69 research outputs found
The X-ray spectrum of Fe XVII revisited with a multi-ion model
The theoretical intensities of the soft X-ray Fe XVII lines arising from
2l-3l' transitions are reexamined using a three-ion collisional-radiative model
that includes the contributions to line formation of radiative recombination
(RR), dielectronic recombination (DR), resonant excitation (RE), and
inner-shell collisional ionization (CI), in addition to the usual contribution
of collisional excitation (CE). These additional processes enhance mostly the
2p-3s lines and not the 2p-3d lines. Under coronal equilibrium conditions, in
the electron temperature range of 400 to 600 eV where the Fe XVII line
emissivities peak, the combined effect of the additional processes is to
enhance the 2p-3s lines at 16.78, 17.05, and 17.10 A, by ~ 25%, 30%, and 55%,
respectively, compared with their traditional, single-ion CE values. The weak
2p-3d line at 15.45 A is also enhanced by up to 20%, while the other 2p-3d
lines are almost unaffected. The effects of DR and RE are found to be dominant
in this temperature range (400 - 600 eV), while that of CI is 3% at the most,
and the contribution of RR is less than 1%. At lower temperatures, where the Fe
XVII / Fe XVIII abundance ratio is high, the RE effect dominates. However, as
the temperature rises and the Fe XVIII abundance increases, the DR effect takes
over.
The newly calculated line powers can reproduce most of the often observed
high values of the (I17.05 + I17.10) / I15.01 intensity ratio. The importance
of ionization and recombination processes to the line strengths also helps to
explain why laboratory measurements in which CE is essentially the sole
mechanism agree well with single-ion calculations, but do not reproduce the
astrophysically observed ratios.Comment: Submitted to Ap
The Chandra Iron-L X-Ray Line Spectrum of Capella
An analysis of the iron L-shell emission in the publicly available spectrum
of the Capella binary system, as obtained by the High Energy Transmission
Grating Spectrometer on board the Chandra X-ray Observatory, is presented. The
atomic-state model, based on the HULLAC code, is shown to be especially
adequate for analyzing high-resolution x-ray spectra of this sort. Almost all
of the spectral lines in the 10 - 18 Angstrom wavelength range are identified.
It is shown that, for the most part, these lines can be attributed to emission
from L-shell iron ions in the Capella coronae. Possibilities for electron
temperature diagnostics using line ratios of Fe16+ are demonstrated. It is
shown that the observed iron-L spectrum can be reproduced almost entirely by
assuming a single electron temperature of kTe= 600 eV. This temperature is
consistent with both the measured fractional ion abundances of iron and with
the temperature derived from ratios of Fe16+ lines. A volume emission measure
of 1053 cm-3 is calculated for the iron L-shell emitting regions of the Capella
coronae indicating a rather small volume of 1029 cm3 for the emitting plasma if
an electron density of 1012 cm-3 is assumed.Comment: Accepted to Ap
Quantum quenches in the anisotropic spin-1/2 Heisenberg chain: different approaches to many-body dynamics far from equilibrium
Recent experimental achievements in controlling ultracold gases in optical
lattices open a new perspective on quantum many-body physics. In these
experimental setups it is possible to study coherent time evolution of isolated
quantum systems. These dynamics reveal new physics beyond the low-energy
properties usually relevant in solid-state many-body systems. In this paper we
study the time evolution of antiferromagnetic order in the Heisenberg chain
after a sudden change of the anisotropy parameter, using various numerical and
analytical methods. As a generic result we find that the order parameter, which
can show oscillatory or non-oscillatory dynamics, decays exponentially except
for the effectively non-interacting case of the XX limit. For weakly ordered
initial states we also find evidence for an algebraic correction to the
exponential law. The study is based on numerical simulations using a numerical
matrix product method for infinite system sizes (iMPS), for which we provide a
detailed description and an error analysis. Additionally, we investigate in
detail the exactly solvable XX limit. These results are compared to
approximative analytical approaches including an effective description by the
XZ-model as well as by mean-field, Luttinger-liquid and sine-Gordon theories.
