7,180 research outputs found
Nuclear absorption and emission in the AGN merger NGC 6240: the hard X-ray view
We present the analysis of four NuSTAR observations of the luminous infrared
galaxy merger NGC 6240, hosting a close pair of highly obscured active galactic
nuclei (AGN). Over a period of about two years, the source exhibits hard X-ray
variability of the order of 20 per cent, peaking around 20 keV. When the two
AGN are resolved with Chandra, column densities in the range cm are estimated for both of them. The exact values
are hard to determine, as they appear to depend on aspects that are sometimes
overlooked in Compton-thick objects, such as the covering factor of the
absorber, iron abundance, and the contamination in the Fe-K band from
foreground hot-gas emission. Nearly spherical covering and slightly subsolar
iron abundance are preferred in this case. While the southern nucleus is
suggested to be intrinsically more powerful, as also implied by the mid-IR and
2-10 keV brightness ratios, solutions involving a similar X-ray luminosity of
the two AGN cannot be ruled out. The observed variability is rather limited
compared to the one revealed by the Swift/BAT light curve, and it can be fully
explained by changes in the continuum flux from the two AGN, without requiring
significant column density variations. NGC 6240 is hereby confirmed to
represent a unique opportunity to investigate the X-ray (and broad-band)
properties of massive galaxy mergers, which were much more frequent in the
early Universe.Comment: 12 pages, 9 figures, 4 tables. Accepted for publication on MNRA
Fluctuations of large-scale jets in the stochastic 2D Euler equation
Two-dimensional turbulence in a rectangular domain self-organises into
large-scale unidirectional jets. While several results are present to
characterize the mean jets velocity profile, much less is known about the
fluctuations. We study jets dynamics in the stochastically forced
two-dimensional Euler equations. In the limit where the average jets velocity
profile evolves slowly with respect to turbulent fluctuations, we employ a
multi-scale (kinetic theory) approach, which relates jet dynamics to the
statistics of Reynolds stresses. We study analytically the Gaussian
fluctuations of Reynolds stresses and predict the spatial structure of the jets
velocity covariance. Our results agree qualitatively well with direct numerical
simulations, clearly showing that the jets velocity profile are enhanced away
from the stationary points of the average velocity profile. A numerical test of
our predictions at quantitative level seems out of reach at the present day
Hairy Black Holes in Massive Gravity: Thermodynamics and Phase Structure
The thermodynamic properties of a static and spherically symmetric hairy
black hole solution arising in massive gravity with spontaneous Lorentz
breaking are investigated. The analysis is carried out by enclosing the black
hole in a spherical cavity whose surface is maintained at a fixed temperature
. It turns out that the ensemble is well-defined only if the "hair"
parameter characterizing the solution is conserved. Under this condition we
compute some relevant thermodynamic quantities, such as the thermal energy and
entropy, and we study the stability and phase structure of the ensemble. In
particular, for negative values of the hair parameter, the phase structure is
isomorphic to the one of Reissner-Nordstrom black holes in the canonical
ensemble. Moreover, the phase-diagram in the plan () has a line of
first-order phase transition that at a critical value of terminates in a
second-order phase transition. Below this line the dominant phase consists of
small, cold black holes that are long-lived and may thus contribute much more
to the energy density of the Universe than what is observationally allowed for
radiating black holes.Comment: 12 pages, 11 figures, relevant references added, match the published
versio
Solvable model of a self-gravitating system
We introduce and discuss an effective model of a self-gravitating system
whose equilibrium thermodynamics can be solved in both the microcanonical and
the canonical ensemble, up to a maximization with respect to a single variable.
Such a model can be derived from a model of self-gravitating particles confined
on a ring, referred to as the self-gravitating ring (SGR) model, allowing a
quantitative comparison between the thermodynamics of the two models. Despite
the rather crude approximations involved in its derivation, the effective model
compares quite well with the SGR model. Moreover, we discuss the relation
between the effective model presented here and another model introduced by
Thirring forty years ago. The two models are very similar and can be considered
as examples of a class of minimal models of self-gravitating systems.Comment: 21 pages, 6 figures; submitted to JSTAT for the special issue on
long-range interaction
From Boltzmann equations to steady wall velocities
By means of a relativistic microscopic approach we calculate the expansion
velocity of bubbles generated during a first-order electroweak phase
transition. In particular, we use the gradient expansion of the Kadanoff-Baym
equations to set up the fluid system. This turns out to be equivalent to the
one found in the semi-classical approach in the non-relativistic limit.
Finally, by including hydrodynamic deflagration effects and solving the Higgs
equations of motion in the fluid, we determine velocity and thickness of the
bubble walls. Our findings are compared with phenomenological models of wall
velocities. As illustrative examples, we apply these results to three theories
providing first-order phase transitions with a particle content in the thermal
plasma that resembles the Standard Model.Comment: 40 pages, 8 figures; v2: added references, version published in JCA
Simulating Cellular Communications in Vehicular Networks: Making SimuLTE Interoperable with Veins
The evolution of cellular technologies toward 5G progressively enables
efficient and ubiquitous communications in an increasing number of fields.
Among these, vehicular networks are being considered as one of the most
promising and challenging applications, requiring support for communications in
high-speed mobility and delay-constrained information exchange in proximity. In
this context, simulation frameworks under the OMNeT++ umbrella are already
available: SimuLTE and Veins for cellular and vehicular systems, respectively.
In this paper, we describe the modifications that make SimuLTE interoperable
with Veins and INET, which leverage the OMNeT++ paradigm, and allow us to
achieve our goal without any modification to either of the latter two. We
discuss the limitations of the previous solution, namely VeinsLTE, which
integrates all three in a single framework, thus preventing independent
evolution and upgrades of each building block.Comment: Published in: A. Foerster, A. Udugama, A. Koensgen, A. Virdis, M.
Kirsche (Eds.), Proc. of the 4th OMNeT++ Community Summit, University of
Bremen - Germany - September 7-8, 201
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