3,819 research outputs found
AdS boundary conditions and the Topologically Massive Gravity/CFT correspondence
The AdS/CFT correspondence provides a new perspective on recurrent questions
in General Relativity such as the allowed boundary conditions at infinity and
the definition of gravitational conserved charges. Here we review the main
insights obtained in this direction over the last decade and apply the new
techniques to Topologically Massive Gravity. We show that this theory is dual
to a non-unitary CFT for any value of its parameter mu and becomes a
Logarithmic CFT at mu = 1.Comment: 10 pages, proceedings for XXV Max Born Symposium, talks given at
Johns Hopkins workshop and Holographic Cosmology workshop at Perimeter
Institute; v2: added reference
Topological regluing of rational functions
Regluing is a topological operation that helps to construct topological
models for rational functions on the boundaries of certain hyperbolic
components. It also has a holomorphic interpretation, with the flavor of
infinite dimensional Thurston--Teichm\"uller theory. We will discuss a
topological theory of regluing, and trace a direction in which a holomorphic
theory can develop.Comment: 38 page
Scattering in the vicinity of relativistic jets: a method for constraining jet parameters
Relativistic jets of radio loud active galactic nuclei (AGN) produce highly
directed, intense beams of radiation. A fraction of this beamed radiation
scatters on the thermal plasma generally surrounding an AGN. The morphology of
the scattered emission can thus provide constraints on the physical properties
of the jet. We present a model to study the feasibility of constraining the
parameters of a jet, especially its inclination angle and bulk Lorentz factor
in this way. We apply our model to the well studied jet of M87 and the
surrounding diffuse gas and find that the observational limits of the surface
brightness measured in the region of the putative counterjet provide the
tightest constraints on the jet parameters consistent with constraints derived
by other methods. We briefly discuss the applicability of our model to other
sources exhibiting relativistic motionsComment: 17 pages, 15 figures, to appear in A&A, 420, 33 (2004
Anomalous HI kinematics in Centaurus A: evidence for jet-induced star formation
We present new 21-cm HI observations performed with ATCA of the large HI
filament located about 15 kpc NE from the centre of Centaurus A and discovered
by Schiminovich et al.(1994). This HI cloud is situated (in projection) near
the radio jet of Centaurus A, as well as near a large filament of ionised gas
of high excitation and turbulent velocities and near regions with young stars.
The higher velocity and spatial resolution of the new data reveals that, apart
from the smooth velocity gradient corresponding to the overall rotation of the
cloud around Centaurus A, HI with anomalous velocities of about 100 km/s is
present at the southern tip of this cloud. This is interpreted as evidence for
an ongoing interaction between the radio jet and the HI cloud. Gas stripped
from the HI cloud gives rise to the large filament of ionised gas and the star
formation regions that are found downstream from the location of the
interaction. The implied flow velocities are very similar to the observed
anomalous HI velocities. Given the amount of HI with anomalous kinematics and
the current star formation rate, the efficiency of jet-induced star formation
is at most of the order of a percent.Comment: Accepted for publication in A&A, 7 pages, 4 figures. The full paper
with high resolution images can be downloaded from
http://www.astron.nl/~morganti/Papers/cena.paper.pd
Triggered Star Formation in a Massive Galaxy at z=3.8: 4C41.17
Spectropolarimetric observations obtained with the W. M. Keck Telescope of
the z=3.8 radio galaxy 4C41.17 show that the UV continuum emission from this
galaxy, which is aligned with the radio axis, is unpolarized (P[2sigma] <
2.4%). This implies that scattered AGN light, which is generally the dominant
contributor to the rest-frame UV emission in z~1 radio galaxies, is unlikely to
be a major component of the UV flux from 4C41.17. The spectrum shows absorption
lines that are similar to those detected in the spectra of the recently
discovered population of star forming galaxies at z~2-3. A galaxian outflow may
contribute partially to the low ionization absorption lines; however, the high
velocity wings of the high ionization lines are unlikely to be dominated by a
galaxian wind since the implied outflow mass is very large. The detection of
stellar absorption lines, the shape of the SiIV profile, the unpolarized
continuum, the inability of any AGN-related processes to account for the UV
flux, and the similarity of the UV continuum spectra of 4C41.17 and the nearby
starburst region NGC 1741B1 suggest that the UV light in 4C41.17 is dominated
by young stars. If so, the implied star-formation rate is roughly
140-1100Msun/yr. We discuss the possibility that star formation in 4C41.17 was
triggered by the radio source. Our data are consistent with the hypothesis that
4C41.17 is undergoing its major epoch of star formation at z~4, and that by z~1
it will have evolved to have spectral and morphological properties similar to
those observed in known z~1 powerful radio galaxies.Comment: 28 pages (Latex text + figures); Accepted for publication in The
Astrophysical Journal (Dec 1, 1997 issue
Potassium Uptake and Recycling to the Soil in Corn and Soybean
Considerable research has been conducted at this farm to study potassium (K) fertilization rates and placement methods on corn and soybean grain yield, K uptake, and soil-test K values. However, no research has investigated K recycling to the soil by maturing plants and crop residue until the next crop is planted. The amount and the timing of the K recycled to the soil should have a significant impact on soil-test K values, and could explain a great deal of usually very high soil-test K temporal variability. Therefore, plots of several field K trials at this farm were used to investigate these issues
