202,510 research outputs found
The phase-diagram of the IGM and the entropy floor of groups and clusters: are clusters born warm?
We point out that two problems of observational cosmology, the facts i) that
> 60% of the baryonic content of the universe is not observed at z=0 and ii)
that the properties of small clusters do not agree with simple expectations,
could be closely related. As shown by recent studies, the shock-heating
associated with the formation of large-scale structures heats the intergalactic
medium (IGM) and leads to a ``warm IGM'' component for the gas. In the same
spirit, we suggest the intracluster medium (ICM) to be a mixture of
galaxy-recycled, metal enriched gas and intergalactic gas, shock-heated by the
collapsing much larger scales. This could be obtained through two processes: 1)
the late infalling gas from the external warm IGM is efficiently mixed within
the halo and brings some additional entropy, or 2) the shocks generated by
larger non-linear scales are also present within clusters and can heat the ICM.
We show that if assumption (1) holds, the entropy brought by the warm IGM is
sufficient to explain the observed properties of clusters, in particular the
entropy floor and the LX-T relation. On the other hand, we briefly note that
the scenario (2) would require a stronger shock-heating because of the larger
density of the ICM as compared with filaments. Our scenario of clusters being
"born warm" can be checked through the predicted redshift evolution of the
entropy floor.Comment: 8 pages, final version published in MNRA
Shocks in coupled socio-ecological systems: what are they and how can we model them?
Coupled socio-ecological systems (SES) are complex systems characterized by self-organization, non-linearities, interactions among heterogeneous elements within each subsystem, and feedbacks across scales and among subsystems. When such a system experiences a shock or a crisis, the consequences are difficult to predict. In this paper we first define what a shock or a crisis means for SES. Depending on where the system boundary is drawn, shocks can be seen as exogenous or endogenous. For example, human intervention in environmental systems could be seen as exogenous, but endogenous in a socio-environmental system. This difference in the origin and nature of shocks has certain consequences for coupled SES and for policies to ameliorate negative consequences of shocks. Having defined shocks, the paper then focuses on modelling challenges when studying shocks in coupled SES. If we are to explore, study and predict the responses of coupled SES to shocks, the models used need to be able to accommodate (exogenous) or produce (endogenous) a shock event. Various modelling choices need to be made. Specifically, the âsuddenâ aspect of a shock suggests the time period over which an event claimed to be a shock occurred might be âquickâ. What does that mean for a discrete event model? Turning to magnitude, what degree of change (in a variable or set of variables) is required for the event to be considered a shock? The âsurprisingâ nature of a shock means that none of the agents in the model should expect the shock to happen, but may need rules enabling them to generate behaviour in exceptional circumstances. This requires a certain design of the agentsâ decision-making algorithms, their perception of a shock, memory of past events and formation of expectations, and the information available to them during the time the shock occurred
A multivariate model for financial indices and an algorithm for detection of jumps in the volatility
We consider a mean-reverting stochastic volatility model which satisfies some
relevant stylized facts of financial markets. We introduce an algorithm for the
detection of peaks in the volatility profile, that we apply to the time series
of Dow Jones Industrial Average and Financial Times Stock Exchange 100 in the
period 1984-2013. Based on empirical results, we propose a bivariate version of
the model, for which we find an explicit expression for the decay over time of
cross-asset correlations between absolute returns. We compare our theoretical
predictions with empirical estimates on the same financial time series, finding
an excellent agreement.Comment: 20 pages, 22 figure
Generalized Flows around Neutron Stars
In this chapter, we present a brief and non-exhaustive review of the
developments of theoretical models for accretion flows around neutron stars. A
somewhat chronological summary of crucial observations and modelling of timing
and spectral properties are given in sections 2 and 3. In section 4, we argue
why and how the Two-Component Advective Flow (TCAF) solution can be applied to
the cases of neutron stars when suitable modifications are made for the NSs. We
showcase some of our findings from Monte Carlo and Smoothed Particle
Hydrodynamic simulations which further strengthens the points raised in section
4. In summary, we remark on the possibility of future works using TCAF for both
weakly magnetic and magnetic Neutron Stars.Comment: 15 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1901.0084
Hydrodynamics of Cloud Collisions in 2D: The Fate of Clouds in a Multi-phase Medium
We have studied head-on collisions between equal-mass, mildly supersonic
(Mach number 1.5) HI clouds, in a standard Two-phase ISM (T_cl = 74 K, n_cl =
22 cm^-3, \chi = 100). We explore the role of various factors, including the
radiative cooling parameter \eta = t_rad/t_coll (t_coll=R_c/v_c), evolutionary
modifications on the cloud structure (by colliding clouds ``evolved'' through
independent motion within the intercloud medium (WIM)), and the symmetry of the
problem (by colliding initially identical clouds, evolved to different ages
before impact). The presence of bow shocks and ram pressure from material in
the cloud wake, developed during such evolution through the WIM, significantly
alters these interactions with respect to the standard case of non-evolved
clouds.
In general, in our adiabatic collisions the clouds are disrupted and convert
their gas into a few low density contrast clumps. By contrast, for symmetric
radiative cases we find that the two clouds coalesce, with almost all the
initial kinetic energy radiated away. On the other hand, for both adiabatic and
radiative collisions, asymmetric collisions have a much greater tendency to
disrupt the two clouds. Fragmentation of the clouds may occur, and
instabilities are in general enhanced. In addition, radiative cooling is less
efficient in our asymmetric interactions, so that those parts of the clouds
that initially seem to merge are more likely to re-expand and fade into the
WIM. Since the majority of real cloud collisions should be asymmetric for one
reason or another, we conclude that most gasdynamical diffuse cloud collisions
will be disruptive, at least in the absence of significant self-gravity or of a
significant magnetic field.Comment: Revised version, published in The Astrophysical Journal; 26 pages
Latex + 9 figures, mpeg animations available at
http://www.msi.umn.edu/Projects/twj/Cloud-Collision.htm
Conformal field theory out of equilibrium: a review
We provide a pedagogical review of the main ideas and results in
non-equilibrium conformal field theory and connected subjects. These concern
the understanding of quantum transport and its statistics at and near critical
points. Starting with phenomenological considerations, we explain the general
framework, illustrated by the example of the Heisenberg quantum chain. We then
introduce the main concepts underlying conformal field theory (CFT), the
emergence of critical ballistic transport, and the CFT scattering construction
of non-equilibrium steady states. Using this we review the theory for energy
transport in homogeneous one-dimensional critical systems, including the
complete description of its large deviations and the resulting (extended)
fluctuation relations. We generalize some of these ideas to one-dimensional
critical charge transport and to the presence of defects, as well as beyond
one-dimensional criticality. We describe non-equilibrium transport in
free-particle models, where connections are made with generalized Gibbs
ensembles, and in higher-dimensional and non-integrable quantum field theories,
where the use of the powerful hydrodynamic ideas for non-equilibrium steady
states is explained. We finish with a list of open questions. The review does
not assume any advanced prior knowledge of conformal field theory,
large-deviation theory or hydrodynamics.Comment: 50 pages + 10 pages of references, 5 figures. v2: minor
modifications. Review article for special issue of JSTAT on nonequilibrium
dynamics in integrable quantum system
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