4,426 research outputs found
Jet-driven and jet-less fireballs from compact binary mergers
During a compact binary merger involving at least one neutron star, a small
fraction of the gravitational energy could be liberated in such a way to
accelerate a small fraction (~ 10^-6) of the neutron star mass in an isotropic
or quasi-isotropic way. In presence of certain conditions, a pair-loaded
fireball can form, which undergoes accelerated expansion reaching relativistic
velocities. As in the standard fireball scenario, internal energy is partly
transformed into kinetic energy. At the photospheric radius, the internal
radiation can escape, giving rise to a pulse that lasts for a time equal to the
delay time since the merger. The subsequent interaction with the interstellar
medium can then convert part of the remaining kinetic energy back into
radiation in a weak isotropic afterglow at all wavelengths. This scenario does
not require the presence of a jet: the associated isotropic prompt and
afterglow emission should be visible for all NS-NS and BH-NS mergers within 90
Mpc, independent of their inclination. The prompt emission is similar to that
expected from an off-axis jet, either structured or much slower than usually
assumed (Gamma ~ 10), or from the jet cocoon. The predicted afterglow emission
properties can discriminate among these scenarios.Comment: 5 pages, 1 figure, revised version submitted to MNRAS Letter
Clathrin Adaptor Complex-interacting Protein Irc6 Functions through the Conserved C-Terminal Domain.
Clathrin coats drive transport vesicle formation from the plasma membrane and in pathways between the trans-Golgi network (TGN) and endosomes. Clathrin adaptors play central roles orchestrating assembly of clathrin coats. The yeast clathrin adaptor-interacting protein Irc6 is an orthologue of human p34, which is mutated in the inherited skin disorder punctate palmoplantar keratoderma type I. Irc6 and p34 bind to clathrin adaptor complexes AP-1 and AP-2 and are members of a conserved family characterized by a two-domain architecture. Irc6 is required for AP-1-dependent transport between the TGN and endosomes in yeast. Here we present evidence that the C-terminal two amino acids of Irc6 are required for AP-1 binding and transport function. Additionally, like the C-terminal domain, the N-terminal domain when overexpressed partially restores AP-1-mediated transport in cells lacking full-length Irc6. These findings support a functional role for Irc6 binding to AP-1. Negative genetic interactions with irc6â are enriched for genes related to membrane traffic and nuclear processes, consistent with diverse cellular roles for Irc6
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Phosphoinositide-mediated clathrin adaptor progression at the trans-Golgi network.
Clathrin-coated vesicles mediate endocytosis and transport between the trans-Golgi network (TGN) and endosomes in eukaryotic cells. Clathrin adaptors play central roles in coat assembly, interacting with clathrin, cargo and membranes. Two main types of clathrin adaptor act in TGN-endosome traffic: GGA proteins and the AP-1 complex. Here we characterize the relationship between GGA proteins, AP-1 and other TGN clathrin adaptors using live-cell and super-resolution microscopy in yeast. We present evidence that GGA proteins and AP-1 are recruited sequentially in two waves of coat assembly at the TGN. Mutations that decrease phosphatidylinositol 4-phosphate (PtdIns(4)P) levels at the TGN slow or uncouple AP-1 coat assembly from GGA coat assembly. Conversely, enhanced PtdIns(4)P synthesis shortens the time between adaptor waves. Gga2p binds directly to the TGN PtdIns(4)-kinase Pik1p and contributes to Pik1p recruitment. These results identify a PtdIns(4)P-based mechanism for regulating progressive assembly of adaptor-specific clathrin coats at the TGN
Improving Table Compression with Combinatorial Optimization
We study the problem of compressing massive tables within the
partition-training paradigm introduced by Buchsbaum et al. [SODA'00], in which
a table is partitioned by an off-line training procedure into disjoint
intervals of columns, each of which is compressed separately by a standard,
on-line compressor like gzip. We provide a new theory that unifies previous
experimental observations on partitioning and heuristic observations on column
permutation, all of which are used to improve compression rates. Based on the
theory, we devise the first on-line training algorithms for table compression,
which can be applied to individual files, not just continuously operating
sources; and also a new, off-line training algorithm, based on a link to the
asymmetric traveling salesman problem, which improves on prior work by
rearranging columns prior to partitioning. We demonstrate these results
experimentally. On various test files, the on-line algorithms provide 35-55%
improvement over gzip with negligible slowdown; the off-line reordering
provides up to 20% further improvement over partitioning alone. We also show
that a variation of the table compression problem is MAX-SNP hard.Comment: 22 pages, 2 figures, 5 tables, 23 references. Extended abstract
