1,222 research outputs found
Simulations of Extreme-Mass-Ratio Inspirals Using Pseudospectral Methods
Extreme-mass-ratio inspirals (EMRIs), stellar-mass compact objects (SCOs)
inspiralling into a massive black hole, are one of the main sources of
gravitational waves expected for the Laser Interferometer Space Antenna (LISA).
To extract the EMRI signals from the expected LISA data stream, which will also
contain the instrumental noise as well as other signals, we need very accurate
theoretical templates of the gravitational waves that they produce. In order to
construct those templates we need to account for the gravitational
backreaction, that is, how the gravitational field of the SCO affects its own
trajectory. In general relativity, the backreaction can be described in terms
of a local self-force, and the foundations to compute it have been laid
recently. Due to its complexity, some parts of the calculation of the
self-force have to be performed numerically. Here, we report on an ongoing
effort towards the computation of the self-force based on time-domain
multi-grid pseudospectral methods.Comment: 6 pages, 4 figures, JPCS latex style. Submitted to JPCS (special
issue for the proceedings of the 7th International LISA Symposium
Three-Body Dynamics with Gravitational Wave Emission
We present numerical three-body experiments that include the effects of
gravitational radiation reaction by using equations of motion that include the
2.5-order post-Newtonian force terms, which are the leading order terms of
energy loss from gravitational waves. We simulate binary-single interactions
and show that close approach cross sections for three 1 solar mass objects are
unchanged from the purely Newtonian dynamics except for close approaches
smaller than 1.0e-5 times the initial semimajor axis of the binary. We also
present cross sections for mergers resulting from gravitational radiation
during three-body encounters for a range of binary semimajor axes and mass
ratios including those of interest for intermediate-mass black holes (IMBHs).
Building on previous work, we simulate sequences of high-mass-ratio three-body
encounters that include the effects of gravitational radiation. The simulations
show that the binaries merge with extremely high eccentricity such that when
the gravitational waves are detectable by LISA, most of the binaries will have
eccentricities e > 0.9 though all will have circularized by the time they are
detectable by LIGO. We also investigate the implications for the formation and
growth of IMBHs and find that the inclusion of gravitational waves during the
encounter results in roughly half as many black holes ejected from the host
cluster for each black hole accreted onto the growing IMBH.Comment: 34 pages, 14 figures, minor corrections to match version accepted by
Ap
The impact of realistic models of mass segregation on the event rate of extreme-mass ratio inspirals and cusp re-growth
One of the most interesting sources of gravitational waves (GWs) for LISA is
the inspiral of compact objects on to a massive black hole (MBH), commonly
referred to as an "extreme-mass ratio inspiral" (EMRI). The small object,
typically a stellar black hole (bh), emits significant amounts of GW along each
orbit in the detector bandwidth. The slowly, adiabatic inspiral of these
sources will allow us to map space-time around MBHs in detail, as well as to
test our current conception of gravitation in the strong regime. The event rate
of this kind of source has been addressed many times in the literature and the
numbers reported fluctuate by orders of magnitude. On the other hand, recent
observations of the Galactic center revealed a dearth of giant stars inside the
inner parsec relative to the numbers theoretically expected for a fully relaxed
stellar cusp. The possibility of unrelaxed nuclei (or, equivalently, with no or
only a very shallow cusp) adds substantial uncertainty to the estimates. Having
this timely question in mind, we run a significant number of direct-summation
body simulations with up to half a million particles to calibrate a much
faster orbit-averaged Fokker-Planck code. We then investigate the regime of
strong mass segregation (SMS) for models with two different stellar mass
components. We show that, under quite generic initial conditions, the time
required for the growth of a relaxed, mass segregated stellar cusp is shorter
than a Hubble time for MBHs with
(i.e. nuclei in the range of LISA). SMS has a significant impact boosting the
