250 research outputs found
Simulating Infinite Vortex Lattices in Superfluids
We present an efficient framework to numerically treat infinite periodic
vortex lattices in rotating superfluids described by the Gross-Pitaevskii
theory. The commonly used split-step Fourier (SSF) spectral methods are
inapplicable to such systems as the standard Fourier transform does not respect
the boundary conditions mandated by the magnetic translation group. We present
a generalisation of the SSF method which incorporates the correct boundary
conditions by employing the so-called magnetic Fourier transform. We test the
method and show that it reduces to known results in the lowest-Landau-level
regime. While we focus on rotating scalar superfluids for simplicity, the
framework can be naturally extended to treat multicomponent systems and systems
under more general `synthetic' gauge fields.Comment: 17 pages, 2 figure
The Local Nanohertz Gravitational-Wave Landscape From Supermassive Black Hole Binaries
Supermassive black hole binaries (SMBHBs) in the 10 million to 10 billion
range form in galaxy mergers, and live in galactic nuclei with large
and poorly constrained concentrations of gas and stars. There are currently no
observations of merging SMBHBs--- it is in fact possible that they stall at
their final parsec of separation and never merge. While LIGO has detected high
frequency GWs, SMBHBs emit GWs in the nanohertz to millihertz band. This is
inaccessible to ground-based interferometers, but possible with Pulsar Timing
Arrays (PTAs). Using data from local galaxies in the 2 Micron All-Sky Survey,
together with galaxy merger rates from Illustris, we find that there are on
average sources emitting GWs in the PTA band, and binaries
which will never merge. Local unresolved SMBHBs can contribute to GW background
anisotropy at a level of , and if the GW background can be
successfully isolated, GWs from at least one local SMBHB can be detected in 10
years.Comment: submitted to Nature Astronomy (reformatted for arXiv
Observing the dynamics of super-massive black hole binaries with Pulsar Timing Arrays
Pulsar Timing Arrays are a prime tool to study unexplored astrophysical
regimes with gravitational waves. Here we show that the detection of
gravitational radiation from individually resolvable super-massive black hole
binary systems can yield direct information about the masses and spins of the
black holes, provided that the gravitational-wave induced timing fluctuations
both at the pulsar and at the Earth are detected. This in turn provides a map
of the non-linear dynamics of the gravitational field and a new avenue to
tackle open problems in astrophysics connected to the formation and evolution
of super-massive black holes. We discuss the potential, the challenges and the
limitations of these observations.Comment: 5 pages, 1 figur
Discovery of a close-separation binary quasar at the heart of a z~0.2 merging galaxy and its implications for low-frequency gravitational waves
Supermassive black hole (SMBH) binaries with masses of ~10^8--10^9 Msun are
expected to dominate the contribution to the as-yet undetected gravitational
wave background (GWB) signal at the nanohertz frequencies accessible to Pulsar
Timing Arrays (PTA). We currently lack firm empirical constraints on the
amplitude of the GWB due to the dearth of confirmed SMBH binaries in the
required mass range. Using HST/WFC3 images, we have discovered a z~0.2 quasar
hosted in a merger remnant with two closely separated (0.13'' or ~430pc)
continuum cores at the heart of the galaxy SDSSJ1010+1413. The two cores are
spatially coincident with two powerful [OIII]-emitting point sources with
quasar-like luminosities (L_AGN ~ 5x10^46 erg/s, suggesting the presence of a
bound SMBH system, each with M_BH > 4x10^8 Msun. We place an upper limit on the
merging timescale of the SMBH pair of 2.5 billion years, roughly the Universe
lookback time at z~0.2. There is likely a population of quasar binaries similar
to SDSSJ1010+1413 that contribute to a stochastic GWB that should be detected
in the next several years. If the GWB is not detected this could indicate that
SMBHs merge only over extremely long timescales, remaining as close separation
binaries for many Hubble times, the so-called `final-parsec problem'.Comment: Accepted to ApJL (in press), 9 pages, 5 figure
The noise properties of 42 millisecond pulsars from the European Pulsar Timing Array and their impact on gravitational wave searches
The sensitivity of Pulsar Timing Arrays to gravitational waves depends on the
noise present in the individual pulsar timing data. Noise may be either
intrinsic or extrinsic to the pulsar. Intrinsic sources of noise will include
rotational instabilities, for example. Extrinsic sources of noise include
contributions from physical processes which are not sufficiently well modelled,
for example, dispersion and scattering effects, analysis errors and
instrumental instabilities. We present the results from a noise analysis for 42
millisecond pulsars (MSPs) observed with the European Pulsar Timing Array. For
characterising the low-frequency, stochastic and achromatic noise component, or
"timing noise", we employ two methods, based on Bayesian and frequentist
statistics. For 25 MSPs, we achieve statistically significant measurements of
their timing noise parameters and find that the two methods give consistent
results. For the remaining 17 MSPs, we place upper limits on the timing noise
amplitude at the 95% confidence level. We additionally place an upper limit on
the contribution to the pulsar noise budget from errors in the reference
terrestrial time standards (below 1%), and we find evidence for a noise
component which is present only in the data of one of the four used telescopes.
