11,533 research outputs found
Radial flow afterburner for event generators and the baryon puzzle
A simple afterburner including radial flow to the randomized transverse
momentum obtained from event generators, Pythia and Hijing, has been
implemented to calculate the ratios and compare them with available
data. A coherent trend of qualitative agreement has been obtained in
collisions and in for various centralities. Those results indicate that
the radial flow does play an important role in the so called baryon puzzle
anomaly.Comment: 11 pages, 5 figures. To appear in Journal of Physics
Jet trails and Mach cones: The interaction of microquasars with the ISM
A sub-set of microquasars exhibit high peculiar velocity with respect to the
local standard of rest due to the kicks they receive when being born in
supernovae. The interaction between the radio plasma released by microquasar
jets from such high-velocity binaries with the ISM must lead to the production
of trails and bow shocks similar to what is observed in narrow-angle tailed
radio galaxies and pulsar wind nebulae. We present a set of numerical
simulations of this interaction that illuminate the long term dynamical
evolution and the observational properties of these microquasar bow shock
nebulae and trails. We find that this interaction always produces a structure
that consists of a bow shock, a trailing neck, and an expanding bubble. Using
our simulations to model emission, we predict that the shock surrounding the
bubble and the neck should be visible in H{\alpha} emission, the interior of
the bubble should be visible in synchrotron radio emission, and only the bow
shock is likely to be detectable in X-ray emission. We construct an analytic
model for the evolution of the neck and bubble shape and compare this model
with observations of X-ray binary SAX J1712.6-3739.Comment: 33 pages, 13 figures, 1 table; Accepted to Ap
Flow effects on the freeze-out phase-space density in heavy ion collisions
The strong longitudinal expansion of the reaction zone formed in relativistic
heavy-ion collisions is found to significantly reduce the spatially averaged
pion phase-space density, compared to naive estimates based on thermal
distributions. This has important implications for data interpretation and
leads to larger values for the extracted pion chemical potential at kinetic
freeze-out.Comment: 5 pages, 3 figures included via epsfig, added discussion of different
transverse density profiles, 1 new figur
A fundamental plane of black hole activity
We examine the disc--jet connection in stellar mass and supermassive black
holes by investigating the properties of their compact emission in the X-ray
and radio bands. We compile a sample of ~100 active galactic nuclei with
measured mass, 5 GHz core emission, and 2-10 keV luminosity, together with 8
galactic black holes with a total of ~50 simultaneous observations in the radio
and X-ray bands. Using this sample, we study the correlations between the radio
(L_{R}) and the X-ray (L_{X}) luminosity and the black hole mass (M). We find
that the radio luminosity is correlated with {\em both} M and L_{X}, at a
highly significant level. In particular, we show that the sources define a
``fundamental plane'' in the three-dimensional (log L_{R},log L_{X},log M)
space, given by log L_{R}=(0.60^{+0.11}_{-0.11}) log L_{X}
+(0.78^{+0.11}_{-0.09}) log M + 7.33^{+4.05}_{-4.07}, with a substantial
scatter of \sigma_{R}=0.88. We compare our results to the theoretical relations
between radio flux, black hole mass, and accretion rate derived by Heinz and
Sunyaev (2003). Such relations depend only on the assumed accretion model and
on the observed radio spectral index. Therefore, we are able to show that the
X-ray emission from black holes accreting at less than a few per cent of the
Eddington rate is unlikely to be produced by radiatively efficient accretion,
and is marginally consistent with optically thin synchrotron emission from the
jet. On the other hand, models for radiatively inefficient accretion flows seem
to agree well with the data.Comment: 21 pages, 8 figures (2 in colour). Revised version accepted for
publication by MNRAS. Improved and extended discussio
A variational approach to moment-closure approximations for the kinetics of biomolecular reaction networks
Approximate solutions of the chemical master equation and the chemical
Fokker-Planck equation are an important tool in the analysis of biomolecular
reaction networks. Previous studies have highlighted a number of problems with
the moment-closure approach used to obtain such approximations, calling it an
ad-hoc method. In this article, we give a new variational derivation of
