9,223 research outputs found
Hybrid Passive Control Strategies for Reducing the Displacements at the Base of Seismic Isolated Structures
In this paper, the use of hybrid passive control strategies to mitigate the seismic response of a base-isolated structure is examined. The control performance of three different types of devices used for reducing base displacements of isolated buildings is investigated. Specifically, the Tuned Mass Damper (TMD), the New Tuned Mass Damper (New TMD) and the Tuned Liquid Column Damper (TLCD), each one associated to a Base Isolated structure (BI), have been considered. The seismic induced vibration control of base-isolated structures equipped with the TMD, New TMD or the TLCD is examined and compared with that of the base-isolated system without devices, using real recorded seismic signals as external input. Data show that the New TMD is the most effective in controlling the response of base-isolated structures so that it can be considered as a practical and appealing means to mitigate the dynamic response of base-isolated structures
Ion-supported tori: a thermal bremsstrahlung model for the X-ray Background
We discuss the possibility that a significant contribution of the hard X-ray
Background is the integrated emission from a population of galaxies undergoing
advection-dominated accretion in their nuclei. Owing to poor coupling between
ions and electrons and to efficient radiative cooling of the electrons, the
accreting plasma is two-temperature, with the ions being generally much hotter
than the electrons and forming an ion-supported torus. We show that the
electron te mperature then saturates at approximately 100keV independent of
model parameters. At this temperature the hard X-ray emission is dominated by
bremsstrahlung radiation. We find that this physical model gives an excellent
fit to the spectrum of the XRB in the 3-60 keV range, provided that there is
some evolution associated with the spectral emissivity which must peak at a
redshift of about 2. We estimate that such galaxies contribute only to a small
fraction of the local X-ray volume emissivity. The model implies a higher mean
black hole mass than is obtained from the evolution of quasars alone.Comment: 7 pages, 7 ps figures, uses mn.sty (included). Submitted for
publication to MNRA
Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4
We present a model for the different X-ray spectral states displayed by
Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral
implications for a magnetically structured corona in which magnetic flares
result from reconnection of flux tubes rising from the accretion disk by the
magnetic buoyancy instability. Using observations of one of the best studied
examples, GX339-4, we identify the geometry and the physical conditions
characterizing each of these states. We find that, in the Soft state, flaring
occurs at small scale heights above the accretion disk. The soft thermal-like
spectrum is the result of heating and consequent re-radiation of the hard
X-rays produced by such flares. The hard tail is produced by Comptonization of
the soft field radiation. Conversely, the hard state is the result of flares
triggered high above the underlying accretion disk which produce X-rays via
Comptonization of either internal synchrotron radiation or soft disk photons.
The spectral characteristics of the different states are naturally accounted
for by the choice of geometry: when flares are triggered high above the disk
the system is photon-starved, hence the hard Comptonized spectrum of the hard
state. Intense flaring close to the disk greatly enhances the soft-photon field
with the result that the spectrum softens. We interpret the two states as being
related to two different phases of magnetic energy dissipation. In the Soft
state, Parker instability in the disk favours the emergence of large numbers of
relatively low magnetic field flux tubes. In the hard state, only intense
magnetic fields become buoyant. The model can also qualitatively account for
the observed short timescale variability and the characteristics of the X-ray
reflected component of the hard state.Comment: submitted to MNRAS, Feb. 1998, 10 pages, 3 figures in MNRAS LaTex
styl
Magnetic flares and the optical variability of the X-ray transient XTE J1118+480
The simultaneous presence of a strong quasi periodic oscillation of period of
about 10 seconds in the optical and X-ray lightcurves of the X-ray transient
XTE J1118+480 suggests that a significant fraction of the optical flux
originates from the inner part of the accretion flow, where most of the X-rays
are produced. We present a model of magnetic flares in an accretion disc corona
where thermal cyclo-synchrotron emission contributes significantly to the
optical emission, while the X-rays are produced by inverse Compton scattering
of the soft photons produced by dissipation in the underlying disc and by the
synchrotron process itself. Given the observational constraints, we estimate
the values for the coronal temperature, optical depth and magnetic field
intensity, as well as the accretion rate for the source. Within our model we
predict a correlation between optical and hard X-ray variability and an
anticorrelation between optical and soft X-rays. We also expect optical
variability on flaring timescales (about tens of milliseconds), with a power
density spectrum similar to the one observed in the X-ray band. Finally we use
both the available optical/EUV/X-ray spectral energy distribution and the low
frequency time variability to discuss limits on the inner radius of the
optically thick disc.Comment: 5 pages, included 1 figure. One reference corrected. Submitted to
MNRA
Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers
We investigate, by means of numerical simulations, the possibility of forming
counter-rotating old stellar components by major mergers between an elliptical
and a spiral galaxy. We show that counter-rotation can appear both in
dissipative and dissipationless retrograde mergers, and it is mostly associated
to the presence of a disk component, which preserves part of its initial spin.
