98 research outputs found
Relative entropy via non-sequential recursive pair substitutions
The entropy of an ergodic source is the limit of properly rescaled 1-block
entropies of sources obtained applying successive non-sequential recursive
pairs substitutions (see P. Grassberger 2002 ArXiv:physics/0207023 and D.
Benedetto, E. Caglioti and D. Gabrielli 2006 Jour. Stat. Mech. Theo. Exp. 09
doi:10.1088/1742.-5468/2006/09/P09011). In this paper we prove that the cross
entropy and the Kullback-Leibler divergence can be obtained in a similar way.Comment: 13 pages , 2 figure
The Anomalous Early Afterglow of GRB 050801
The ROTSE-IIIc telescope at the H.E.S.S. site, Namibia, obtained the earliest
detection of optical emission from a Gamma-Ray Burst (GRB), beginning only 21.8
s from the onset of Swift GRB 050801. The optical lightcurve does not fade or
brighten significantly over the first ~250 s, after which there is an
achromatic break and the lightcurve declines in typical power-law fashion. The
Swift/XRT also obtained early observations starting at 69 s after the burst
onset. The X-ray lightcurve shows the same features as the optical lightcurve.
These correlated variations in the early optical and X-ray emission imply a
common origin in space and time. This behavior is difficult to reconcile with
the standard models of early afterglow emission.Comment: 5 pages, 1 figure. Accepted for publication in ApJ Letter
Turbulent Magnetic Field Amplification from Spiral SASI Modes: Implications for Core-Collapse Supernovae and Proto-Neutron Star Magnetization
We extend our investigation of magnetic field evolution in three-dimensional
flows driven by the stationary accretion shock instability (SASI) with a suite
of higher-resolution idealized models of the post-bounce core-collapse
supernova environment. Our magnetohydrodynamic simulations vary in initial
magnetic field strength, rotation rate, and grid resolution. Vigorous
SASI-driven turbulence inside the shock amplifies magnetic fields
exponentially; but while the amplified fields reduce the kinetic energy of
small-scale flows, they do not seem to affect the global shock dynamics. The
growth rate and final magnitude of the magnetic energy are very sensitive to
grid resolution, and both are underestimated by the simulations. Nevertheless
our simulations suggest that neutron star magnetic fields exceeding G
can result from dynamics driven by the SASI, \emph{even for non-rotating
progenitors}.Comment: 28 pages, 17 figures, accepted for publication in the Ap
Probing the Core-Collapse Supernova Mechanism with Gravitational Waves
The mechanism of core-collapse supernova explosions must draw on the energy
provided by gravitational collapse and transfer the necessary fraction to the
kinetic and internal energy of the ejecta. Despite many decades of concerted
theoretical effort, the detailed mechanism of core-collapse supernova
explosions is still unknown, but indications are strong that multi-D processes
lie at its heart. This opens up the possibility of probing the supernova
mechanism with gravitational waves, carrying direct dynamical information from
the supernova engine deep inside a dying massive star. I present a concise
overview of the physics and primary multi-D dynamics in neutrino-driven,
magnetorotational, and acoustically-driven core-collapse supernova explosion
scenarios. Discussing and contrasting estimates for the gravitational-wave
emission characteristics of these mechanisms, I argue that their
gravitational-wave signatures are clearly distinct and that the observation (or
non-observation) of gravitational waves from a nearby core-collapse event could
put strong constraints on the supernova mechanism.Comment: 13 pages, 5 figures. Submitted to the special issue of Class. Quant.
Grav. for the 13th Gravitational Wave Data Analysis Workshop (GWDAW13). A
version with high-resolution figures is available from
http://stellarcollapse.org/papers/OTT_gwdaw13.pd
Multi-wavelength Observations of the Type IIb Supernova 2009mg
We present Swift UVOT and XRT observations, and visual wavelength
spectroscopy of the Type IIb supernova (SN) 2009mg, discovered in the Sb galaxy
ESO 121-G26. The observational properties of SN 2009mg are compared to the
prototype Type IIb SNe 1993J and 2008ax, with which we find many similarities.