moment-closure equations which provides us with an intuitive understanding of
their properties and failure modes and allows us to correct some of these
problems. We use mixtures of product-Poisson distributions to obtain a flexible
parametric family which solves the commonly observed problem of divergences at
low system sizes. We also extend the recently introduced entropic matching
approach to arbitrary ansatz distributions and Markov processes, demonstrating
that it is a special case of variational moment closure. This provides us with
a particularly principled approximation method. Finally, we extend the above
approaches to cover the approximation of multi-time joint distributions,
resulting in a viable alternative to process-level approximations which are
often intractable.Comment: Minor changes and clarifications; corrected some typo
Impact of tangled magnetic fields on AGN-blown bubbles
There is growing consensus that feedback from AGN is the main mechanism
responsible for stopping cooling flows in clusters of galaxies. AGN are known
to inflate buoyant bubbles that supply mechanical power to the intracluster gas
(ICM). High Reynolds number hydrodynamical simulations show that such bubbles
get entirely disrupted within 100 Myr, as they rise in cluster atmospheres,
which is contrary to observations. This artificial mixing has consequences for
models trying to quantify the amount of heating and star formation in cool core
clusters of galaxies. It has been suggested that magnetic fields can stabilize
bubbles against disruption. We perform MHD simulations of fossil bubbles in the
presence of tangled magnetic fields using the high order PENCIL code. We focus
on the physically-motivated case where thermal pressure dominates over magnetic
pressure and consider randomly oriented fields with and without maximum
helicity and a case where large scale external fields drape the bubble.We find
that helicity has some stabilizing effect. However, unless the coherence length
of magnetic fields exceeds the bubble size, the bubbles are quickly shredded.
As observations of Hydra A suggest that lengthscale of magnetic fields may be
smaller then typical bubble size, this may suggest that other mechanisms, such
as viscosity, may be responsible for stabilizing the bubbles. However, since
Faraday rotation observations of radio lobes do not constrain large scale ICM
fields well if they are aligned with the bubble surface, the draping case may
be a viable alternative solution to the problem. A generic feature found in our
simulations is the formation of magnetic wakes where fields are ordered and
amplified. We suggest that this effect could prevent evaporation by thermal
conduction of cold Halpha filaments observed in the Perseus cluster.Comment: accepted for publication in MNRAS, (downgraded resolution figures,
color printing recommended
Effects of fluctuations on the initial eccentricity from the Color Glass Condensate in heavy ion collisions
We introduce a modified form of the Kharzeev-Levin-Nardi (KLN) approach for
nuclear collisions. The new ansatz for the unintegrated gluon distribution
function preserves factorization, and the saturation scale is bound from below
by that for a single nucleon. It also reproduces the correct scaling with the
number of collisions at high transverse momentum. The corresponding Monte Carlo
implementation allows us to account for fluctuations of the hard sources
(nucleons) in the transverse plane. We compute various definitions of the
eccentricity within the new approach, which are relevant for the interpretation
of the elliptic flow. Our approach predicts breaking of the scaling of the
eccentricity with the Glauber eccentricity at the level of about 30%.Comment: 9 pages, 10 figures, Updated version as accepted by Phys.Rev.
Elliptic Flow and Initial Eccentricity in Cu+Cu and Au+Au Collisions at RHIC
We present a systematic study of elliptic flow as a function of centrality,
pseudorapidity, transverse momentum and energy for Cu+Cu and Au+Au collisions
from the PHOBOS experiment. New data on elliptic flow in Cu+Cu collisions at
22.4 GeV are shown. Elliptic flow scaled by participant eccentricity is found
to be similar for both systems when collisions with the same number of
participants or the same average area density are compared. This similarity is
observed over a wide range in pseudorapidity and transverse momentum,
indicating that participant eccentricity is the relevant quantity for
generating the azimuthal asymmetry leading to the observed elliptic flow.Comment: 5 pages, 4 figures, the 19th International Conference On Ultra
relativistic Nucleus-Nucleus Collisions (Quark Matter 2006), Shanghai China,
Nov. 14-20, 200
- …