159 research outputs found
Viscous Torque and Dissipation in the Inner Region of a Thin Accretion Disk: Implications for Measuring Black Hole Spin
We consider a simple Newtonian model of a steady accretion disk around a
black hole. The model is based on height-integrated hydrodynamic equations,
alpha-viscosity, and a pseudo-Newtonian potential that results in an innermost
stable circular orbit (ISCO) that closely approximates the one predicted by GR.
We find that the hydrodynamic models exhibit increasing deviations from the
standard disk model of Shakura & Sunyaev as disk thickness H/R or the value of
alpha increases. The latter is an analytical model in which the viscous torque
is assumed to vanish at the ISCO. We consider the implications of the results
for attempts to estimate black hole spin by using the standard disk model to
fit continuum spectra of black hole accretion disks. We find that the error in
the spin estimate is quite modest so long as H/R < 0.1 and alpha < 0.2. At
worst the error in the estimated value of the spin parameter is 0.1 for a
non-spinning black hole; the error is much less for a rapidly spinning hole. We
also consider the density and disk thickness contrast between the gas in the
disk and that inside the ISCO. The contrast needs to be large if black hole
spin is to be successfully estimated by fitting the relativistically-broadened
X-ray line profile of fluorescent iron emission from reflection off an
accretion disk. In our hydrodynamic models, the contrast in density and
thickness is low when H/R>0.1, sugesting that the iron line technique may be
most reliable in extemely thin disks. We caution that these results have been
obtained with a viscous hydrodynamic model and need to be confirmed with MHD
simulations of radiatively cooled thin disks.Comment: 32 pages, 10 figures; accepted by Ap
Totally laparoscopic combined freehand ileocystoplasty and malone procedures
Background and Purpose: Cecostomy performed together with open enterocystoplasty can allow continent bowel evacuation in children with neurogenic dysfunction. We present the first report on a combined approach to fecal and urinary incontinence in children with myelomeningocele that was performed exclusively by freehand laparoscopy. Patients and Method: We treated six dysrhaphic patients for total urinary and fecal incontinence by laparoscopy. Through five ports, a selected segment of ileum was isolated with cautery. A single-layer intestinal anastomosis, fashioning of the U-shaped patch, and anastomosis to the opened bladder dome were all done by endocorporeal freehand suturing. The tip of the appendix was simply brought to the skin via a trocar site. Results: The procedures took 5 to 8.5 hours. Patients remained hospitalized for 5 to 16 days (median 5 days). At 13 to 16 months' follow-up, all patients remain continent of urine, and nearly perfect fecal continence has resulted on antegrade enema. Leak from the ileal anastomosis in one patient resolved rapidly with conservative management. One short retrocecal appendix later developed stenosis and was replaced by a tubed cecal flap. Conclusion: Apart from its cosmetic advantage, this procedure is notable for addressing all evacuation problems at one session. Our suturing time seems reasonable compared with open sutured precedents. Use of a gastrointestinal stapling device for anastomosis would have significantly increased the cost while not necessarily guaranteeing against complications. We present this laparoscopic combination as an effective alternative to its open counterpart. © Mary Ann Liebert, Inc
Transcriptomics technologies
Transcriptomics technologies are the techniques used to study an organism’s transcriptome, the sum of all of its RNA transcripts. The information content of an organism is recorded in the DNA of its genome and expressed through transcription. Here, mRNA serves as a transient intermediary molecule in the information network, whilst noncoding RNAs perform additional diverse functions. A transcriptome captures a snapshot in time of the total transcripts present in a cell. The first attempts to study the whole transcriptome began in the early 1990s, and technological advances since the late 1990s have made transcriptomics a widespread discipline. Transcriptomics has been defined by repeated technological innovations that transform the field. There are two key contemporary techniques in the field: microarrays, which quantify a set of predetermined sequences, and RNA sequencing (RNA-Seq), which uses high-throughput sequencing to capture all sequences. Measuring the expression of an organism’s genes in different tissues, conditions, or time points gives information on how genes are regulated and reveals details of an organism’s biology. It can also help to infer the functions of previously unannotated genes. Transcriptomic analysis has enabled the study of how gene expression changes in different organisms and has been instrumental in the understanding of human disease. An analysis of gene expression in its entirety allows detection of broad coordinated trends which cannot be discerned by more targeted assays.Rohan Lowe, Neil Shirley, Mark Bleackley, Stephen Dolan, Thomas Shafe
The Race Between Stars and Quasars in Reionizing Cosmic Hydrogen
The cosmological background of ionizing radiation has been dominated by
quasars once the Universe aged by ~2 billion years. At earlier times (redshifts
z>3), the observed abundance of bright quasars declined sharply, implying that
cosmic hydrogen was reionized by stars instead. Here, we explain the physical
origin of the transition between the dominance of stars and quasars as a
generic feature of structure formation in the concordance LCDM cosmology. At
early times, the fraction of baryons in galaxies grows faster than the maximum
(Eddington-limited) growth rate possible for quasars. As a result, quasars were
not able to catch up with the rapid early growth of stellar mass in their host
galaxies.Comment: 5 pages, 1 figure, Accepted for publication in JCA
Can accretion disk properties distinguish gravastars from black holes?
