some results from the NNARX model are presented to illustrate the feasibility of using such
a model as ionospheric storm forecasting technique
Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
We report new precision measurements of the properties of our Galaxy's
supermassive black hole. Based on astrometric (1995-2007) and radial velocity
(2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fully
unconstrained Keplerian orbit for the short period star S0-2 provides values
for Ro of 8.0+-0.6 kpc, M_bh of 4.1+-0.6x10^6 Mo, and the black hole's radial
velocity, which is consistent with zero with 30 km/s uncertainty. If the black
hole is assumed to be at rest with respect to the Galaxy, we can further
constrain the fit and obtain Ro = 8.4+-0.4 kpc and M_bh = 4.5+-0.4x10^6 Mo.
More complex models constrain the extended dark mass distribution to be less
than 3-4x10^5 Mo within 0.01 pc, ~100x higher than predictions from stellar and
stellar remnant models. For all models, we identify transient astrometric
shifts from source confusion and the assumptions regarding the black hole's
radial motion as previously unrecognized limitations on orbital accuracy and
the usefulness of fainter stars. Future astrometric and RV observations will
remedy these effects. Our estimates of Ro and the Galaxy's local rotation
speed, which it is derived from combining Ro with the apparent proper motion of
Sgr A*, (theta0 = 229+-18 km/s), are compatible with measurements made using
other methods. The increased black hole mass found in this study, compared to
that determined using projected mass estimators, implies a longer period for
the innermost stable orbit, longer resonant relaxation timescales for stars in
the vicinity of the black hole and a better agreement with the M_bh-sigma
relation.Comment: ApJ, accepted (26 pages, 16 figures, 7 tables
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