60,793 research outputs found
Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars
A new statistical method for pinpointing the massive black hole (BH) in the
Galactic Center on the IR grid is presented and applied to astrometric IR
observations of stars close to the BH. This is of interest for measuring the IR
emission from the BH, in order to constrain accretion models; for solving the
orbits of stars near the BH, in order to measure the BH mass and to search for
general relativistic effects; and for detecting the fluctuations of the BH away
from the dynamical center of the stellar cluster, in order to study the stellar
potential. The BH lies on the line connecting the two images of any background
source it gravitationally lenses, and so the intersection of these lines fixes
its position. A combined search for a lensing signal and for the BH shows that
the most likely point of intersection coincides with the center of acceleration
of stars orbiting the BH. This statistical detection of lensing by the BH has a
random probability of ~0.01. It can be verified by deep IR stellar
spectroscopy, which will determine whether the most likely lensed image pair
candidates (listed here) have identical spectra.Comment: 4 pages, 2 figures, submitted to ApJ
Kinematics of the Broad Line Region in M81
A new model is presented which explains the origin of the broad emission
lines observed in the LINER/Seyfert nucleus of M81 in terms of a steady state
spherically symmetric inflow, amounting to 1 x 10^-5 Msun/yr, which is
sufficient to explain the luminosity of the AGN. The emitting volume has an
outer radius of ~1 pc, making it the largest broad line region yet to be
measured, and it contains a total mass of ~ 5 x 10^-2 Msun of dense, ~ 10^8
cm^-3, ionized gas, leading to a very low filling factor of ~ 5 x 10^-9. The
fact that the BLR in M81 is so large may explain why the AGN is unable to
sustain the ionization seen there. Thus, the AGN in M81 is not simply a scaled
down quasar.Comment: Accepted for Publication in ApJ 7/21/0
Interaction induced dimerization in zigzag single wall carbon nanotubes
We derive a low-energy effective model of metallic zigzag carbon nanotubes at
half filling. We show that there are three important features characterizing
the low-energy properties of these systems: the long-range Coulomb interaction,
umklapp scattering and an explicit dimerization generated by interactions. The
ratio of the dimerization induced gap and the Mott gap induced by the umklapp
interactions is dependent on the radius of the nanotube and can drive the
system through a quantum phase transition with SU(2)_1 quantum symmetry. We
consider the physical properties of the phases on either side of this
transition which should be relevant for realistic nanotubes.Comment: 8 pages, 5 figure
On the problem of inflation in nonlinear multidimensional cosmological models
We consider a multidimensional cosmological model with nonlinear quadratic
and quartic actions. As a matter source, we include a monopole form
field, D-dimensional bare cosmological constant and tensions of branes located
in fixed points. In the spirit of the Universal Extra Dimensions models, the
Standard Model fields are not localized on branes but can move in the bulk. We
define conditions which ensure the stable compactification of the internal
space in zero minimum of the effective potentials. Such effective potentials
may have rather complicated form with a number of local minima, maxima and
saddle points. Then, we investigate inflation in these models. It is shown that
and models can have up to 10 and 22 e-foldings, respectively. These
values are not sufficient to solve the homogeneity and isotropy problem but big
enough to explain the recent CMB data. Additionally, model can provide
conditions for eternal topological inflation. However, the main drawback of the
given inflationary models consists in a value of spectral index which is
less than observable now . For example, in the case of
model we find .Comment: 18 pages, RevTex4, References are correcte
Uniform spin chain physics arising from NCN bridges in CuNCN: surprises on the way from copper oxides to their nitride analogs
We report on the unexpected uniform spin chain physics in CuNCN, the
insulating nitride analog of copper oxides. Based on full-potential band
structure calculations, we derive the relevant microscopic parameters, estimate
individual exchange couplings, and establish a realistic spin model of this
compound. The structure of CuNCN contains chains of edge-sharing CuN(4)
squares. As a surprise, in contrast to analogous [CuO(2)] chains in
"edge-sharing" cuprates, the leading magnetic interactions J ~ 2500 K run
perpendicular to the structural [CuN(2)] chains via bridging NCN groups. The
resulting spin model of a uniform chain is in agreement with the experimentally
observed temperature-independent magnetic susceptibility below 300 K. The
nearest-neighbor and next-nearest-neighbor interactions along the structural
[CuN(2)] chains are J(1) ~ -500 K and J(2) ~ 100 K, respectively. Despite the
frustrating nature of J(1) and J(2), we assign the anomaly at 70 K to
long-range magnetic ordering, which is likely collinear with antiparallel and
parallel arrangement of spins along the 'c' and 'a' directions, respectively.
The pronounced one-dimensionality of the spin system should lead to a reduction
in the ordered moment and to a suppression of the transition anomaly in the
specific heat, thus impeding the experimental observation of the long-range
ordering. Our results suggest CuNCN as a promising material for ballistic heat
transport within spin chains, while the sizable bandwidth W ~ 3 eV may lead to
a metal-insulator transition and other exotic properties under high pressure.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.
Disentangling the timescales behind the non-perturbative heavy quark potential
The static part of the heavy quark potential has been shown to be closely
related to the spectrum of the rectangular Wilson loop. In particular the
lowest lying positive frequency peak encodes the late time evolution of the
two-body system, characterized by a complex potential. While initial studies
assumed a perfect separation of early and late time physics, where a simple
Lorentian (Breit-Wigner) shape suffices to describe the spectral peak, we argue
that scale decoupling in general is not complete. Thus early time, i.e.
non-potential effects, significantly modify the shape of the lowest peak. We
derive on general grounds an improved peak distribution that reflects this
fact. Application of the improved fit to non-perturbative lattice QCD spectra
now yields a potential that is compatible with a transition to a deconfined
screening plasma.Comment: 5 pages, 3 figure
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
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