This reveals which aspects of non-equilibrium dynamics can as in equilibrium be
described by low-energy theories and which are the novel phenomena specific to
quantum quench dynamics. The relevance of the energetically high part of the
spectrum is illustrated by means of a full numerical diagonalization of the
Hamiltonian.Comment: 28 page
GROWTH on S190814bv: Deep Synoptic Limits on the Optical/Near-infrared Counterpart to a Neutron Star-Black Hole Merger
On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron star-black hole (NSBH) merger, or potentially a low-mass binary BH merger. Due to the low false-alarm rate and the precise localization (23 deg2 at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to an NSBH merger. To search for potential counterparts, the GROWTH Collaboration performed real-time image subtraction on six nights of public Dark Energy Camera images acquired in the 3 weeks following the merger, covering >98% of the localization probability. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be M ej < 0.04 M oË at polar viewing angles, or M ej < 0.03 M oË if the opacity is Îș < 2 cm2g-1. Assuming a tidal deformability for the NS at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be Ï < 0.7 for mass ratios Q < 6, with weaker constraints for more compact NSs. © 2020. The American Astronomical Society
BLOOM: A 176B-Parameter Open-Access Multilingual Language Model
Large language models (LLMs) have been shown to be able to perform new tasks
based on a few demonstrations or natural language instructions. While these
capabilities have led to widespread adoption, most LLMs are developed by
resource-rich organizations and are frequently kept from the public. As a step
towards democratizing this powerful technology, we present BLOOM, a
176B-parameter open-access language model designed and built thanks to a
collaboration of hundreds of researchers. BLOOM is a decoder-only Transformer
language model that was trained on the ROOTS corpus, a dataset comprising
hundreds of sources in 46 natural and 13 programming languages (59 in total).
We find that BLOOM achieves competitive performance on a wide variety of
benchmarks, with stronger results after undergoing multitask prompted
finetuning. To facilitate future research and applications using LLMs, we
publicly release our models and code under the Responsible AI License
Global Retinoblastoma Presentation and Analysis by National Income Level.
Importance: Early diagnosis of retinoblastoma, the most common intraocular cancer, can save both a child's life and vision. However, anecdotal evidence suggests that many children across the world are diagnosed late. To our knowledge, the clinical presentation of retinoblastoma has never been assessed on a global scale. Objectives: To report the retinoblastoma stage at diagnosis in patients across the world during a single year, to investigate associations between clinical variables and national income level, and to investigate risk factors for advanced disease at diagnosis. Design, Setting, and Participants: A total of 278 retinoblastoma treatment centers were recruited from June 2017 through December 2018 to participate in a cross-sectional analysis of treatment-naive patients with retinoblastoma who were diagnosed in 2017. Main Outcomes and Measures: Age at presentation, proportion of familial history of retinoblastoma, and tumor stage and metastasis. Results: The cohort included 4351 new patients from 153 countries; the median age at diagnosis was 30.5 (interquartile range, 18.3-45.9) months, and 1976 patients (45.4%) were female. Most patients (nâ=â3685 [84.7%]) were from low- and middle-income countries (LMICs). Globally, the most common indication for referral was leukocoria (nâ=â2638 [62.8%]), followed by strabismus (nâ=â429 [10.2%]) and proptosis (nâ=â309 [7.4%]). Patients from high-income countries (HICs) were diagnosed at a median age of 14.1 months, with 656 of 666 (98.5%) patients having intraocular retinoblastoma and 2 (0.3%) having metastasis. Patients from low-income countries were diagnosed at a median age of 30.5 months, with 256 of 521 (49.1%) having extraocular retinoblastoma and 94 of 498 (18.9%) having metastasis. Lower national income level was associated with older presentation age, higher proportion of locally advanced disease and distant metastasis, and smaller proportion of familial history of retinoblastoma. Advanced disease at diagnosis was more common in LMICs even after adjusting for age (odds ratio for low-income countries vs upper-middle-income countries and HICs, 17.92 [95% CI, 12.94-24.80], and for lower-middle-income countries vs upper-middle-income countries and HICs, 5.74 [95% CI, 4.30-7.68]). Conclusions and Relevance: This study is estimated to have included more than half of all new retinoblastoma cases worldwide in 2017. Children from LMICs, where the main global retinoblastoma burden lies, presented at an older age with more advanced disease and demonstrated a smaller proportion of familial history of retinoblastoma, likely because many do not reach a childbearing age. Given that retinoblastoma is curable, these data are concerning and mandate intervention at national and international levels. Further studies are needed to investigate factors, other than age at presentation, that may be associated with advanced disease in LMICs
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