Gravastars, hypothetic astrophysical objects, consisting of a dark energy
condensate surrounded by a strongly correlated thin shell of anisotropic
matter, have been proposed as an alternative to the standard black hole picture
of general relativity. Observationally distinguishing between astrophysical
black holes and gravastars is a major challenge for this latter theoretical
model. In the context of stationary and axially symmetrical geometries, a
possibility of distinguishing gravastars from black holes is through the
comparative study of thin accretion disks around rotating gravastars and
Kerr-type black holes, respectively. In the present paper, we consider
accretion disks around slowly rotating gravastars, with all the metric tensor
components estimated up to the second order in the angular velocity. Due to the
differences in the exterior geometry, the thermodynamic and electromagnetic
properties of the disks (energy flux, temperature distribution and equilibrium
radiation spectrum) are different for these two classes of compact objects,
consequently giving clear observational signatures. In addition to this, it is
also shown that the conversion efficiency of the accreting mass into radiation
is always smaller than the conversion efficiency for black holes, i.e.,
gravastars provide a less efficient mechanism for converting mass to radiation
than black holes. Thus, these observational signatures provide the possibility
of clearly distinguishing rotating gravastars from Kerr-type black holes.Comment: 12 pages, 12 figures. V2: 14 pages, significant discussion and
references added, to appear in Class.Quant.Gra
Modeling of non-stationary accretion disks in X-ray novae A 0620-00 and GRS 1124-68 during outburst
We address the task of modeling soft X-ray and optical light curves of X-ray
novae in the high/soft state. The analytic model of viscous evolution of an
externally truncated accretion \alpha-disk is used. Relativistic effects near a
Kerr black hole and self-irradiation of an accretion disk are taken into
account. The model is applied to the outbursts of X-ray nova Monocerotis 1975
(A 0620-00) and X-ray nova Muscae 1991 (GRS 1124-68). Comparison of
observational data with the model yields constraints on the angular momentum
(the Kerr parameter) of the black holes in A 0620-00 and GRS 1124-68: 0.3-0.6
and \leq 0.4, and on the viscosity parameter \alpha of the disks: 0.7-0.95 and
0.55-0.75. We also conclude that the accretion disks should have an effective
geometrical thickness 1.5-2 times greater than the theoretical value of the
distance between the photometric layers.Comment: 12 pages, 11 figures, 1 table, accepted for publication in A&A (minor
changens following the referee's comments, five references added
Orbital resonances in discs around braneworld Kerr black holes
Rotating black holes in the brany universe of the Randall-Sundrum type are
described by the Kerr geometry with a tidal charge b representing the
interaction of the brany black hole and the bulk spacetime. For b<0 rotating
black holes with dimensionless spin a>1 are allowed. We investigate the role of
the tidal charge b in the orbital resonance model of QPOs in black hole
systems. The orbital Keplerian, the radial and vertical epicyclic frequencies
of the equatorial, quasicircular geodetical motion are given and their radial
profiles are discussed. The resonant conditions are given in three
astrophysically relevant situations: for direct (parametric) resonances, for
the relativistic precession model, and for some trapped oscillations of the
warped discs, with resonant combinational frequencies. It is shown, how b could
influence matching of the observational data indicating the 3:2 frequency ratio
observed in GRS 1915+105 microquasar with prediction of the orbital resonance
model; limits on allowed range of the black hole parameters a and b are
established. The "magic" dimensionless black hole spin enabling presence of
strong resonant phenomena at the radius where \nu_K:\nu_{\theta}:\nu_r=3:2:1 is
determined in dependence on b. Such strong resonances could be relevant even in
sources with highly scattered resonant frequencies, as those expected in Sgr
A*. The specific values of a and b are given also for existence of specific
radius where \nu_K:\nu_{\theta}:\nu_r=s:t:u with 5>=s>t>u being small natural
numbers. It is shown that for some ratios such situation is impossible in the
field of black holes. We can conclude that analysing the microquasars
high-frequency QPOs in the framework of orbital resonance models, we can put
relevant limits on the tidal charge of brany Kerr black holes.Comment: 31 pages, 19 figures, to appear in General Relativity and Gravitatio
Long XMM observation of the Narrow-Line Seyfert 1 galaxy IRAS13224-3809: rapid variability, high spin and a soft lag
Results are presented from a 500ks long XMM-Newton observation of the
Narrow-Line Seyfert 1 galaxy IRAS13224-3809. The source is rapidly variable on
timescales down to a few 100s. The spectrum shows strong broad Fe-K and L
emission features which are interpreted as arising from reflection from the
inner parts of an accretion disc around a rapidly spinning black hole. Assuming
a power-law emissivity for the reflected flux and that the innermost radius
corresponds to the innermost stable circular orbit, the black hole spin is
measured to be 0.988 with a statistical precision better than one per cent.
Systematic uncertainties are discussed. A soft X-ray lag of 100s confirms this
scenario. The bulk of the power-law continuum source is located at a radius of
2-3 gravitational radii.Comment: 7 pages, 14 figures, submitted to MNRA
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