368 research outputs found
RAM: A Relativistic Adaptive Mesh Refinement Hydrodynamics Code
We have developed a new computer code, RAM, to solve the conservative
equations of special relativistic hydrodynamics (SRHD) using adaptive mesh
refinement (AMR) on parallel computers. We have implemented a
characteristic-wise, finite difference, weighted essentially non-oscillatory
(WENO) scheme using the full characteristic decomposition of the SRHD equations
to achieve fifth-order accuracy in space. For time integration we use the
method of lines with a third-order total variation diminishing (TVD)
Runge-Kutta scheme. We have also implemented fourth and fifth order Runge-Kutta
time integration schemes for comparison. The implementation of AMR and
parallelization is based on the FLASH code. RAM is modular and includes the
capability to easily swap hydrodynamics solvers, reconstruction methods and
physics modules. In addition to WENO we have implemented a finite volume module
with the piecewise parabolic method (PPM) for reconstruction and the modified
Marquina approximate Riemann solver to work with TVD Runge-Kutta time
integration. We examine the difficulty of accurately simulating shear flows in
numerical relativistic hydrodynamics codes. We show that under-resolved
simulations of simple test problems with transverse velocity components produce
incorrect results and demonstrate the ability of RAM to correctly solve these
problems. RAM has been tested in one, two and three dimensions and in
Cartesian, cylindrical and spherical coordinates. We have demonstrated
fifth-order accuracy for WENO in one and two dimensions and performed detailed
comparison with other schemes for which we show significantly lower convergence
rates. Extensive testing is presented demonstrating the ability of RAM to
address challenging open questions in relativistic astrophysics.Comment: ApJS in press, 21 pages including 18 figures (6 color figures
A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic WR Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scale spectroscopic
variability in apparently single Wolf-Rayet stars brighter than ~12.5. In this
first paper we characterize the various forms of variability detected and
distinguish several separate groups. For each star in our sample, we obtained
4-5 high-resolution spectra with a signal-to-noise ratio ~100. Our ultimate
goal is to identify new candidates presenting variability that potentially
comes from Co-rotating Interaction Regions (CIR).
Out of a sample of 25 stars, 10 were found to display large-scale changes of
which 4 are of CIR-type (WR1, WR115, WR120 and WR134). The star WR134 was
already known to show such changes from previous studies. Three WN8 stars
present a different type of large-scale variability and we believe deserve a
group of their own. Also, all three WC9d stars in our sample present
large-scale variability, but it remains to be checked if these are binaries, as
many dust-making WR stars are double. Finally, of the remaining stars, 10 were
found to show small-amplitude spectral changes which we attribute to normal
line-profile variability due to inhomogeneities in the wind, and 5 were found
to show no spectral variability, as far as can be concluded from the data in
hand.
Follow-up studies are required to identify potential periods for our
candidates showing CIR-type changes and eventually estimate a rotation rate for
these WR stars.Comment: 25 pages, 13 figure
Flavor conversion of cosmic neutrinos from hidden jets
High energy cosmic neutrino fluxes can be produced inside relativistic jets
under the envelopes of collapsing stars. In the energy range E ~ (0.3 - 1e5)
GeV, flavor conversion of these neutrinos is modified by various matter effects
inside the star and the Earth. We present a comprehensive (both analytic and
numerical) description of the flavor conversion of these neutrinos which
includes: (i) oscillations inside jets, (ii) flavor-to-mass state transitions
in an envelope, (iii) loss of coherence on the way to observer, and (iv)
oscillations of the mass states inside the Earth. We show that conversion has
several new features which are not realized in other objects, in particular
interference effects ("L- and H- wiggles") induced by the adiabaticity
violation. The neutrino-neutrino scattering inside jet and inelastic neutrino
interactions in the envelope may produce some additional features at E > 1e4
GeV. We study dependence of the probabilities and flavor ratios in the
matter-affected region on angles theta13 and theta23, on the CP-phase delta, as
well as on the initial flavor content and density profile of the star. We show
that measurements of the energy dependence of the flavor ratios will, in
principle, allow to determine independently the neutrino and astrophysical
parameters.Comment: 56 pages, 19 figures. Minor changes. Accepted by JHEP
Neutrino Spectra from Accretion Disks: Neutrino General Relativistic Effects and the Consequences for Nucleosynthesis
Black hole accretion disks have been proposed as good candidates for a range
of interesting nucleosynthesis, including the r-process. The presence of the
black hole influences the neutrino fluxes and affects the nucleosynthesis
resulting from the interaction of the emitted neutrinos and hot outflowing
material ejected from the disk. We study the impact of general relativistic
effects on the neutrinos emitted from black hole accretion disks. We present
abundances obtained by considering null geodesics and energy shifts for two
different disk models. We find that both the bending of the neutrino
trajectories and the energy shifts have important consequences for the
nucleosynthetic outcomeComment: 18 pages, 17 figures, submitted to Ap
Can a Large Neutron Excess Help Solve the Baryon Loading Problem in Gamma-Ray Burst Fireballs?