EMRI rates by a factor of for our fiducial models of Milky Way type
galactic nuclei.Comment: Accepted by CQG, minor changes, a bit expande
Identification of a novel anti-σE factor in Neisseria meningitidis
<p>Abstract</p> <p>Background</p> <p>Fine tuning expression of genes is a prerequisite for the strictly human pathogen <it>Neisseria meningitidis </it>to survive hostile growth conditions and establish disease. Many bacterial species respond to stress by using alternative σ factors which, in complex with RNA polymerase holoenzyme, recognize specific promoter determinants. σ<sup>E</sup>, encoded by <it>rpoE </it>(NMB2144) in meningococci, is known to be essential in mounting responses to environmental challenges in many pathogens. Here we identified genes belonging to the σ<sup>E </sup>regulon of meningococci.</p> <p>Results</p> <p>We show that meningococcal σ<sup>E </sup>is part of the polycistronic operon NMB2140-NMB2145 and autoregulated. In addition we demonstrate that σ<sup>E </sup>controls expression of methionine sulfoxide reductase (MsrA/MsrB). Moreover, we provide evidence that the activity of σ<sup>E </sup>is under control of NMB2145, directly downstream of <it>rpoE</it>. The protein encoded by NMB2145 is structurally related to anti-sigma domain (ASD) proteins and characterized by a zinc containing anti-σ factor (ZAS) motif, a hall mark of a specific class of Zn<sup>2+-</sup>binding ASD proteins acting as anti-σ factors. We demonstrate that Cys residues in ZAS, as well as the Cys residue on position 4, are essential for anti-σ<sup>E </sup>activity of NMB2145, as found for a minority of members of the ZAS family that are predicted to act in the cytoplasm and responding to oxidative stimuli. However, exposure of cells to oxidative stimuli did not result in altered expression of σ<sup>E</sup>.</p> <p>Conclusions</p> <p>Together, our results demonstrate that meningococci express a functional transcriptionally autoregulated σ<sup>E </sup>factor, the activity of which is controlled by a novel meningococcal anti-σ factor belonging to the ZAS family.</p
The origin of variability of the intermediate-mass black-hole ULX system HLX-1 in ESO 243-49
The ultra-luminous intermediate-mass black-hole system HLX-1 in the ESO
243-49 galaxy exhibits variability with a possible recurrence time of a few
hundred days. Finding the origin of this variability would constrain the still
largely unknown properties of this extraordinary object. Since it exhibits an
intensity-hardness behavior characteristic of black-hole X-ray transients, we
have analyzed the variability of HLX-1 in the framework of the disk instability
model that explains outbursts of such systems. We find that the long-term
variability of HLX-1 is unlikely to be explained by a model in which outbursts
are triggered by thermal-viscous instabilities in an accretion disc. Possible
alternatives include the instability in a radiation-pressure dominated disk but
we argue that a more likely explanation is a modulated mass-transfer due to
tidal stripping of a star on an eccentric orbit around the intermediate-mass
black hole. We consider an evolutionary scenario leading to the creation of
such a system and estimate the probability of its observation. We conclude,
using a simplified dynamical model of the post-collapse cluster, that no more
than 1/100 to 1/10 of Mbh < 10^4 Msun IMBHs - formed by run-away stellar
mergers in the dense collapsed cores of young clusters - could have a few times
1 Msun Main-Sequence star evolve to an AGB on an orbit eccentric enough for
mass transfer at periapse, while avoiding collisional destruction or being
scattered into the IMBH by 2-body encounters. The finite but low probability of
this configuration is consistent with the uniqueness of HLX-1. We note,
however, that the actual response of a standard accretion disk to bursts of
mass transfer may be too slow to explain the observations unless the orbit is
close to parabolic (and hence even rarer) and/or additional heating, presumably
linked to the highly time-dependent gravitational potential, are invoked.Comment: 8 pages, 2 figures. Additional figure, extended discussion. To be
published in ApJ, June 10, 2011, v734 -
Understanding the importance of transient resonances in extreme mass ratio inspirals
Extreme mass ratio inspirals (EMRIs) occur when a compact object orbits a
much larger one, like a solar-mass black hole around a supermassive black hole.
