1,949 research outputs found
Spherical non-linear absorption of cosmological scalar fields onto a black hole
In this paper we track the non-linear spherical evolution of a massless
scalar field onto a Schwarzschild black hole space-time as a first
approximation to the accretion of cosmologically motivated classical scalar
fields. We perform an analysis related to wave packets described by wave number
and width. We study various values of the wave number k, and found that for k =
0 and width packets bigger than the Schwarzschild radius, the absorption is not
total. In the cases we studied for k > 0, the black hole absorbs the total
amount of energy density of the scalar field moving toward the horizon. Our
results indicate that assuming spherical symmetry, in the non-linear regime,
there are cases for which scalar fields are allowed to survive outside black
holes and may eventually have life-times consistent with cosmological time
scales.Comment: 7 revtex pages, accepted for publication in Phys. Rev.
Occurrence of Mixed Phase in Bi0.5Sr0.5Mn0.9Cr0.1O3 bulk sample: Electron Paramagnetic Resonance and Magnetization Studies
We study the effects of 10% Cr substitution in Mn sites of Bi0.5Sr0.5MnO3 on
the antiferromagnetic (AFM) (TN ~ 110 K) transition using structural, magnetic
and electron paramagnetic resonance (EPR) techniques. Field cooled (FC) and
zero field cooled (ZFC) magnetization measurements done from 400 K down to 4 K
show that the compound is in the paramagnetic (PM) phase till 50 K where it
undergoes a transition to a short range ferromagnetic phase (FM). Electron
paramagnetic resonance measurements performed in the temperature range 300 K
till 80 K conform with the magnetization measurements as symmetric signals are
observed owing to the paramagnetic phase. Below 80 K, signals become
asymmetric. Electron paramagnetic resonance intensity peaks at ~ 110 K, the
decreasing intensity below this temperature confirming the presence of
antiferromagnetism. We conclude that below 50 K the magnetization and EPR
results are consistent with a cluster glass phase of BSMCO, where ferromagnetic
clusters coexist with an antiferromagnetic background
Rotation curves of ultralight BEC dark matter halos with rotation
We study the rotation curves of ultralight BEC dark matter halos. These halos
are long lived solutions of initially rotating BEC fluctuations. In order to
study the implications of the rotation characterizing these long-lived
configurations we consider the particular case of a boson mass
and no self-interaction. We find that these halos
successfully fit samples of rotation curves (RCs) of LSB galaxies.Comment: 7 pages, 10 eps figures, 1 tables. Accepted for publication in
General Relativity and Gravitatio
Cold, old and metal-poor: New stellar substructures in the Milky Way's dwarf spheroidals
Dwarf spheroidal galaxies (dSph) orbiting the Milky Way are complex objects
often with complicated star formation histories and internal dynamics. In this
work, we search for stellar substructures in four of the classical dSph
satellites of the Milky Way: Sextans, Carina, Leo I, and Leo II. We apply two
methods to search for stellar substructure: the minimum spanning tree method,
which helps us to find and quantify spatially connected structures, and the
"brute-force" method, which is able to find elongated stellar substructures. We
detected the previously known substructure in Sextans, and also found a new
stellar substructure within Sextans. Furthermore, we identified a new stellar
substructure close to the core radius of the Carina dwarf galaxy. We report a
detection of one substructure in Leo I and two in Leo II, but we note that we
are dealing with a low number of stars in the samples used. Such old stellar
substructures in dSph galaxies could help us to shed light on the nature of the
dark matter halos, within which such structures form, evolve, and survive.Comment: Accepted for publication in ApJ, 13 pages, 6 figure
Is the flip-flop behaviour of accretion shock cones on to black holes an effect of coordinates?
