4,583 research outputs found
The Enigmatic Radio Afterglow of GRB 991216
We present wide-band radio observations spanning from 1.4 GHz to 350 GHz of
the afterglow of GRB 991216, taken from 1 to 80 days after the burst. The
optical and X-ray afterglow of this burst were fairly typical and are explained
by a jet fireball. In contrast, the radio light curve is unusual in two
respects: (a) the radio light curve does not show the usual rise to maximum
flux on timescales of weeks and instead appears to be declining already on day
1 and (b) the power law indices show significant steepening from the radio
through the X-ray bands. We show that the standard fireball model, in which the
afterglow is from a forward shock, is unable to account for (b) and we conclude
that the bulk of the radio emission must arise from a different source. We
consider two models, neither of which can be ruled out with the existing data.
In the first (conventional) model, the early radio emission is attributed to
emission from the reverse shock as in the case of GRB 990123. We predict that
the prompt optical emission would have been as bright (or brighter) than 8th
magnitude. In the second (exotic) model, the radio emission originates from the
forward shock of an isotropically energetic fireball (10^54 erg) expanding into
a tenuous medium (10^-4 cm^-3). The resulting fireball would remain
relativistic for months and is potentially resolvable with VLBI techniques.
Finally, we note that the near-IR bump of the afterglow is similar to that seen
in GRB 971214 and no fireball model can explain this bump.Comment: ApJ, submitte
Compound state resonances in the collinear collision of an atom with a diatomic oscillator
Detection of fast radio transients with multiple stations: a case study using the Very Long Baseline Array
Recent investigations reveal an important new class of transient radio
phenomena that occur on sub-millisecond timescales. Often transient surveys'
data volumes are too large to archive exhaustively. Instead, an on-line
automatic system must excise impulsive interference and detect candidate events
in real-time. This work presents a case study using data from multiple
geographically distributed stations to perform simultaneous interference
excision and transient detection. We present several algorithms that
incorporate dedispersed data from multiple sites, and report experiments with a
commensal real-time transient detection system on the Very Long Baseline Array
(VLBA). We test the system using observations of pulsar B0329+54. The
multiple-station algorithms enhanced sensitivity for detection of individual
pulses. These strategies could improve detection performance for a future
generation of geographically distributed arrays such as the Australian Square
Kilometre Array Pathfinder and the Square Kilometre Array.Comment: 12 pages, 14 figures. Accepted for Ap
The Broadband Afterglow of GRB980329
We present radio observations of the afterglow of the bright gamma-ray burst
GRB980329 made between one month and several years after the burst, a
re-analysis of previously published submillimeter data, and late-time optical
and near-infrared (NIR) observations of the host galaxy. From the absence of a
spectral break in the optical/NIR colors of the host galaxy, we exclude the
earlier suggestion that GRB980329 lies at a redshift of z >~5. We combine our
data with the numerous multi-wavelength observations of the early afterglow,
fit a comprehensive afterglow model to the entire broadband dataset, and derive
fundamental physical parameters of the blast-wave and its host environment.
Models for which the ejecta expand isotropically require both a high
circumburst density and extreme radiative losses from the shock. No low density
model (n << 10 cm^{-3}) fits the data. A burst with a total energy of ~ 10^{51}
erg, with the ejecta narrowly collimated to an opening angle of a few degrees,
driven into a surrounding medium with density ~ 20 cm^{-3}, provides a
satisfactory fit to the lightcurves over a range of redshifts.Comment: 27 pages, incl. 6 figures, minor revisions (e.g. added/updated
references) Accepted by Ap
Structure and expression of the human globin genes and murine histocompatibility antigen genes
A Continuous-Time Dynamic Factor Model for Intensive Longitudinal Data Arising from Mobile Health Studies
Intensive longitudinal data (ILD) collected in mobile health (mHealth)
studies contain rich information on multiple outcomes measured frequently over
time that have the potential to capture short-term and long-term dynamics.
Motivated by an mHealth study of smoking cessation in which participants
self-report the intensity of many emotions multiple times per day, we propose a
dynamic factor model that summarizes the ILD as a low-dimensional,
interpretable latent process. This model consists of two submodels: (i) a
measurement submodel -- a factor model -- that summarizes the multivariate
longitudinal outcome as lower-dimensional latent variables and (ii) a
structural submodel -- an Ornstein-Uhlenbeck (OU) stochastic process -- that
captures the temporal dynamics of the multivariate latent process in continuous
time. We derive a closed-form likelihood for the marginal distribution of the
outcome and the computationally-simpler sparse precision matrix for the OU
process. We propose a block coordinate descent algorithm for estimation.