The orbit has 3 frequencies which evolve through the inspiral. If the orbital
radial frequency and polar frequency become commensurate, the system passes
through a transient resonance. Evolving through resonance causes a jump in the
evolution of the orbital parameters. We study these jumps and their impact on
EMRI gravitational-wave detection. Jumps are smaller for lower eccentricity
orbits; since most EMRIs have small eccentricities when passing through
resonances, we expect that the impact on detection will be small. Neglecting
the effects of transient resonances leads to a loss of ~4% of detectable
signals for an astrophysically motivated population of EMRIs.Comment: 2 pages, 0 figures; to appear in the proceedings of the 11th
International LISA Symposiu
Stellar Dynamics and Black Holes
Chandrasekhar's most important contribution to stellar dynamics was the
concept of dynamical friction. I briefly review that work, then discuss some
implications of Chandrasekhar's theory of gravitational encounters for motion
in galactic nuclei.Comment: Talk presented at the "Chandrasekhar Centenary Conference" (2010
Detection of chromosome aberrations in the human interphase nucleus by visualization of specific target DNAs with radioactive and non-radioactive in situ hybridization techniques: diagnosis of trisomy 18 with probe L1.84
The localization of chromosome 18 in human interphase nuclei is demonstrated by use of radioactive and nonradioactive in situ hybridization techniques with a DNA clone designated L1.84. This clone represents a distinct subpopulation of the repetitive human alphoid DNA family, located in the centric region of chromosome 18. Under stringent hybridization conditions hybridization of L1.84 is restricted to chromosome 18 and reflects the number of these chromosomes present in the nuclei, namely, two in normal diploid human cells and three in nuclei from cells with trisomy 18. Under conditions of low stringency, cross-hybridization with other subpopulations of the alphoid DNA family occurs in the centromeric regions of the whole chromosome complement, and numerous hybridization sites are detected over interphase nuclei. Detection of chromosome-specific target DNAs by non-radioactive in situ hybridization with appropriate DNA probes cloned from individual chromosomal subregions presents a rapid means of identifying directly numerical or even structural chromosome aberrations in the interphase nucleus. Present limitations and future applications of interphase cytogenetics are discussed
A symmetry-preserving second-order time-accurate PISO-based method
A new conservative symmetry-preserving second-order time-accurate PISO-based pressure-velocity coupling for solving the incompressible Navier-Stokes equations on unstructured collocated grids is presented in this paper. This new method for implicit time stepping is an extension of the conservative symmetry-preserving incremental-pressure projection method for explicit time stepping and unstructured collocated meshes of Trias et al. [35]. In order to assess and compare both methods, we have implemented them within one unified solver in the open source code OpenFOAM where we use a Butcher array to prescribe the Runge-Kutta method. Thus, by changing the entries of the Butcher array, explicit and diagonally implicit Runge-Kutta schemes can be combined into one solver. We assess the energy conservation properties of the implemented discretisation methods and the temporal consistency of the selected Runge-Kutta schemes using Taylor-Green vortex and lid-driven cavity flow test cases. Finally, we use a more complex turbulent channel flow test case in order to further assess the performance of the presented new conservative symmetry-preserving incremental-pressure PISO-based method. Although both implemented methods are based on a symmetry-preserving discretisation, we show they still produce a small amount of numerical dissipation when the total pressure is directly solved from a Poisson equation. When an incremental-pressure approach is used, where a pressure correction is solved from a Poisson equation, both methods are effectively fully-conservative. For high-fidelity simulations of incompressible turbulent flows, it is highly desirable to use fully-conservative methods. For such simulations, the presented numerical methods are therefore expected to have large added value, since they pave the way for the execution of truly energy-conservative high-fidelity simulations in complex geometries. Furthermore, both methods are implemented in OpenFOAM, which is widely used within the CFD community, so that a large part of this community can benefit from the developed and implemented numerical methods
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