We study numerically the relativistic Bondi-Hoyle accretion of an ideal gas
onto a Kerr fixed background space-time on the equatorial plane with s-lab
symmetry. We use both Kerr-Schild (KS) and Boyer-Lindquist (BL) coordinates. We
particularly focus on the study of the flip-flop motion of the shock cone
formed when the gas is injected at supersonic speed. The development of the
flip-flop instability of the shock cone in the relativistic regime was reported
recently for the first time. We reproduce the flip-flop behaviour found in the
past when BL coordinates are used, and perform similar numerical experiments
using horizon penetrating KS coordinates. We find that when using KS
coordinates the shock cone oscillates, however such oscillations are not of the
flip-flop type and their amplitude decrease with resolution.Comment: 8 pages, 9 eps figures, accepted for publication in MNRA
Horizon growth of supermassive black hole seeds fed with collisional dark matter
We present the accretion of collisional dark matter on a supermassive black
hole seed. The analysis is based on the numerical solution of the fully coupled
system of Einstein-Euler equations for spherically symmetric flow, where the
dark matter is modeled as a perfect fluid that obeys an ideal gas equation of
state. As the black hole actually grows, the accretion rate of dark matter
corresponds to the black hole apparent horizon growth rate. We analyse cases
with infall velocity as high as and an environment density of
, which are rather extreme conditions. Being the
radial flux the maximum accretion case, our results show that the accretion of
an ideal gas, eventually collisional dark matter, does not contribute
significantly to SMBH masses. This result favors models predicting SMBHs were
formed already with supermasses. We show that despite the fact that we are
solving the full general relativistic system, for the parameter space studied
our results are surprisingly similar to those obtained using the Bondi formula,
which somehow certifies its use as a good approximation of a fully evolving
space-time with spherical symmetry at short scales at least for dark matter
densities. Additionally, we study the density profile of the gas and find that
the presence of SMBHs redistributes the gas near the event horizon with a cuspy
profile, whereas beyond a small fraction of a parsec it is not-cuspy anymore.Comment: 11 pages, 6 eps figures, 3 tables. Accepted for publication in MNRA
Evolution of a mass-less test scalar field on Boson Stars space-times
We numerically solve the mass-less test scalar field equation on the
space-time background of boson stars and black holes. In order to do so, we use
a numerical domain that contains future null infinity. We achieve this
construction using a scri-fixing conformal compactification technique based on
hyperboloidal constant mean curvature foliations of the space-time and solve
the conformally invariant wave equation. We present two results: the scalar
field shows oscillations of the quasi- normal-mode type found for black holes
only for boson star configurations that are compact, and no signs of tail decay
is found in the parameter space we explored. Even though our results do not
correspond to the master equation of perturbations of boson star solutions,
they indicate that the parameter space of boson stars as black hole mimickers
is restricted to compact configurations.Comment: 9 pages, 15 eps figures, revtex
CAFE: A New Relativistic MHD Code
We introduce CAFE, a new independent code designed to solve the equations of
Relativistic ideal Magnetohydrodynamics (RMHD) in 3D. We present the standard
tests for a RMHD code and for the Relativistic Hydrodynamics (RHD) regime since
we have not reported them before. The tests include the 1D Riemann problems
related to blast waves, head-on collision of streams and states with transverse
velocities, with and without magnetic field, which is aligned or transverse,
constant or discontinuous across the initial discontinuity. Among the 2D and 3D
tests, without magnetic field we include the 2D Riemann problem, a one
dimensional shock tube along a diagonal, the high speed Emery wind tunnel, the
Kelvin-Helmholtz instability, a set of jets and a 3D spherical blast wave,
whereas in the presence of a magnetic field we show the magnetic rotor, the
cylindrical explosion, a case of Kelvin-Helmholtz instability and a 3D magnetic
field advection loop. The code uses High Resolution Shock Capturing methods and
we present the error analysis for a combination that uses the HLLE flux formula
combined with linear, PPM and fifth order WENO reconstructors. We use the
flux-CT and the divergence cleaning methods to control the divergence free
magnetic field constraint.Comment: 30 pages, 110 png figures, 4 tables. Accepted for publication in the
Astrophysical Journal Supplement. More numerical details, tests and
additional reference
Revisiting spherically symmetric relativistic hydrodynamics
In this paper we revise two classical examples of Relativistic Hydrodynamics
in order to illustrate in detail the numerical methods commonly used in fluid
dynamics, specifically those designed to deal with shocks, which are based on a
finite volume approximation. The two cases we consider are the relativistic
blast wave problem and the evolution of a Tolman-Oppenheimer-Volkoff star
model, in spherical symmetry. In the first case we illustrate the
implementation of relativistic Euler's equations on a fixed background
space-time, whereas in the second case we also show how to couple the evolution
of the fluid to the evolution of the space-time.Comment: Prepared with educative purposes, 15 pages, 34 eps figure
PBH mass growth through radial accretion during the radiation dominated era
We model the radial accretion of radiation on Primordial Black Holes (PBH) by
numerically solving Einstein's equations coupled to an ultrarelativistic ideal
gas with equation of state . We calculate the final mass of a black
hole by the integration of the accreted radiation energy density during the
leptonic era between to after the Big Bang. Our
results indicate that small PBHs with initial masses between to
may grow up to hundreds of solar masses, and thus can be SMBH
seeds. On the other hand, PBHs formed at with initial mass between
900 and , by the time show masses of to
which are masses of seeds or already formed SMBHs. The fact
that we consider only radial flow implies that our results work well as
limiting cases, and it is expected that under more general scenarios the
accretion rates may change significantly. Nevertheless we show that it is
possible that SMBHs can be PBHs that grew due to the accretion of radiation.Comment: 15 pages, 6 eps figures. Accepted for publication in JCA
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