580 research outputs found
Tracing Ghost Cavities with Low Frequency Radio Observations
We present X-ray and multi-frequency radio observations of the central radio
sources in several X-ray cavity systems. We show that targeted radio
observations are key to determining if the lobes are being actively fed by the
central AGN. Low frequency observations provide a unique way to study both the
lifecycle of the central radio source as well as its energy input into the ICM
over several outburst episodes.Comment: 6 pages, 4 figures, To appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", eds. H. Boehringer, P.
Schuecker, G. W. Pratt & A. Finoguenov (ESO Astrophysics Symposia,
Springer-Verlag), Garching (Germany), August 200
Cold Feedback in Cooling-Flow Galaxy Clusters
We put forward an alternative view to the Bondi-driven feedback between
heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies
and clusters. We adopt the popular view that the heating is due to an active
galactic nucleus (AGN), i.e. a central black hole accreting mass and launching
jets and/or winds. We propose that the feedback occurs with the entire cool
inner region (5-30 kpc). A moderate cooling flow does exist here, and
non-linear over-dense blobs of gas cool fast and are removed from the ICM
before experiencing the next major AGN heating event. Some of these blobs may
not accrete on the central black hole, but may form stars and cold molecular
clouds. We discuss the conditions under which the dense blobs may cool to low
temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
Assessing the importance of a self-generated detachment process in river biofilm models
1. Epilithic biofilm biomass was measured for 14 months in two sites, located up- and downstream of the city of Toulouse in the Garonne River (south-west France). Periodical sampling provided a biomass data set to compare with simulations from the model of Uehlinger, Bürher and Reichert (1996: Freshwater Biology, 36, 249–263.), in order to evaluate the impact of hydraulic disturbance.
2. Despite differences in application conditions (e.g. river size, discharge, frequency of disturbance), the base equation satisfactorily predicted biomass between low and high water periods of the year, suggesting that the flood disturbance regime may be considered a universal mechanism controlling periphyton biomass.
3. However modelling gave no agreement with biomass dynamics during the 7-month long low water period that the river experienced. The influence of other biomass-regulating factors (temperature, light and soluble reactive phosphorus) on temporal biomass dynamics was weak.
4. Implementing a supplementary mechanism corresponding to a temperature-dependent self-generated loss because of heterotrophic processes allowed us to accurately reproduce the observed pattern: a succession of two peaks. This case study suggests that during typical summer low water periods (flow stability and favourable temperature) river biofilm modelling requires self-generated detachment to be considered
Mirror quiescence and high-sensitivity position measurements with feedback
We present a detailed study of how phase-sensitive feedback schemes can be
used to improve the performance of optomechanical devices. Considering the case
of a cavity mode coupled to an oscillating mirror by the radiation pressure, we
show how feedback can be used to reduce the position noise spectrum of the
mirror, cool it to its quantum ground state, or achieve position squeezing.