appears in Proc. 13th ACM-SIAM SODA, pp. 213-222, 200
On the logarithmic behaviour in N=4 SYM theory
We show that the logarithmic behaviour seen in perturbative and non
perturbative contributions to Green functions of gauge-invariant composite
operators in N=4 SYM with SU(N) gauge group can be consistently interpreted in
terms of anomalous dimensions of unprotected operators in long multiplets of
the superconformal group SU(2,2|4). In order to illustrate the point we analyse
the short-distance behaviour of a particularly simple four-point Green function
of the lowest scalar components of the N=4 supercurrent multiplet. Assuming the
validity of the Operator Product Expansion, we are able to reproduce the known
value of the one-loop anomalous dimension of the single-trace operators in the
Konishi supermultiplet. We also show that it does not receive any
non-perturbative contribution from the one-instanton sector. We briefly comment
on double- and multi-trace operators and on the bearing of our results on the
AdS/SCFT correspondence.Comment: 18 pages, Late
Properties of the Konishi multiplet in N=4 SYM theory
We study perturbative and non-perturbative properties of the Konishi
multiplet in N=4 SYM theory in D=4 dimensions. We compute two-, three- and
four-point Green functions with single and multiple insertions of the lowest
component of the multiplet, and of the lowest component of the supercurrent
multiplet. These computations require a proper definition of the renormalized
operator and lead to an independent derivation of its anomalous dimension. The
O(g^2) value found in this way is in agreement with previous results. We also
find that instanton contributions to the above correlators vanish. From our
results we are able to identify some of the lowest dimensional gauge-invariant
composite operators contributing to the OPE of the correlation functions we
have computed. We thus confirm the existence of an operator belonging to the
representation 20', which has vanishing anomalous dimension at order g^2 and
g^4 in perturbation theory as well as at the non-perturbative level, despite
the fact that it does not obey any of the known shortening conditions.Comment: 23 pages, latex, no figure
Interpreting GRB170817A as a giant flare from a jet-less double neutron-star merger
We show that the delay between GRB170817A and GW170817 is incompatible with
de-beamed emission from an off-axis relativistic jet. The prompt emission and
the subsequent radio and X-ray observations can instead be interpreted within a
giant-flare-like scenario, being the result of a relativistic outflow driven by
the ultra-strong magnetic field produced by magnetohydrodynamic amplification
during the merger of the progenitor double neutron-star binary. Within such
picture, the data indicate that the outflow must be endowed with a steep
velocity profile, with a relatively fast tail extending to Gamma~8. Since the
conditions for the launch of such an outflow are quite general, and the
presence of a velocity profile is a natural expectation of the acceleration
process, most neutron star binary mergers should feature this quasi-isotropic,
hard X-ray emission component, that can be a powerful guide to the discovery of
additional kilonovae associated to relatively nearby gravitational wave events.Comment: 6 pages, 2 figures, accepted by Astronomy and Astrophysic
Quantum and Superquantum Nonlocal Correlations
We present a simple hidden variable model for the singlet state of a pair of
qubits, characterized by two kinds, hierarchically ordered, of hidden
variables. We prove that, averaging over both types of variables, one
reproduces all the quantum mechanical correlations of the singlet state. On the
other hand, averaging only over the hidden variables of the lower level, one
obtains a general formal theoretical scheme exhibiting correlations stronger
than the quantum ones, but with faster-than-light communication forbidden. This
result is interesting by itself since it shows that a violation of the quantum
bound for nonlocal correlations can be implemented in a precise physical manner
and not only mathematically, and it suggests that resorting to two levels of
nonlocal hidden variables might led to a deeper understanding of the physical
principles at the basis of quantum nonlocality.Comment: 5 pages, 1 figure. Submitted for publicatio
A general hidden variable model for the two-qubits system
We generalize Bell's hidden variable model describing the singlet state of a
two-qubits system by extending it to arbitrary states and observables. As in
the original work, we assume a uniform, state-independent probability
distribution for the hidden variables which are identified with the unit
vectors of a 3-dimensional real space. By slightly modifying our model, we
provide also a minimal hidden variable description of the two-qubits system,
relying on a single hidden variable. We discuss the main features and the
implications of the model.Comment: 4 pages, submitted for publicatio
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