515 research outputs found
The Environments of the Most Energetic Gamma-Ray Bursts
We analyze the properties of a sample of long gamma-ray bursts (LGRBs)
detected by the Fermi satellite that have a spectroscopic redshift and good
follow-up coverage at both X-ray and optical/nIR wavelengths. The evolution of
LGRB afterglows depends on the density profile of the external medium, enabling
us to separate wind or ISM-like environments based on the observations. We do
this by identifying the environment that provides the best agreement between
estimates of , the index of the underlying power-law distribution of
electron energies, as determined by the behavior of the afterglow in different
spectral/temporal regimes. At 11 rest-frame hours after trigger, we find a
roughly even split between ISM-like and wind-like environments. We further find
a 2 separation in the prompt emission energy distributions of wind-like
and ISM-like bursts. We investigate the underlying physical parameters of the
shock, and calculate the (degenerate) product of density and magnetic field
energy (). We show that must be to avoid
implied densities comparable to the intergalactic medium. Finally, we find that
the most precisely constrained observations disagree on by more than would
be expected based on observational errors alone. This suggests additional
sources of error that are not incorporated in the standard afterglow theory.
For the first time, we provide a measurement of this intrinsic error which can
be represented as an error in the estimate of of magnitude .
When this error is included in the fits, the number of LGRBs with an identified
environment drops substantially, but the equal division between the two types
remains.Comment: 31 pages (+14 appendix), 9 figures, 6 tables. Accepted for
publication in Ap
Fallback accretion on to a newborn magnetar : long GRBs with giant X-ray flares
Flares in the X-ray afterglow of gamma-ray bursts (GRBs) share more characteristics with the prompt emission than the afterglow, such as pulse profile and contained fluence. As a result, they are believed to originate from late-time activity of the central engine and can be used to constrain the overall energy budget. In this paper, we collect a sample of 19 long GRBs observed by Swift-XRT that contain giant flares in their X-ray afterglows. We fit this sample with a version of the magnetar propeller model, modified to include fallback accretion. This model has already successfully reproduced extended emission in short GRBs. Our best fits provide a reasonable morphological match to the light curves. However, 16 out of 19 of the fits require efficiencies for the propeller mechanism that approach 100%. The high efficiency parameters are a direct result of the high energy contained in the flares and the extreme duration of the dipole component, which forces either slow spin periods or low magnetic fields. We find that even with the inclusion of significant fallback accretion, in all but a few cases it is energetically challenging to produce prompt emission, afterglow and giant flares within the constraints of the rotational energy budget of a magnetar
A multi-messenger model for neutron star - black hole mergers
We present a semi-analytic model for predicting kilonova light curves from
the mergers of neutron stars with black holes (NSBH). The model is integrated
into the MOSFiT platform, and can generate light curves from input binary
properties and nuclear equation-of-state considerations, or incorporate
measurements from gravitational wave (GW) detectors to perform multi-messenger
parameter estimation. The rapid framework enables the generation of NSBH
kilonova distributions from binary populations, light curve predictions from GW
data, and statistically meaningful comparisons with an equivalent BNS model in
MOSFiT. We investigate a sample of kilonova candidates associated with
cosmological short gamma-ray bursts, and demonstrate that they are broadly
consistent with being driven by NSBH systems, though most have limited data. We
also perform fits to the very well sampled GW170817, and show that the
inability of an NSBH merger to produce lanthanide-poor ejecta results in a
significant underestimate of the early (< 2 days) optical emission. Our model
indicates that NSBH-driven kilonovae may peak up to a week after merger at
optical wavelengths for some observer angles. This demonstrates the need for
early coverage of emergent kilonovae in cases where the GW signal is either
ambiguous or absent; they likely cannot be distinguished from BNS mergers by
the light curves alone from ~2 days after the merger. We also discuss the
detectability of our model kilonovae with the Vera C. Rubin Observatory's
Legacy Survey of Space and Time (LSST).Comment: 14 pages, 6 figures, 2 tables. Accepted for publication in MNRAS.
This is the author's final submitted version. The model code is available
through MOSFiT at https://github.com/guillochon/MOSFi
The Heumann-Hotzel model for aging revisited
Since its proposition in 1995, the Heumann-Hotzel model has remained as an
obscure model of biological aging. The main arguments used against it were its
apparent inability to describe populations with many age intervals and its
failure to prevent a population extinction when only deleterious mutations are
present. We find that with a simple and minor change in the model these
difficulties can be surmounted. Our numerical simulations show a plethora of
interesting features: the catastrophic senescence, the Gompertz law and that
postponing the reproduction increases the survival probability, as has already
been experimentally confirmed for the Drosophila fly.Comment: 11 pages, 5 figures, to be published in Phys. Rev.
