213 research outputs found
Gamma-ray emission from dark matter wakes of recoiled black holes
A new scenario for the emission of high-energy gamma-rays from dark matter
annihilation around massive black holes is presented. A black hole can leave
its parent halo, by means of gravitational radiation recoil, in a merger event
or in the asymmetric collapse of its progenitor star. A recoiled black hole
which moves on an almost-radial orbit outside the virial radius of its central
halo, in the cold dark matter background, reaches its apapsis in a finite time.
Near or at the apapsis passage, a high-density wake extending over a large
radius of influence, forms around the black hole. It is shown that significant
gamma-ray emission can result from the enhancement of neutralino annihilation
in these wakes. At its apapsis passage, a black hole is shown to produce a
flash of high-energy gamma-rays whose duration is determined by the mass of the
black hole and the redshift at which it is ejected. The ensemble of such black
holes in the Hubble volume is shown to produce a diffuse high-energy gamma-ray
background whose magnitude is compared to the diffuse emission from dark matter
haloes alone.Comment: version to appear in Astrophysical Journal letters (labels on Fig. 3
corrected
Lattice density-functional theory of surface melting: the effect of a square-gradient correction
I use the method of classical density-functional theory in the
weighted-density approximation of Tarazona to investigate the phase diagram and
the interface structure of a two-dimensional lattice-gas model with three
phases -- vapour, liquid, and triangular solid. While a straightforward
mean-field treatment of the interparticle attraction is unable to give a stable
liquid phase, the correct phase diagram is obtained when including a suitably
chosen square-gradient term in the system grand potential. Taken this theory
for granted, I further examine the structure of the solid-vapour interface as
the triple point is approached from low temperature. Surprisingly, a novel
phase (rather than the liquid) is found to grow at the interface, exhibiting an
unusually long modulation along the interface normal. The conventional
surface-melting behaviour is recovered only by artificially restricting the
symmetries being available to the density field.Comment: 16 pages, 6 figure
Determining the Physical Lens Parameters of the Binary Gravitational Microlensing Event MOA-2009-BLG-016
We report the result of the analysis of the light curve of the microlensing
event MOA-2009-BLG-016. The light curve is characterized by a short-duration
anomaly near the peak and an overall asymmetry. We find that the peak anomaly
is due to a binary companion to the primary lens and the asymmetry of the light
curve is explained by the parallax effect caused by the acceleration of the
observer over the course of the event due to the orbital motion of the Earth
around the Sun. In addition, we detect evidence for the effect of the finite
size of the source near the peak of the event, which allows us to measure the
angular Einstein radius of the lens system. The Einstein radius combined with
the microlens parallax allows us to determine the total mass of the lens and
the distance to the lens. We identify three distinct classes of degenerate
solutions for the binary lens parameters, where two are manifestations of the
previously identified degeneracies of close/wide binaries and positive/negative
impact parameters, while the third class is caused by the symmetric cycloid
shape of the caustic. We find that, for the best-fit solution, the estimated
mass of the lower-mass component of the binary is (0.04 +- 0.01) M_sun,
implying a brown-dwarf companion. However, there exists a solution that is
worse only by \Delta\chi^2 ~ 3 for which the mass of the secondary is above the
hydrogen-burning limit. Unfortunately, resolving these two degenerate solutions
will be difficult as the relative lens-source proper motions for both are
similar and small (~ 1 mas/yr) and thus the lens will remain blended with the
source for the next several decades.Comment: 7 pages, 2 tables, and 5 figure
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192
We report the detection of an extrasolar planet of mass ratio q ~ 2 x 10^(-4)
in microlensing event MOA-2007-BLG-192. The best fit microlensing model shows
both the microlensing parallax and finite source effects, and these can be
combined to obtain the lens masses of M = 0.060 (+0.028 -0.021) M_sun for the
primary and m = 3.3 (+4.9 -1.6) M_earth for the planet. However, the
observational coverage of the planetary deviation is sparse and incomplete, and
the radius of the source was estimated without the benefit of a source star
color measurement. As a result, the 2-sigma limits on the mass ratio and finite
source measurements are weak. Nevertheless, the microlensing parallax signal
clearly favors a sub-stellar mass planetary host, and the measurement of finite
source effects in the light curve supports this conclusion. Adaptive optics
images taken with the Very Large Telescope (VLT) NACO instrument are consistent
with a lens star that is either a brown dwarf or a star at the bottom of the
main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations
will either confirm that the primary is a brown dwarf or detect the low-mass
lens star and enable a precise determination of its mass. In either case, the
lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a
companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is
probably the lowest mass exoplanet found to date, aside from the lowest mass
pulsar planet.Comment: Accepted for publication in the Astrophysical Journal. Scheduled for
the Sept. 1, 2008 issu
Microlensing Event MOA-2007-BLG-400: Exhuming the Buried Signature of a Cool, Jovian-Mass Planet
We report the detection of the cool, Jovian-mass planet MOA-2007-BLG-400Lb.
The planet was detected in a high-magnification microlensing event (with peak
magnification A_max = 628) in which the primary lens transited the source,
resulting in a dramatic smoothing of the peak of the event. The angular extent
of the region of perturbation due to the planet is significantly smaller than
the angular size of the source, and as a result the planetary signature is also
smoothed out by the finite source size. Thus the deviation from a single-lens
fit is broad and relatively weak (~ few percent). Nevertheless, we demonstrate
that the planetary nature of the deviation can be unambiguously ascertained
from the gross features of the residuals, and detailed analysis yields a fairly
precise planet/star mass ratio of q = 0.0026+/-0.0004, in accord with the large
significance (\Delta\chi^2=1070) of the detection. The planet/star projected
separation is subject to a strong close/wide degeneracy, leading to two
indistinguishable solutions that differ in separation by a factor of ~8.5.
Upper limits on flux from the lens constrain its mass to be M < 0.75 M_Sun
(assuming it is a main-sequence star). A Bayesian analysis that includes all
available observational constraints indicates a primary in the Galactic bulge
with a mass of ~0.2-0.5 M_Sun and thus a planet mass of ~ 0.5-1.3 M_Jupiter.
The separation and equilibrium temperature are ~0.6-1.1AU (~5.3-9.7AU) and
~103K (~34K) for the close (wide) solution. If the primary is a main-sequence
star, follow-up observations would enable the detection of its light and so a
measurement of its mass and distance.Comment: 30 pages, 6 figures, Submitted to Ap
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