However, minor differences are discernible including SN 2009mg not exhibiting
an initial fast decline or u-band upturn as observed in the comparison objects,
and its rise to maximum is somewhat slower leading to slightly broader light
curves. The late-time temporal index of SN 2009mg, determined from 40 days
post-explosion, is consistent with the decay rate of SN 1993J, but inconsistent
with the decay of 56Co. This suggests leakage of gamma-rays out of the ejecta
and a stellar mass on the small side of the mass distribution. Our XRT
non-detection provides an upper limit on the mass-loss rate of the progenitor
of <1.5x10^-5 Msun per yr. Modelling of the SN light curve indicates a kinetic
energy of 0.15 (+0.02,-0.13) x10^51 erg, an ejecta mass of 0.56(+0.10,-0.26)
Msun and a 56Ni mass of 0.10\pm0.01 Msun.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Evolution and Nucleosynthesis of Very Massive Stars
In this chapter, after a brief introduction and overview of stellar
evolution, we discuss the evolution and nucleosynthesis of very massive stars
(VMS: M>100 solar masses) in the context of recent stellar evolution model
calculations. This chapter covers the following aspects: general properties,
evolution of surface properties, late central evolution, and nucleosynthesis
including their dependence on metallicity, mass loss and rotation. Since very
massive stars have very large convective cores during the main-sequence phase,
their evolution is not so much affected by rotational mixing, but more by mass
loss through stellar winds. Their evolution is never far from a homogeneous
evolution even without rotational mixing. All VMS at metallicities close to
solar end their life as WC(-WO) type Wolf-Rayet stars. Due to very important
mass loss through stellar winds, these stars may have luminosities during the
advanced phases of their evolution similar to stars with initial masses between
60 and 120 solar masses. A distinctive feature which may be used to disentangle
Wolf-Rayet stars originating from VMS from those originating from lower initial
masses is the enhanced abundances of neon and magnesium at the surface of WC
stars. At solar metallicity, mass loss is so strong that even if a star is born
with several hundred solar masses, it will end its life with less than 50 solar
masses (using current mass loss prescriptions). At the metallicity of the LMC
and lower, on the other hand, mass loss is weaker and might enable star to
undergo pair-instability supernovae.Comment: 42 pages, 20 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
Exploring Broadband GRB Behavior During gamma-ray Emission
The robotic ROTSE-III telescope network detected prompt optical emission
contemporaneous with the gamma-ray emission of Swift events GRB051109A and
GRB051111. Both datasets have continuous coverage at high signal-to-noise
levels from the prompt phase onwards, thus the early observations are readily
compared to the Swift XRT and BAT high energy detections. In both cases, the
optical afterglow is established, declining steadily during the prompt
emission. For GRB051111, there is evidence of an excess optical component
during the prompt emission. The component is consistent with the flux
spectrally extrapolated from the gamma-rays, using the gamma-ray spectral
index. A compilation of spectral information from previous prompt detections
shows that such a component is unusual. The existence of two prompt optical
components - one connected to the high-energy emission, the other to separate
afterglow flux, as indicated in GRB051111 - is not compatible with a simple
``external-external'' shock model for the GRB and its afterglow.Comment: ApJ accepted. 32 pages (in preprint form), 5 tables, 5 figure
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
Gravitational Waves from Gravitational Collapse
Gravitational wave emission from the gravitational collapse of massive stars
has been studied for more than three decades. Current state of the art
numerical investigations of collapse include those that use progenitors with
realistic angular momentum profiles, properly treat microphysics issues,
account for general relativity, and examine non--axisymmetric effects in three
dimensions. Such simulations predict that gravitational waves from various
phenomena associated with gravitational collapse could be detectable with
advanced ground--based and future space--based interferometric observatories.Comment: 68 pages including 13 figures; revised version accepted for
publication in Living Reviews in Relativity (http://www.livingreviews.org
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