18,851 research outputs found
Bi-Directional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant
We present measurements of bi-directional motions in the jets of the radio
galaxy 1946+708 at z=0.101. This is a Compact Symmetric Object with striking
S-symmetry. Sensitive 15 GHz observations reveal a compact component at the
center of symmetry with a strongly inverted spectrum, that we identify as the
core. From five 4.9 GHz observations spread over 4 years we have determined the
velocities of four compact jet components. If simple kinematic models can be
applied then the inclination of the source and the bulk jet velocity can be
directly determined for any assumed value of the Hubble constant. Conversely,
the measurements already place constraints on the Hubble constant, and we show
how further observations of 1946+708 can yield an increasingly accurate
determination of H_0.Comment: in press at ApJ Letters, 12 page LaTex document includes 5 postscript
figure
Coupled Cluster Channels in the Homogeneous Electron Gas
We discuss diagrammatic modifications to the coupled cluster doubles (CCD)
equations, wherein different groups of terms out of rings, ladders,
crossed-rings and mosaics can be removed to form approximations to the coupled
cluster method, of interest due to their similarity with various types of
random phase approximations. The finite uniform electron gas is benchmarked for
14- and 54-electron systems at the complete basis set limit over a wide density
range and performance of different flavours of CCD are determined. These
results confirm that rings generally overcorrelate and ladders generally
undercorrelate; mosaics-only CCD yields a result surprisingly close to CCD. We
use a recently developed numerical analysis [J. J. Shepherd and A. Gr\"uneis,
Phys. Rev. Lett. 110, 226401 (2013)] to study the behaviours of these methods
in the thermodynamic limit. We determine that the mosaics, on forming the
Brueckner Hamltonian, open a gap in the effective one-particle eigenvalues at
the Fermi energy. Numerical evidence is presented which shows that methods
based on this renormalisation have convergent energies in the thermodynamic
limit including mosaic-only CCD, which is just a renormalised MP2. All other
methods including only a single channel, namely ladder-only CCD, ring-only CCD
and crossed-ring-only CCD, appear to yield divergent energies; incorporation of
mosaic terms prevents this from happening.Comment: 9 pages, 4 figures, 1 table. Comments welcome: [email protected]
The two sided parsec scale structure of the Low Luminosity Active Galactic Nucleus in NGC 4278
We present new Very Long Baseline Interferometry observations of the LINER
galaxy NGC 4278. The observations were taken with the Very Long Baseline Array
(VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz
and have a linear resolution of <0.1 pc. Our radio data reveal a two sided
structure, with symmetric S-shaped jets emerging from a flat spectrum core. We
fit the jet brightness with gaussian components, which we identify from a
previous observation taken five years before. By comparing the positions of the
components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76
+/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the
range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically
symmetric and its appearance is governed by Doppler beaming effects, we find
that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the
line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could
be oriented at a larger angle and asymmetries could be related to the jet
interaction with the surrounding medium. We also present new simultaneous VLA
observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and
2003. The radio spectrum can be fit by a relatively steep power-law (alpha =
0.54). We find significant variability at 5 GHz. All these arguments indicate
that the radiation from NGC 4278 is emitted via the synchrotron process by
relativistic particles accelerated by a supermassive black hole. Despite a much
lower power, this is the same process that takes place in ordinary radio loud
AGNs.Comment: 29 pages, 9 figures, ApJ accepte
Resolved CO(1-0) Nuclei in IRAS 14348-1447: Evidence for Massive Bulge Progenitors to Ultraluminous Infrared Galaxies
High-resolution, CO(1-0) interferometry of the ultraluminous infrared galaxy
IRAS 14348-1447 is presented. The merger system has a molecular gas mass of
\~3x10^10 solar masses and a projected nuclear separation of 4.8 kpc (3.5"),
making it one of the most molecular gas-rich galaxies known and an ideal
candidate for studying the intermediate stages of an ultraluminous merger
event. The CO morphology shows two molecular gas components associated with the
stellar nuclei of the progenitors, consistent with the idea that the molecular
disks are gravitationally bound by the dense bulges of the progenitor galaxies
as the interaction proceeds. In contrast, less luminous infrared galaxies
observed to date with projected nuclear separations of ~<5 kpc show a dominant
CO component between the stellar nuclei. This discrepancy may be an indication
that the progenitors of mergers with lower infrared luminosity do not possess
massive bulges, and that the gas is stripped during the initial encounter of
their progenitors. A comparison of the CO and radio luminosities of the NE and
SW component show them to have comparable radio and CO flux ratios of
f(NE)/f(SW) ~0.6, possibly indicating that the amount of star-forming molecular
gas in the progenitors is correlated with the supernovae rate. The estimate of
molecular gas masses of the nuclei and the extent of the radio emission are
used to infer that the nuclei of IR 14348-1447 have gas densities comparable to
the cores of elliptical galaxies.Comment: LaTex, 5 pages with 1 postscript and 1 jpg figure, ApJ Letters, in
pres
Entanglement cost of generalised measurements
Bipartite entanglement is one of the fundamental quantifiable resources of
quantum information theory. We propose a new application of this resource to
the theory of quantum measurements. According to Naimark's theorem any rank 1
generalised measurement (POVM) M may be represented as a von Neumann
measurement in an extended (tensor product) space of the system plus ancilla.
By considering a suitable average of the entanglements of these measurement
directions and minimising over all Naimark extensions, we define a notion of
entanglement cost E_min(M) of M.
We give a constructive means of characterising all Naimark extensions of a
given POVM. We identify various classes of POVMs with zero and non-zero cost
and explicitly characterise all POVMs in 2 dimensions having zero cost. We
prove a constant upper bound on the entanglement cost of any POVM in any
dimension. Hence the asymptotic entanglement cost (i.e. the large n limit of
the cost of n applications of M, divided by n) is zero for all POVMs.
The trine measurement is defined by three rank 1 elements, with directions
symmetrically placed around a great circle on the Bloch sphere. We give an
analytic expression for its entanglement cost. Defining a normalised cost of
any d-dimensional POVM by E_min(M)/log(d), we show (using a combination of
analytic and numerical techniques) that the trine measurement is more costly
than any other POVM with d>2, or with d=2 and ancilla dimension 2. This
strongly suggests that the trine measurement is the most costly of all POVMs.Comment: 20 pages, plain late
VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey
We present VLBA observations and a statistical analysis of 5 GHz VLBI
polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum
(CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293
extragalactic radio sources, gives us the unique opportunity to compare a broad
range of source properties for quasars, galaxies and BL Lacertae objects. We
focus primarily on jet properties, specifically the correlation between the jet
axis angle and the polarization angle in the core and jet. A strong correlation
is found for the electric vector polarization angle in the cores of quasars to
be perpendicular to the jet axis. Contrary to previous claims, no correlation
is found between the jet polarization angle and the jet axis in either quasars
or BL Lac objects. With this large, homogeneous sample we are also able to
investigate cosmological issues and AGN evolution.Comment: Accepted to the Astrophysical Journal: 37 pages, 14 figure
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