Evidence for the Gompertz Curve in the Income Distribution of Brazil 1978-2005
This work presents an empirical study of the evolution of the personal income
distribution in Brazil. Yearly samples available from 1978 to 2005 were studied
and evidence was found that the complementary cumulative distribution of
personal income for 99% of the economically less favorable population is well
represented by a Gompertz curve of the form , where
is the normalized individual income. The complementary cumulative
distribution of the remaining 1% richest part of the population is well
represented by a Pareto power law distribution . This
result means that similarly to other countries, Brazil's income distribution is
characterized by a well defined two class system. The parameters , ,
, were determined by a mixture of boundary conditions,
normalization and fitting methods for every year in the time span of this
study. Since the Gompertz curve is characteristic of growth models, its
presence here suggests that these patterns in income distribution could be a
consequence of the growth dynamics of the underlying economic system. In
addition, we found out that the percentage share of both the Gompertzian and
Paretian components relative to the total income shows an approximate cycling
pattern with periods of about 4 years and whose maximum and minimum peaks in
each component alternate at about every 2 years. This finding suggests that the
growth dynamics of Brazil's economic system might possibly follow a
Goodwin-type class model dynamics based on the application of the
Lotka-Volterra equation to economic growth and cycle.Comment: 22 pages, 15 figures, 4 tables. LaTeX. Accepted for publication in
"The European Physical Journal B
Exact Solution of an Evolutionary Model without Ageing
We introduce an age-structured asexual population model containing all the
relevant features of evolutionary ageing theories. Beneficial as well as
deleterious mutations, heredity and arbitrary fecundity are present and managed
by natural selection. An exact solution without ageing is found. We show that
fertility is associated with generalized forms of the Fibonacci sequence, while
mutations and natural selection are merged into an integral equation which is
solved by Fourier series. Average survival probabilities and Malthusian growth
exponents are calculated indicating that the system may exhibit mutational
meltdown. The relevance of the model in the context of fissile reproduction
groups as many protozoa and coelenterates is discussed.Comment: LaTeX file, 15 pages, 2 ps figures, to appear in Phys. Rev.
The diversity of kilonova emission in short gamma-ray bursts
The historic first joint detection of both gravitational-wave and electromagnetic emission from a binary neutron star merger cemented the association between short gamma-ray bursts (SGRBs) and compact object mergers, as well as providing a well-sampled multi-wavelength light curve of a radioactive kilonova (KN) for the first time. Here, we compare the optical and near-infrared light curves of this KN, AT 2017gfo, to the counterparts of a sample of nearby (z < 0.5) SGRBs to characterize their diversity in terms of their brightness distribution. Although at similar epochs AT 2017gfo appears fainter than every SGRB-associated KN claimed so far, we find three bursts (GRBs 050509B, 061201, and 080905A) where, if the reported redshifts are correct, deep upper limits rule out the presence of a KN similar to AT 2017gfo by several magnitudes. Combined with the properties of previously claimed KNe in SGRBs this suggests considerable diversity in the properties of KN drawn from compact object mergers, despite the similar physical conditions that are expected in many NSâNS mergers. We find that observer angle alone is not able to explain this diversity, which is likely a product of the merger type (NSâNS versus NSâBH) and the detailed properties of the binary (mass ratio, spins etc.). Ultimately disentangling these properties should be possible through observations of SGRBs and gravitational-wave sources, providing direct measurements of heavy element enrichment throughout the universe
âItâs hard to tellâ. The challenges of scoring patients on standardised outcome measures by multidisciplinary teams: a case study of Neurorehabilitation
Background
Interest is increasing in the application of standardised outcome measures in clinical practice. Measures designed for use in research may not be sufficiently precise to be used in monitoring individual patients. However, little is known about how clinicians and in particular, multidisciplinary teams, score patients using these measures. This paper explores the challenges faced by multidisciplinary teams in allocating scores on standardised outcome measures in clinical practice.
Methods
Qualitative case study of an inpatient neurorehabilitation team who routinely collected standardised outcome measures on their patients. Data were collected using non participant observation, fieldnotes and tape recordings of 16 multidisciplinary team meetings during which the measures were recited and scored. Eleven clinicians from a range of different professions were also interviewed. Data were analysed used grounded theory techniques.
Results
We identified a number of instances where scoring the patient was 'problematic'. In 'problematic' scoring, the scores were uncertain and subject to revision and adjustment. They sometimes required negotiation to agree on a shared understanding of concepts to be measured and the guidelines for scoring. Several factors gave rise to this problematic scoring. Team members' knowledge about patients' problems changed over time so that initial scores had to be revised or dismissed, creating an impression of deterioration when none had occurred. Patients had complex problems which could not easily be distinguished from each other and patients themselves varied in their ability to perform tasks over time and across different settings. Team members from different professions worked with patients in different ways and had different perspectives on patients' problems. This was particularly an issue in the scoring of concepts such as anxiety, depression, orientation, social integration and cognitive problems.
Conclusion
From a psychometric perspective these problems would raise questions about the validity, reliability and responsiveness of the scores. However, from a clinical perspective, such characteristics are an inherent part of clinical judgement and reasoning. It is important to highlight the challenges faced by multidisciplinary teams in scoring patients on standardised outcome measures but it would be unwarranted to conclude that such challenges imply that these measures should not be used in clinical practice for decision making about individual patients. However, our findings do raise some concerns about the use of such measures for performance management
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