We point out that the baryon-loading problem in Gamma-Ray Burst (GRB) models
can be amelioriated if a significant fraction of the baryons which inertially
confine the fireball are converted to neutrons. A high neutron fraction in some
circumstances can result in a reduced transfer of energy from relativistic
light particles in the fireball to baryons. The energy needed to produce the
required relativistic flow in the GRB is consequently reduced, in some cases by
orders of magnitude. This could be relevant to GRB models because a high
neutron-to-proton ratio has been calculated in neutron star-merger fireball
environments. Significant neutron excess also could occur near compact objects
with high neutrino fluxes.Comment: 5 pages, 2 figures, to appear in Phys. Rev. Let
EM counterparts of recoiling black holes: general relativistic simulations of non-Keplerian discs
We investigate the dynamics of a circumbinary disc that responds to the loss
of mass and to the recoil velocity of the black hole produced by the merger of
a binary system of supermassive black holes. We perform the first
two-dimensional general relativistic hydrodynamics simulations of
\textit{extended} non-Keplerian discs and employ a new technique to construct a
"shock detector", thus determining the precise location of the shocks produced
in the accreting disc by the recoiling black hole. In this way we can study how
the properties of the system, such as the spin, mass and recoil velocity of the
black hole, affect the mass accretion rate and are imprinted on the
electromagnetic emission from these sources. We argue that the estimates of the
bremsstrahlung luminosity computed without properly taking into account the
radiation transfer yield cooling times that are unrealistically short. At the
same time we show, through an approximation based on the relativistic
isothermal evolution, that the luminosity produced can reach a peak value above
at about after the merger
of a binary with total mass and persist for several days
at values which are a factor of a few smaller. If confirmed by more
sophisticated calculations such a signal could indeed lead to an
electromagnetic counterpart of the merger of binary black-hole system.Comment: 17 pages, 11 figures, accepted by A&A, movies available at
http://numrel.aei.mpg.de/Visualisations/Archive/BinaryBlackHoles/EMCounterparts/EMCounterparts.htm
Gamma Ray Bursts as Probes of Quantum Gravity
Gamma ray bursts (GRBs) are short and intense pulses of -rays
arriving from random directions in the sky. Several years ago Amelino-Camelia
et al. pointed out that a comparison of time of arrival of photons at different
energies from a GRB could be used to measure (or obtain a limit on) possible
deviations from a constant speed of light at high photons energies. I review
here our current understanding of GRBs and reconsider the possibility of
performing these observations.Comment: Lectures given at the 40th winter school of theretical physics:
Quantum Gravity and Phenomenology, Feb. 2004 Polan
The electromagnetic model of Gamma Ray Bursts
I describe electromagnetic model of gamma ray bursts and contrast its main
properties and predictions with hydrodynamic fireball model and its
magnetohydrodynamical extension. The electromagnetic model assumes that
rotational energy of a relativistic, stellar-mass central source
(black-hole--accretion disk system or fast rotating neutron star) is converted
into magnetic energy through unipolar dynamo mechanism, propagated to large
distances in a form of relativistic, subsonic, Poynting flux-dominated wind and
is dissipated directly into emitting particles through current-driven
instabilities. Thus, there is no conversion back and forth between internal and
bulk energies as in the case of fireball model. Collimating effects of magnetic
hoop stresses lead to strongly non-spherical expansion and formation of jets.
