216 research outputs found
Energy exchange between relativistic fluids: the polytropic case
We present a simple, analytic and straightforward method to elucidate the
effects produced by polytropic fluids on any other gravitational source, no
matter its nature, for static and spherically symmetric spacetimes. As a direct
application, we study the interaction between polytropes and perfect fluids
coexisting inside a self-gravitating stellar objectComment: 10 pages, 12 figure
Black hole spin inferred from 3:2 epicyclic resonance model of high-frequency quasi-periodic oscillations
Estimations of black hole spin in the three Galactic microquasars GRS
1915+105, GRO J1655-40, and XTE J1550-564 have been carried out based on
spectral and timing X-ray measurements and various theoretical concepts. Among
others, a non-linear resonance between axisymmetric epicyclic oscillation modes
of an accretion disc around a Kerr black hole has been considered as a model
for the observed high-frequency quasi-periodic oscillations (HF QPOs).
Estimates of spin predicted by this model have been derived based on the
geodesic approximation of the accreted fluid motion. Here we assume accretion
flow described by the model of a pressure-supported torus and carry out related
corrections to the mass-spin estimates. We find that for dimensionless black
hole spin a<0.9, the resonant eigenfrequencies are very close to those
calculated for the geodesic motion. Their values slightly grow with increasing
torus thickness. These findings agree well with results of a previous study
carried out in the pseudo-Newtonian approximation. The situation becomes
different for a>0.9, in which case the resonant eigenfrequencies rapidly
decrease as the torus thickness increases. We conclude that the assumed
non-geodesic effects shift the lower limit of the spin, implied for the three
microquasars by the epicyclic model and independently measured masses, from
a~0.7 to a~0.6. Their consideration furthermore confirms compatibility of the
model with the rapid spin of GRS 1915+105 and provides highly testable
predictions of the QPO frequencies. Individual sources with a moderate spin
(a<0.9) should exhibit a smaller spread of the measured 3:2 QPO frequencies
than sources with a near-extreme spin (a~1). This should be further examined
using the large amount of high-resolution data expected to become available
with the next generation of X-ray instruments, such as the proposed Large
Observatory for X-ray Timing (LOFT).Comment: 6 pages, 4 figures, accepted by Astronomy & Astrophysic
Electromagnetic Properties of Kerr-Anti-de Sitter Black Holes
We examine the electromagnetic properties of Kerr-anti-de Sitter (Kerr-AdS)
black holes in four and higher spacetime dimensions. Assuming that the black
holes may carry a test electric charge we show that the Killing one-form which
represents the difference between the timelike generators in the spacetime and
in the reference background can be used as a potential one-form for the
associated electromagnetic field. In four dimensions the potential one-form and
the Kerr-AdS metric with properly re-scaled mass parameter solve the
Einstein-Maxwell equations, thereby resulting in the familiar Kerr-Newman-AdS
solution. We solve the quartic equation governing the location of the event
horizons of the Kerr-Newman-AdS black holes and present closed analytic
expressions for the radii of the horizons. We also compute the gyromagnetic
ratio for these black holes and show that it corresponds to g=2 just as for
ordinary black holes in asymptotically flat spacetime. Next, we compute the
gyromagnetic ratio for the Kerr-AdS black holes with a single angular momentum
and with a test electric charge in all higher dimensions. The gyromagnetic
ratio crucially depends on the dimensionless ratio of the rotation parameter to
the curvature radius of the AdS background. At the critical limit, when the
boundary Einstein universe is rotating at the speed of light, it tends to g=2
irrespective of the spacetime dimension. Finally, we consider the case of a
five dimensional Kerr-AdS black hole with two angular momenta and show that it
possesses two distinct gyromagnetic ratios in accordance with its two
orthogonal 2-planes of rotation. In the special case of two equal angular
momenta, the two gyromagnetic ratios merge into one leading to g=4 at the
maximum angular velocities of rotation.Comment: Typos corrected; 31 pages, REVTe
Isotropization and change of complexity by gravitational decoupling
We employ the gravitational decoupling appro- ach for static and spherically symmetric systems to develop a simple and powerful method in order to (a) continuously isotropize any anisotropic solution of the Einstein field equa- tions, and (b) generate new solutions for self-gravitating dis- tributions with the same or vanishing complexity factor. A few working examples are given for illustrative purposes
Unstable fields in Kerr spacetimes
We show that both the interior region of a Kerr black
hole and the Kerr naked singularity admit unstable solutions of the
Teukolsky equation for any value of the spin weight. For every harmonic number
there is at least one axially symmetric mode that grows exponentially in time
and decays properly in the radial directions. These can be used as Debye
potentials to generate solutions for the scalar, Weyl spinor, Maxwell and
linearized gravity field equations on these backgrounds, satisfying appropriate
spatial boundary conditions and growing exponentially in time, as shown in
detail for the Maxwell case. It is suggested that the existence of the unstable
modes is related to the so called "time machine" region, where the axial
Killing vector field is time-like, and the Teukolsky equation, restricted to
axially symmetric fields, changes its character from hyperbolic to elliptic
Particle motion in the field of a five-dimensional charged black hole
In this paper, we have investigated the geodesics of neutral particles near a
five-dimensional charged black hole using a comparative approach. The effective
potential method is used to determine the location of the horizons and to study
radial and circular trajectories. This also helps us to analyze the stability
of radial and circular orbits. The radius of the innermost stable circular
orbits have also been determined. Contrary to the case of massive particles for
which, the circular orbits may have up to eight possible values of specific
radius, we find that the photons will only have two distinct values for the
specific radii of circular trajectories. Finally we have used the dynamical
systems analysis to determine the critical points and the nature of the
trajectories for the timelike and null geodesics.Comment: 15 pages, accepted for publication in Astrophysics and Space Scienc
Hospital-Level Nicu Capacity, Utilization, and 30-Day Outcomes in Texas
IMPORTANCE: Risk-adjusted neonatal intensive care unit (NICU) utilization and outcomes vary markedly across regions and hospitals. The causes of this variation are poorly understood.