Finally, we apply our method to the mHealth data to summarize the dynamics of
18 different emotions as two latent processes. These latent processes are
interpreted by behavioral scientists as the psychological constructs of
positive and negative affect and are key in understanding vulnerability to
lapsing back to tobacco use among smokers attempting to quit.Comment: Main text is 19 pages with 4 figures and 1 table. Supporting material
is 25 page
Responsive Sensory Evaluation to Develop Flexible Taste-Masked Paediatric Primaquine Tablets against Malaria for Low-Resource Settings
Primaquine is an important antimalarial drug for malaria transmission blocking and radical cure, but it is not currently available in child-friendly formulations in appropriate doses. Adult-strength tablets are often crushed and dissolved in water to obtain the required dose, which exposes the drug’s bitter taste. As part of the developing paediatric primaquine (DPP) project, this study adopted a responsive sensory pharmaceutics approach by integrating real-time formulation development and pre-clinical taste assessment to develop palatable, flavour-infused primaquine tablets. A design of experiment (DoE) approach was used to screen different taste-masking agents and excipient blends with trained, expert sensory assessors, with quinine hydrochloride as a model bitter tastant. The taste-masking efficacy of selected prototype formulation blends was validated with naïve assessors using the highest 15 mg primaquine dose. The mean bitterness intensity rating, measured on a discrete 11-point scale, was halved from 7.04 for the unflavoured control to 2.74–3.70 for the formulation blends. Sucralose had the biggest impact on bitterness suppression and improving palatability. Two different flavouring systems have been developed, and their acceptability in paediatric patients will be assessed as part of upcoming validation field clinical trials in Africa
Spectroscopic Identification of Massive Galaxies at z~2.3 with Strongly Suppressed Star Formation
We present first results of a spectroscopic survey targeting K-selected
galaxies at z=2.0-2.7 using the GNIRS instrument on Gemini-South. We obtained
near-infrared spectra with a wavelength coverage of 1.0-2.5 micron for 26
K-bright galaxies (K<19.7) selected from the MUSYC survey using photometric
redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies
using rest-frame optical emission lines or the redshifted Balmer/4000 Angstrom
break. Twenty galaxies have spectroscopic redshifts in the range 2.0<z<2.7, for
which bright emission lines like Halpha and [OIII] fall in atmospheric windows.
Surprisingly, we detected no emission lines for nine of these 20 galaxies. The
median 2 sigma upper limit on the rest-frame equivalent width of Halpha for
these nine galaxies is ~10 Angstrom. The stellar continuum emission of these
same nine galaxies is best fitted by evolved stellar population models. The
best-fit star formation rate (SFR) is zero for five out of nine galaxies, and
consistent with zero within 1 sigma for the remaining four. Thus, both the
Halpha measurements and the independent stellar continuum modeling imply that
45% of our K-selected galaxies are not forming stars intensely. This high
fraction of galaxies without detected line emission and low SFRs may imply that
the suppression of star formation in massive galaxies occurs at higher redshift
than is predicted by current CDM galaxy formation models. However, obscured
star formation may have been missed, and deep mid-infrared imaging is needed to
clarify this situation.Comment: Accepted for publication in ApJ Letter
Discovery of the INTEGRAL X/Gamma-ray transient IGR J00291+5934: a Comptonised accreting ms pulsar ?
We report the discovery of a high-energy transient with the IBIS/ISGRI
detector on board the INTEGRAL observatory. The source, namely IGR J00291+5934,
was first detected on 2nd December 2004 in the routine monitoring of the
IBIS/ISGRI 20--60 keV images. The observations were conducted during Galactic
Plane Scans, which are a key part of the INTEGRAL Core Programme observations.
After verifying the basic source behaviour, the discovery was announced on 3rd
December. The transient shows a hard Comptonised spectrum, with peak energy
release at about 20 keV and a total luminosity of ~ 0.9E36 erg/s in the 5--100
keV range, assuming a distance of 3 kpc. Following the INTEGRAL announcement of
the discovery of IGR J00291+5934, a number of observations were made by other
instruments. We summarise the results of those observations and, together with
the INTEGRAL data, identifiy IGR J00291+5934 as the 6th member of a class of
accreting X-ray millisecond pulsars.Comment: Accepted for publication as an A&A Letter 24/01/2005. 5 pages, 2
figure
- …