Forming Galaxies with MOND
Beginning with a simple model for the growth of structure, I consider the
dissipationless evolution of a MOND-dominated region in an expanding Universe
by means of a spherically symmetric N-body code. I demonstrate that the final
virialized objects resemble elliptical galaxies with well-defined relationships
between the mass, radius, and velocity dispersion. These calculations suggest
that, in the context of MOND, massive elliptical galaxies may be formed early
(z > 10) as a result of monolithic dissipationless collapse. Then I reconsider
the classic argument that a galaxy of stars results from cooling and
fragmentation of a gas cloud on a time scale shorter than that of dynamical
collapse. Qualitatively, the results are similar to that of the traditional
picture; moreover, the existence, in MOND, of a density-temperature relation
for virialized, near isothermal objects as well as a mass-temperature relation
implies that there is a definite limit to the mass of a gas cloud where this
condition can be met-- an upper limit corresponding to that of presently
observed massive galaxies.Comment: 9 pages, 9 figures, revised in response to comments of referee. Table
added, extended discussion, accepted MNRA
Helium Star/Black Hole Mergers: a New Gamma-Ray Burst Model
We present a model for gamma-ray bursts (GRB's) in which a stellar mass black
hole acquires a massive accretion disk by merging with the helium core of its
red giant companion. The black hole enters the helium core after it, or its
neutron star progenitor, first experiences a common envelope phase that carries
it inwards through the hydrogen envelope. Accretion of the last several solar
masses of helium occurs on a time scale of roughly a minute and provides a
neutrino luminosity of approximately 10^51 - 10^52 erg/s. Neutrino
annihilation, 0.01% to 0.1% efficient, along the rotational axis then gives a
baryon loaded fireball of electron-positron pairs and radiation (about
10 erg total) whose beaming and relativistic interaction with
circumstellar material makes the GRB (e.g., Rees & Meszaros 1992). The useful
energy can be greatly increased if energy can be extracted from the rotational
energy of the black hole by magnetic interaction with the disk. Such events
should occur at a rate comparable to that of merging neutron stars and black
hole neutron star pairs and may be responsible for long complex GRB's, but not
short hard ones.Comment: 11 pages total, 2 Figures - altered and revised for ApJ letters,
accepte
Fe K\alpha emission from photoionized slabs: the impact of the iron abundance
Iron K\alpha emission from photoionized and optically thick material is
observed in a variety of astrophysical environments including X-ray binaries,
active galactic nuclei, and possibly gamma-ray bursts. This paper presents
calculations showing how the equivalent width (EW) of the Fe K line depends on
the iron abundance of the illuminated gas and its ionization state -- two
variables subject to significant cosmic scatter. Reflection spectra from a
constant density slab which is illuminated with a power-law spectrum with
photon-index \Gamma are computed using the code of Ross & Fabian. When the Fe K
EW is measured from the reflection spectra alone, we find that it can reach
values greater than 6 keV if the Fe abundance is about 10 times solar and the
illuminated gas is neutral. EWs of about 1 keV are obtained when the gas is
ionized. In contrast, when the EW is measured from the incident+reflected
spectrum, the largest EWs are ~800 keV and are found when the gas is ionized.
When \Gamma is increased, the Fe K line generally weakens, but significant
emission can persist to larger ionization parameters. The iron abundance has
its greatest impact on the EW when it is less than 5 times solar. When the
abundance is further increased, the line strengthens only marginally.
Therefore, we conclude that Fe K lines with EWs much greater than 800 eV are
unlikely to be produced by gas with a supersolar Fe abundance. These results
should be useful in interpreting Fe K emission whenever it arises from
optically thick fluorescence.Comment: 5 pages, 5 figures, accepted by MNRAS Letter
Yet Another Model of Gamma-Ray Bursts
Sari and Piran have demonstrated that the time structure of gamma-ray bursts
must reflect the time structure of their energy release. A model which
satisfies this condition uses the electrodynamic emission of energy by the
magnetized rotating ring of dense matter left by neutron star coalescence; GRB
are essentially fast, high field, differentially rotating pulsars. The energy
densities are large enough that the power appears as an outflowing equilibrium
pair plasma, which produces the burst by baryon entrainment and subsequent
internal shocks. I estimate the magnetic field and characteristic time scale
for its rearrangement, which determines the observed time structure of the
burst. There may be quasi-periodic oscillations at the rotational frequencies,
which are predicted to range up to 5770 Hz (in a local frame). This model is
one of a general class of electrodynamic accretion models which includes the
Blandford and Lovelace model of AGN, and which can also be applied to black
hole X-ray sources of stellar mass. The apparent efficiency of nonthermal
particle acceleration is predicted to be 10--50%, but higher values are
possible if the underlying accretion flow is super-Eddington. Applications to
high energy gamma-ray observations of AGN are briefly discussed.Comment: 21pp, latex, uses aaspp4.st
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