OBJECTIVE: to assess the association of hospital-level NICU bed capacity with utilization and outcomes in newborn cohorts with differing levels of health risk.
DESIGN, SETTING, AND PARTICIPANTS: This population-based retrospective cohort study included all Medicaid-insured live births in Texas from 2010 to 2014 using linked vital records and maternal and newborn claims data. Participants were Medicaid-insured singleton live births (LBs) with birth weights of at least 400 g and gestational ages between 22 and 44 weeks. Newborns were grouped into 3 cohorts: very low birth weight (VLBW; \u3c1500 \u3eg), late preterm (LPT; 34-36 weeks\u27 gestation), and nonpreterm newborns (NPT; ≥37 weeks\u27 gestation). Data analysis was conducted from January 2022 to October 2023.
EXPOSURE: Hospital NICU capacity measured as reported NICU beds/100 LBs, adjusted (ie, allocated) for transfers.
MAIN OUTCOMES AND MEASURES: NICU admissions and special care days; inpatient mortality and 30-day postdischarge adverse events (ie, mortality, emergency department visit, admission, observation stay).
RESULTS: The overall cohort of 874 280 single LBs included 9938 VLBW (5054 [50.9%] female; mean [SD] birth weight, 1028.9 [289.6] g; mean [SD] gestational age, 27.6 [2.6] wk), 63 160 LPT (33 684 [53.3%] female; mean [SD] birth weight, 2664.0 [409.4] g; mean [SD] gestational age, 35.4 [0.8] wk), and 801 182 NPT (407 977 [50.9%] female; mean [SD] birth weight, 3318.7 [383.4] g; mean [SD] gestational age, 38.9 [1.0] wk) LBs. Median (IQR) NICU capacity was 0.84 (0.57-1.30) allocated beds/100 LB/year. For VLBW newborns, NICU capacity was not associated with the risk of NICU admission or number of special care days. For LPT newborns, birth in hospitals with the highest compared with the lowest category of capacity was associated with a 17% higher risk of NICU admission (adjusted risk ratio [aRR], 1.17; 95% CI, 1.01-1.33). For NPT newborns, risk of NICU admission was 55% higher (aRR, 1.55; 95% CI, 1.22-1.97) in the highest- vs the lowest-capacity hospitals. The number of special care days for LPT and NPT newborns was 21% (aRR, 1.21; 95% CI,1.08-1.36) and 37% (aRR, 1.37; 95% CI, 1.08-1.74) higher in the highest vs lowest capacity hospitals, respectively. Among LPT and NPT newborns, NICU capacity was associated with higher inpatient mortality and 30-day postdischarge adverse events.
CONCLUSIONS AND RELEVANCE: In this cohort study of Medicaid-insured newborns in Texas, greater hospital NICU bed supply was associated with increased NICU utilization in newborns born LPT and NPT. Higher capacity was not associated with lower risk of adverse events. These findings raise important questions about how the NICU is used for newborns with lower risk
A two-mass expanding exact space-time solution
In order to understand how locally static configurations around
gravitationally bound bodies can be embedded in an expanding universe, we
investigate the solutions of general relativity describing a space-time whose
spatial sections have the topology of a 3-sphere with two identical masses at
the poles. We show that Israel junction conditions imply that two spherically
symmetric static regions around the masses cannot be glued together. If one is
interested in an exterior solution, this prevents the geometry around the
masses to be of the Schwarzschild type and leads to the introduction of a
cosmological constant. The study of the extension of the Kottler space-time
shows that there exists a non-static solution consisting of two static regions
surrounding the masses that match a Kantowski-Sachs expanding region on the
cosmological horizon. The comparison with a Swiss-Cheese construction is also
discussed.Comment: 15 pages, 5 figures. Replaced to match the published versio
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