In turn, the external regions of the two interacting galaxies acquire part of
the orbital angular momentum, due to the action of tidal forces exerted on each
galaxy by the companion.Comment: 6 pages, 15 figures. Accepted on Astronomy & Astrophysic
Quantifying stellar radial migration in an N-body simulation: blurring, churning, and the outer regions of galaxy discs
Radial stellar migration in galactic discs has received much attention in
studies of galactic dynamics and chemical evolution, but remains a dynamical
phenomenon that needs to be fully quantified. In this work, using a Tree-SPH
simulation of an Sb-type disc galaxy, we quantify the effects of blurring
(epicyclic excursions) and churning (change of guiding radius). We quantify
migration (either blurring or churning) both in terms of flux (the number of
migrators passing at a given radius), and by estimating the population of
migrators at a given radius at the end of the simulation compared to
non-migrators, but also by giving the distance over which the migration is
effective at all radii. We confirm that the corotation of the bar is the main
source of migrators by churning in a bar-dominated galaxy, its intensity being
directly linked to the episode of a strong bar, in the first 1-3 Gyr of the
simulation. We show that within the outer Lindblad resonance (OLR), migration
is strongly dominated by churning, while blurring gains progressively more
importance towards the outer disc and at later times. Most importantly, we show
that the OLR limits the exchange of angular momentum, separating the disc in
two distinct parts with minimal or null exchange, except in the transition
zone, which is delimited by the position of the OLR at the epoch of the
formation of the bar, and at the final epoch. We discuss the consequences of
these findings for our understanding of the structure of the Milky Way disc.
Because the Sun is situated slightly outside the OLR, we suggest that the solar
vicinity may have experienced very limited churning from the inner disc.Comment: Accepted for publication in Astronomy and Astrophysics (acceptance
date: 27/04/15), 24 pages, 24 figure
Hiding its age: the case for a younger bulge
The determination of the age of the bulge has led to two contradictory
results. On the one side, the color-magnitude diagrams in different bulge
fields seem to indicate a uniformly old (10 Gyr) population. On the other
side, individual ages derived from dwarfs observed through microlensing events
seem to indicate a large spread, from 2 to 13 Gyr. Because the
bulge is now recognised as being mainly a boxy peanut-shaped bar, it is
suggested that disk stars are one of its main constituents, and therefore also
stars with ages significantly younger than 10 Gyr. Other arguments as well
point to the fact that the bulge cannot be exclusively old, and in particular
cannot be a burst population, as it is usually expected if the bulge was the
fossil remnant of a merger phase in the early Galaxy. In the present study, we
show that given the range of metallicities observed in the bulge, a uniformly
old population would be reflected into a significant spread in color at the
turn-off which is not observed. Inversely, we demonstrate that the correlation
between age and metallicity expected to hold for the inner disk would conspire
to form a color-magnitude diagram with a remarkably small spread in color, thus
mimicking the color-magnitude diagram of a uniformly old population. If stars
younger than 10 Gyr are part of the bulge, as must be the case if the bulge has
been mainly formed through dynamical instabilities in the disk, then a very
small spread at the turn-off is expected, as seen in the observations.Comment: 11 pages, 11 figures. Accepted for publication in A&
Two-temperature coronae in active galactic nuclei
We show that coronal magnetic dissipation in thin active sheets that sandwich
standard thin accretion disks in active galactic nuclei may account for
canonical electron temperatures of a few K if protons acquire most
of the dissipated energy. Coulomb collisions transfer energy from the ions to
the electrons, which subsequently cool rapidly by inverse-Compton scattering.
In equilibrium, the proton energy density likely exceeds that of the magnetic
field and both well exceed the electron and photon energy densities. The
Coulomb energy transfer from protons to electrons is slow enough to maintain a
high proton temperature, but fast enough to explain observed rapid X-ray
variabilities in Seyferts. The K electron temperature is insensitive
to the proton temperature when the latter is K.Comment: 5 pages LaTex, and 2 .ps figures, submitted to MNRAS, 4/9
X-ray Dichroism and the Pseudogap Phase of Cuprates
A recent polarized x-ray absorption experiment on the high temperature
cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity
symmetry below the temperature, T*, where a pseudogap appears in photoemission.
We critically analyze the x-ray data, and conclude that a parity-breaking
signal of the kind suggested is unlikely based on the crystal structures
reported in the literature. Possible other origins of the observed dichroism
signal are discussed. We propose x-ray scattering experiments that can be done
in order to determine whether such alternative interpretations are valid or
not.Comment: final version to be published in Phys Rev B: some calculational
details added, clarification of XNLD contamination and biaxiality, more
discussion on possible space groups and previous optics result
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