625 research outputs found
Extinction in Lotka-Volterra model
Competitive birth-death processes often exhibit an oscillatory behavior. We
investigate a particular case where the oscillation cycles are marginally
stable on the mean-field level. An iconic example of such a system is the
Lotka-Volterra model of predator-prey competition. Fluctuation effects due to
discreteness of the populations destroy the mean-field stability and eventually
drive the system toward extinction of one or both species. We show that the
corresponding extinction time scales as a certain power-law of the population
sizes. This behavior should be contrasted with the extinction of models stable
in the mean-field approximation. In the latter case the extinction time scales
exponentially with size.Comment: 11 pages, 17 figure
A high-density relativistic reflection origin for the soft and hard X-ray excess emission from Mrk 1044
We present the first results from a detailed spectral-timing analysis of a
long (130 ks) XMM-Newton observation and quasi-simultaneous NuSTAR and
Swift observations of the highly-accreting narrow-line Seyfert 1 galaxy Mrk
1044. The broadband (0.350 keV) spectrum reveals the presence of a strong
soft X-ray excess emission below 1.5 keV, iron K emission
complex at 67 keV and a `Compton hump' at 1530 keV. We find
that the relativistic reflection from a high-density accretion disc with a
broken power-law emissivity profile can simultaneously explain the soft X-ray
excess, highly ionized broad iron line and the Compton hump. At low frequencies
( Hz), the power-law continuum dominated 1.55 keV band
lags behind the reflection dominated 0.31 keV band, which is explained with
a combination of propagation fluctuation and Comptonization processes, while at
higher frequencies ( Hz), we detect a soft lag which is
interpreted as a signature of X-ray reverberation from the accretion disc. The
fractional root-mean-squared (rms) variability of the source decreases with
energy and is well described by two variable components: a less variable
relativistic disc reflection and a more variable direct coronal emission. Our
combined spectral-timing analyses suggest that the observed broadband X-ray
variability of Mrk~1044 is mainly driven by variations in the location or
geometry of the optically thin, hot corona.Comment: 23 pages, 19 figures, 4 tables, Published in MNRA
Discovery of soft and hard X-ray time lags in low-mass AGNs
The scaling relations between the black hole (BH) mass and soft lag
properties for both active galactic nuclei (AGNs) and BH X-ray binaries
(BHXRBs) suggest the same underlying physical mechanism at work in accreting BH
systems spanning a broad range of mass. However, the low-mass end of AGNs has
never been explored in detail. In this work, we extend the existing scaling
relations to lower-mass AGNs, which serve as anchors between the normal-mass
AGNs and BHXRBs. For this purpose, we construct a sample of low-mass AGNs
() from the XMM-Newton archive and
measure frequency-resolved time delays between the soft (0.3-1 keV) and hard
(1-4 keV) X-ray emissions. We report that the soft band lags behind the hard
band emission at high frequencies Hz, which is
interpreted as a sign of reverberation from the inner accretion disc in
response to the direct coronal emission. At low frequencies ( Hz), the hard band lags behind the soft band variations, which we
explain in the context of the inward propagation of luminosity fluctuations
through the corona. Assuming a lamppost geometry for the corona, we find that
the X-ray source of the sample extends at an average height and radius of and , respectively. Our results confirm that the
scaling relations between the BH mass and soft lag amplitude/frequency derived
for higher-mass AGNs can safely extrapolate to lower-mass AGNs, and the
accretion process is indeed independent of the BH mass.Comment: 11 pages, 5 figures, 4 tables, Published in MNRA
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Uprooting (Our) Whiteness
Social work education reinforces hegemonic Whiteness through pedagogies and practices that rely on an entitlement to and harvesting of Black, Indigenous, and People of Colorâs lived experiences for the purpose of its tacit audience: White students. Despite this exploitative and harmful reliance on objectified lived experiences, White students continue to lack critical understanding of their racial positionality and connections to racism. Uprooting Whiteness requires sitting with what it means for White people to be âa White problem.â Drawing on the work of Yancy, we (group co-facilitators; our dean of diversity, equity, and inclusion; and three MSW student participants) describe the creation, organization, facilitation, and experiences of the first year of the Space for Uprooting Whitenessâa biweekly space where White social work students examine and uproot their relationship to White supremacy and domination. We argue for White social workers to take collective responsibility for racism in and beyond our institutionsârequiring interrogation of our everyday practices and their (inter)dependence with and on systems of domination. This paper ends with three experiential narratives from student participants in the space and implications of critical intragroup dialogic pedagogy among White students in social work education and beyond
Status of Muon Collider Research and Development and Future Plans
The status of the research on muon colliders is discussed and plans are
outlined for future theoretical and experimental studies. Besides continued
work on the parameters of a 3-4 and 0.5 TeV center-of-mass (CoM) energy
collider, many studies are now concentrating on a machine near 0.1 TeV (CoM)
that could be a factory for the s-channel production of Higgs particles. We
discuss the research on the various components in such muon colliders, starting
from the proton accelerator needed to generate pions from a heavy-Z target and
proceeding through the phase rotation and decay ()
channel, muon cooling, acceleration, storage in a collider ring and the
collider detector. We also present theoretical and experimental R & D plans for
the next several years that should lead to a better understanding of the design
and feasibility issues for all of the components. This report is an update of
the progress on the R & D since the Feasibility Study of Muon Colliders
presented at the Snowmass'96 Workshop [R. B. Palmer, A. Sessler and A.
Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics
(Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].Comment: 95 pages, 75 figures. Submitted to Physical Review Special Topics,
Accelerators and Beam
Theory and Applications of Non-Relativistic and Relativistic Turbulent Reconnection
Realistic astrophysical environments are turbulent due to the extremely high
Reynolds numbers. Therefore, the theories of reconnection intended for
describing astrophysical reconnection should not ignore the effects of
turbulence on magnetic reconnection. Turbulence is known to change the nature
of many physical processes dramatically and in this review we claim that
magnetic reconnection is not an exception. We stress that not only
astrophysical turbulence is ubiquitous, but also magnetic reconnection itself
induces turbulence. Thus turbulence must be accounted for in any realistic
astrophysical reconnection setup. We argue that due to the similarities of MHD
turbulence in relativistic and non-relativistic cases the theory of magnetic
reconnection developed for the non-relativistic case can be extended to the
relativistic case and we provide numerical simulations that support this
conjecture. We also provide quantitative comparisons of the theoretical
predictions and results of numerical experiments, including the situations when
turbulent reconnection is self-driven, i.e. the turbulence in the system is
generated by the reconnection process itself. We show how turbulent
reconnection entails the violation of magnetic flux freezing, the conclusion
that has really far reaching consequences for many realistically turbulent
astrophysical environments. In addition, we consider observational testing of
turbulent reconnection as well as numerous implications of the theory. The
former includes the Sun and solar wind reconnection, while the latter include
the process of reconnection diffusion induced by turbulent reconnection, the
acceleration of energetic particles, bursts of turbulent reconnection related
to black hole sources as well as gamma ray bursts. Finally, we explain why
turbulent reconnection cannot be explained by turbulent resistivity or derived
through the mean field approach.Comment: 66 pages, 24 figures, a chapter of the book "Magnetic Reconnection -
Concepts and Applications", editors W. Gonzalez, E. N. Parke
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Biological, clinical and population relevance of 95 loci for blood lipids.
Plasma concentrations of total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol and triglycerides are among the most important risk factors for coronary artery disease (CAD) and are targets for therapeutic intervention. We screened the genome for common variants associated with plasma lipids in >100,000 individuals of European ancestry. Here we report 95 significantly associated loci (P < 5 x 10(-8)), with 59 showing genome-wide significant association with lipid traits for the first time. The newly reported associations include single nucleotide polymorphisms (SNPs) near known lipid regulators (for example, CYP7A1, NPC1L1 and SCARB1) as well as in scores of loci not previously implicated in lipoprotein metabolism. The 95 loci contribute not only to normal variation in lipid traits but also to extreme lipid phenotypes and have an impact on lipid traits in three non-European populations (East Asians, South Asians and African Americans). Our results identify several novel loci associated with plasma lipids that are also associated with CAD. Finally, we validated three of the novel genes-GALNT2, PPP1R3B and TTC39B-with experiments in mouse models. Taken together, our findings provide the foundation to develop a broader biological understanding of lipoprotein metabolism and to identify new therapeutic opportunities for the prevention of CAD
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