Long and short GRBs may develop in a qualitatively similar way, except that in
case of long bursts ejecta expansion has a relatively short, non-relativistic,
strongly dissipative stage inside the star. Electromagnetic and fireball models
(as well as strongly and weakly magnetized fireballs) lead to different early
afterglow dynamics, before deceleration time. Finally, I discuss the models in
view of latest observational data in the Swift era.Comment: solicited contribution to Focus Issue of New Journal of Physics, 27
pages, 4 figure
A multi-colour study of the dark GRB 000210 host galaxy and its environment
We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark
gamma-ray burst (GRB) of February 10, 2000. These observations represent the
most exhaustive photometry given to date of any GRB host galaxy. A grid of
spectral templates have been fitted to the Spectral Energy Distribution (SED)
of the host. The derived photometric redshift is z=0.842^+0.054_-0.042, which
is in excellent agreement with the spectroscopic redshift (z=0.8463+/-0.0002)
proposed by Piro et al. (2002) based on a single emission line. Furthermore, we
have determined the photometric redshift of all the galaxies in an area of
6'x6' around the host galaxy, in order to check for their overdensity in the
environment of the host. We find that the GRB 000210 host galaxy is a
subluminous galaxy (L ~ 0.5+/-0.2 L*), with no companions above our detection
threshold of 0.18+/-0.06 L*. Based on the restframe ultraviolet flux a star
formation rate of 2.1+/-0.2 Solar Masses per year is estimated. The best fit to
the SED is obtained for a starburst template with an age of 0.181^+0.037_-0.026
Gyr and a very low extinction (Av~0). We discuss the implications of the
inferred low value of Av and the age of the dominant stellar population for the
non detection of the GRB 000210 optical afterglow.Comment: 10 pages with 4 encapsulated PostScript figures included. Accepted
for publication in Astronomy & Astrophysic
A Relativistic Type Ibc Supernova Without a Detected Gamma-ray Burst
Long duration gamma-ray bursts (GRBs) mark the explosive death of some
massive stars and are a rare sub-class of Type Ibc supernovae (SNe Ibc). They
are distinguished by the production of an energetic and collimated relativistic
outflow powered by a central engine (an accreting black hole or neutron star).
Observationally, this outflow is manifested in the pulse of gamma-rays and a
long-lived radio afterglow. To date, central engine-driven SNe have been
discovered exclusively through their gamma-ray emission, yet it is expected
that a larger population goes undetected due to limited satellite sensitivity
or beaming of the collimated emission away from our line-of-sight. In this
framework, the recovery of undetected GRBs may be possible through radio
searches for SNe Ibc with relativistic outflows. Here we report the discovery
of luminous radio emission from the seemingly ordinary Type Ibc SN 2009bb,
which requires a substantial relativistic outflow powered by a central engine.
The lack of a coincident GRB makes SN 2009bb the first engine-driven SN
discovered without a detected gamma-ray signal. A comparison with our extensive
radio survey of SNe Ibc reveals that the fraction harboring central engines is
low, ~1 percent, measured independently from, but consistent with, the inferred
rate of nearby GRBs. Our study demonstrates that upcoming optical and radio
surveys will soon rival gamma-ray satellites in pinpointing the nearest
engine-driven SNe. A similar result for a different supernova is reported
independently.Comment: To appear in Nature on Jan 28 2010. Embargoed for discussion in the
press until 13:00 US Eastern Time on Jan 27 (Accepted version, 27 pages,
Manuscript and Suppl. Info.
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