5,466 research outputs found
How else can we detect Fast Radio Bursts?
We discuss possible electromagnetic signals accompanying Fast Radio Bursts
(FRBs) that are expected in the scenario where FRBs originate in neutron star
magnetospheres. For models involving Crab-like giant pulses, no appreciable
contemporaneous emission is expected at other wavelengths. Magnetar giant
flares, driven by the reconfiguration of the magnetosphere, however, can
produce both contemporaneous bursts at other wavelengths as well as
afterglow-like emission. We conclude that the best chances are: (i) prompt
short GRB-like emission; (ii) a contemporaneous optical flash that can reach
naked eye peak luminosity (but only for a few milliseconds); (iii) a high
energy afterglow emission. Case (i) could be tested by coordinated radio and
high-energy experiments. Case (ii) could be seen in a coordinated radio-optical
surveys, \eg\ by the Palomar Transient Factory in a 60-second frame as a
transient object of magnitude with an expected optical detection rate
of about 0.1~hr, an order of magnitude higher than in radio. Shallow,
but large-area sky surveys such as ASAS-SN and EVRYSCOPE could also detect
prompt optical flashes from the more powerful Lorimer-burst clones. The best
constraints on the optical-to-radio power for this kind of emission could be
provided by future observations with facilities like LSST. Case (iii) might be
seen in relatively rare cases that the relativistically ejected magnetic blob
is moving along the line of sight
Survival Probabilities at Spherical Frontiers
Motivated by tumor growth and spatial population genetics, we study the
interplay between evolutionary and spatial dynamics at the surfaces of
three-dimensional, spherical range expansions. We consider range expansion
radii that grow with an arbitrary power-law in time:
, where is a growth exponent, is the
initial radius, and is a characteristic time for the growth, to be
affected by the inflating geometry. We vary the parameters and
to capture a variety of possible growth regimes. Guided by recent results for
two-dimensional inflating range expansions, we identify key dimensionless
parameters that describe the survival probability of a mutant cell with a small
selective advantage arising at the population frontier. Using analytical
techniques, we calculate this probability for arbitrary . We compare
our results to simulations of linearly inflating expansions (
spherical Fisher-Kolmogorov-Petrovsky-Piscunov waves) and treadmilling
populations (, with cells in the interior removed by apoptosis or a
similar process). We find that mutations at linearly inflating fronts have
survival probabilities enhanced by factors of 100 or more relative to mutations
at treadmilling population frontiers. We also discuss the special properties of
"marginally inflating" expansions.Comment: 35 pages, 11 figures, revised versio
Pcf theory and cardinal invariants of the reals
The additivity spectrum ADD(I) of an ideal I is the set of all regular
cardinals kappa such that there is an increasing chain {A_alpha:alpha<kappa\}
in the ideal I such that the union of the chain is not in I.
We investigate which set A of regular cardinals can be the additivity
spectrum of certain ideals.
Assume that I=B or I=N, where B denotes the sigma-ideal generated by the
compact subsets of the Baire space omega^omega, and N is the ideal of the null
sets.
For countable sets we give a full characterization of the additivity spectrum
of I: a non-empty countable set A of uncountable regular cardinals can be
ADD(I) in some c.c.c generic extension iff A=pcf(A).Comment: 9 page
Electrostatic Point Charge Fitting as an Inverse Problem: Revealing the Underlying Ill-Conditioning
Atom-centered point charge model of the molecular electrostatics---a major
workhorse of the atomistic biomolecular simulations---is usually parameterized
by least-squares (LS) fitting of the point charge values to a reference
electrostatic potential, a procedure that suffers from numerical instabilities
due to the ill-conditioned nature of the LS problem. Here, to reveal the
origins of this ill-conditioning, we start with a general treatment of the
point charge fitting problem as an inverse problem, and construct an
analytically soluble model with the point charges spherically arranged
according to Lebedev quadrature naturally suited for the inverse electrostatic
problem. This analytical model is contrasted to the atom-centered point-charge
model that can be viewed as an irregular quadrature poorly suited for the
problem. This analysis shows that the numerical problems of the point charge
fitting are due to the decay of the curvatures corresponding to the
eigenvectors of LS sum Hessian matrix. In part, this ill-conditioning is
intrinsic to the problem and related to decreasing electrostatic contribution
of the higher multipole moments, that are, in the case of Lebedev grid model,
directly associated with the Hessian eigenvectors. For the atom-centered model,
this association breaks down beyond the first few eigenvectors related to the
high-curvature monopole and dipole terms; this leads to even wider spread-out
of the Hessian curvature values. Using these insights, it is possible to
alleviate the ill-conditioning of the LS point-charge fitting without
introducing external restraints and/or constraints. Also, as the analytical
Lebedev grid PC model proposed here can reproduce multipole moments up to a
given rank, it may provide a promising alternative to including explicit
multipole terms in a force field
Probing a Secluded U(1) at B-factories
A secluded U(1) gauge field, kinetically mixed with Standard Model
hypercharge, provides a `portal' mediating interactions with a hidden sector at
the renormalizable level, as recently exploited in the context of WIMP dark
matter. The secluded U(1) symmetry-breaking scale may naturally be suppressed
relative to the weak scale, and so this sector is efficiently probed by medium
energy electron-positron colliders. We study the collider signatures of the
minimal secluded U(1) model, focusing on the reach of B-factory experiments
such as BaBar and BELLE. In particular, we show that Higgs-strahlung in the
secluded sector can lead to multi-lepton signatures which probe the natural
range for the kinetic mixing angle of 10^(-2)-10^(-3) over a large portion of
the kinematically accessible parameter space.Comment: 14 pages, 3 figure
New limits on extragalactic magnetic fields from rotation measures
We take advantage of the wealth of rotation measures data contained in the
NRAO VLA Sky Survey catalogue to derive new, statistically robust, upper limits
on the strength of extragalactic magnetic fields. We simulate the extragalactic
magnetic field contribution to the rotation measures for a given field strength
and correlation length, by assuming that the electron density follows the
distribution of Lyman- clouds. Based on the observation that rotation
measures from distant radio sources do not exhibit any trend with redshift,
while the extragalactic contribution instead grows with distance, we constrain
fields with Jeans' length coherence length to be below 1.7~nG at the
level, and fields coherent across the entire observable Universe below 0.65~nG.
These limits do not depend on the particular origin of these cosmological
fields.Comment: 5 pages, 3 figures -- v2 to match PRL versio
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