Finally, we estimate that the timing noise of individual pulsars reduces the
sensitivity of this data set to an isotropic, stochastic GW background by a
factor of >9.1 and by a factor of >2.3 for continuous GWs from resolvable,
inspiralling supermassive black-hole binaries with circular orbits.Comment: Accepted for publication by the Monthly Notices of the Royal
Astronomical Societ
HCC development is associated to peripheral insulin resistance in a mouse model of NASH
NAFLD is the most common liver disease worldwide but it is the potential evolution to NASH and eventually to hepatocellular carcinoma (HCC), even in the absence of cirrhosis, that makes NAFLD of such clinical importance. Aim: we aimed to create a mouse model reproducing the pathological spectrum of NAFLD and to investigate the role of possible co-factors in promoting HCC. Methods: mice were treated with a choline-deficient L-amino-acid-defined-diet (CDAA) or its control (CSAA diet) and subjected to a low-dose i.p. injection of CCl 4 or vehicle. Insulin resistance was measured by the euglycemic-hyperinsulinemic clamp method. Steatosis, fibrosis and HCC were evaluated by histological and molecular analysis. Results: CDAA-treated mice showed peripheral insulin resistance at 1 month. At 1-3 months, extensive steatosis and fibrosis were observed in CDAA and CDAA+CCl4 groups. At 6 months, equal increase in steatosis and fibrosis was observed between the two groups, together with the appearance of tumor. At 9 months of treatment, the 100% of CDAA+ CCl4 treated mice revealed tumor versus 40% of CDAA mice. Insulin-like Growth Factor-2 (IGF-2) and Osteopontin (SPP-1) were increased in CDAA mice versus CSAA. Furthermore, Immunostaining for p-AKT, p-c-Myc and Glypican-3 revealed increased positivity in the tumors. Conclusions: the CDAA model promotes the development of HCC from NAFLD-NASH in the presence of insulin resistance but in the absence of cirrhosis. Since this condition is increasingly recognized in humans, our study provides a model that may help understanding mechanisms of carcinogenesis in NAFLD. © 2014 De Minicis et al
Mapping gravitational-wave backgrounds using methods from CMB analysis: Application to pulsar timing arrays
We describe an alternative approach to the analysis of gravitational-wave
backgrounds, based on the formalism used to characterise the polarisation of
the cosmic microwave background. In contrast to standard analyses, this
approach makes no assumptions about the nature of the background and so has the
potential to reveal much more about the physical processes that generated it.