Then, we show that even though feedback is not able to improve the sensitivity
of stationary position spectral measurements, it is possible to design a
nonstationary strategy able to increase this sensitivity.Comment: 25 pages, 11 figure
AGN effect on cooling flow dynamics
We analyzed the feedback of AGN jets on cooling flow clusters using
three-dimensional AMR hydrodynamic simulations. We studied the interaction of
the jet with the intracluster medium and creation of low X-ray emission
cavities (Bubbles) in cluster plasma. The distribution of energy input by the
jet into the system was quantified in its different forms, i.e. internal,
kinetic and potential. We find that the energy associated with the bubbles, (pV
+ gamma pV/(gamma-1)), accounts for less than 10 percent of the jet energy.Comment: "Accepted for publication in Astrophysics & Space Science
Measuring Black Hole Spin using X-ray Reflection Spectroscopy
I review the current status of X-ray reflection (a.k.a. broad iron line)
based black hole spin measurements. This is a powerful technique that allows us
to measure robust black hole spins across the mass range, from the stellar-mass
black holes in X-ray binaries to the supermassive black holes in active
galactic nuclei. After describing the basic assumptions of this approach, I lay
out the detailed methodology focusing on "best practices" that have been found
necessary to obtain robust results. Reflecting my own biases, this review is
slanted towards a discussion of supermassive black hole (SMBH) spin in active
galactic nuclei (AGN). Pulling together all of the available XMM-Newton and
Suzaku results from the literature that satisfy objective quality control
criteria, it is clear that a large fraction of SMBHs are rapidly-spinning,
although there are tentative hints of a more slowly spinning population at high
(M>5*10^7Msun) and low (M<2*10^6Msun) mass. I also engage in a brief review of
the spins of stellar-mass black holes in X-ray binaries. In general,
reflection-based and continuum-fitting based spin measures are in agreement,
although there remain two objects (GROJ1655-40 and 4U1543-475) for which that
is not true. I end this review by discussing the exciting frontier of
relativistic reverberation, particularly the discovery of broad iron line
reverberation in XMM-Newton data for the Seyfert galaxies NGC4151, NGC7314 and
MCG-5-23-16. As well as confirming the basic paradigm of relativistic disk
reflection, this detection of reverberation demonstrates that future large-area
X-ray observatories such as LOFT will make tremendous progress in studies of
strong gravity using relativistic reverberation in AGN.Comment: 19 pages. To appear in proceedings of the ISSI-Bern workshop on "The
Physics of Accretion onto Black Holes" (8-12 Oct 2012). Revised version adds
a missing source to Table 1 and Fig.6 (IRAS13224-3809) and corrects the
referencing of the discovery of soft lags in 1H0707-495 (which were in fact
first reported in Fabian et al. 2009
The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling
We investigate the effect that Rashba spin-orbit coupling has on the low
energy behaviour of a two dimensional magnetic impurity system. It is shown
that the Kondo effect, the screening of the magnetic impurity at temperatures T
< T_K, is robust against such spin-orbit coupling, despite the fact that the
spin of the conduction electrons is no longer a conserved quantity. A proposal
is made for how the spin-orbit coupling may change the value of the Kondo
temperature T_K in such systems and the prospects of measuring this change are
discussed. We conclude that many of the assumptions made in our analysis
invalidate our results as applied to recent experiments in semi-conductor
quantum dots but may apply to measurements made with magnetic atoms placed on
metallic surfaces.Comment: 22 pages, 1 figure; reference update
Cold Gas in Cluster Cores
I review the literature's census of the cold gas in clusters of galaxies.
Cold gas here is defined as the gas that is cooler than X-ray emitting
temperatures (~10^7 K) and is not in stars. I present new Spitzer IRAC and MIPS
observations of Abell 2597 (PI: Sparks) that reveal significant amounts of warm
dust and star formation at the level of 5 solar masses per year. This rate is
inconsistent with the mass cooling rate of 20 +/- 5 solar masses per year
inferred from a FUSE [OVI] detection.Comment: 10 pages, conference proceeding
Generalizations of Gronwall-Bihari Inequalities on Time Scales
We establish some nonlinear integral inequalities for functions defined on a
time scale. The results extend some previous Gronwall and Bihari type
inequalities on time scales. Some examples of time scales for which our results
can be applied are provided. An application to the qualitative analysis of a
nonlinear dynamic equation is discussed.Comment: This is a preprint of an article accepted (16/May/2008) for
publication in the "Journal of Difference Equations and Applications"; J.
Difference Equ. Appl. is available online at http://www.informaworld.co
X-ray Spectroscopy of Cooling Clusters
We review the X-ray spectra of the cores of clusters of galaxies. Recent high
resolution X-ray spectroscopic observations have demonstrated a severe deficit
of emission at the lowest X-ray temperatures as compared to that expected from
simple radiative cooling models. The same observations have provided compelling
evidence that the gas in the cores is cooling below half the maximum
temperature. We review these results, discuss physical models of cooling
clusters, and describe the X-ray instrumentation and analysis techniques used
to make these observations. We discuss several viable mechanisms designed to
cancel or distort the expected process of X-ray cluster cooling.Comment: To appear in Physics Reports, 71 pages, 20 figure
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