An arbitrary background can be decomposed into modes whose angular dependence
on the sky is given by gradients and curls of spherical harmonics. We derive
the pulsar timing overlap reduction functions for the individual modes, which
are given by simple combinations of spherical harmonics evaluated at the pulsar
locations. We show how these can be used to recover the components of an
arbitrary background, giving explicit results for both isotropic and
anisotropic uncorrelated backgrounds. We also find that the response of a
pulsar timing array to curl modes is identically zero, so half of the
gravitational-wave sky will never be observed using pulsar timing, no matter
how many pulsars are included in the array. An isotropic, unpolarised and
uncorrelated background can be accurately represented using only three modes,
and so a search of this type will be only slightly more complicated than the
standard cross-correlation search using the Hellings and Downs overlap
reduction function. However, by measuring the components of individual modes of
the background and checking for consistency with isotropy, this approach has
the potential to reveal much more information. Each individual mode on its own
describes a background that is correlated between different points on the sky.
A measurement of the components that indicates the presence of correlations in
the background on large angular scales would suggest startling new physics.Comment: 48 pages, 16 figures, to appear in Phys. Rev. D; v2 contains various
changes in response to the referee report and is consistent with published
versio
The NANOGrav 11-Year Data Set: Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries
Observations indicate that nearly all galaxies contain supermassive black
holes (SMBHs) at their centers. When galaxies merge, their component black
holes form SMBH binaries (SMBHBs), which emit low-frequency gravitational waves
(GWs) that can be detected by pulsar timing arrays (PTAs). We have searched the
recently-released North American Nanohertz Observatory for Gravitational Waves
(NANOGrav) 11-year data set for GWs from individual SMBHBs in circular orbits.
As we did not find strong evidence for GWs in our data, we placed 95\% upper
limits on the strength of GWs from such sources as a function of GW frequency
and sky location. We placed a sky-averaged upper limit on the GW strain of at nHz. We also developed a
technique to determine the significance of a particular signal in each pulsar
using ``dropout' parameters as a way of identifying spurious signals in
measurements from individual pulsars. We used our upper limits on the GW strain
to place lower limits on the distances to individual SMBHBs. At the
most-sensitive sky location, we ruled out SMBHBs emitting GWs with
nHz within 120 Mpc for , and
within 5.5 Gpc for . We also determined that
there are no SMBHBs with emitting
GWs in the Virgo Cluster. Finally, we estimated the number of potentially
detectable sources given our current strain upper limits based on galaxies in
Two Micron All-Sky Survey (2MASS) and merger rates from the Illustris
cosmological simulation project. Only 34 out of 75,000 realizations of the
local Universe contained a detectable source, from which we concluded it was
unsurprising that we did not detect any individual sources given our current
sensitivity to GWs.Comment: 10 pages, 11 figures. Accepted by Astrophysical Journal. Please send
any comments/questions to S. J. Vigeland ([email protected]
Multi-Messenger Gravitational Wave Searches with Pulsar Timing Arrays: Application to 3C66B Using the NANOGrav 11-year Data Set
When galaxies merge, the supermassive black holes in their centers may form
binaries and, during the process of merger, emit low-frequency gravitational
radiation in the process. In this paper we consider the galaxy 3C66B, which was
used as the target of the first multi-messenger search for gravitational waves.
Due to the observed periodicities present in the photometric and astrometric
data of the source of the source, it has been theorized to contain a
supermassive black hole binary. Its apparent 1.05-year orbital period would
place the gravitational wave emission directly in the pulsar timing band. Since
the first pulsar timing array study of 3C66B, revised models of the source have
been published, and timing array sensitivities and techniques have improved
dramatically. With these advances, we further constrain the chirp mass of the
potential supermassive black hole binary in 3C66B to less than using data from the NANOGrav 11-year data set. This
upper limit provides a factor of 1.6 improvement over previous limits, and a
factor of 4.3 over the first search done. Nevertheless, the most recent orbital
model for the source is still consistent with our limit from pulsar timing
array data. In addition, we are able to quantify the improvement made by the
inclusion of source properties gleaned from electromagnetic data to `blind'
pulsar timing array searches. With these methods, it is apparent that it is not
necessary to obtain exact a priori knowledge of the period of a binary to gain
meaningful astrophysical inferences.Comment: 14 pages, 6 figures. Accepted by Ap
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