202 research outputs found
The detection of cluster magnetic fields via radio source depolarisation
It has been well established that galaxy clusters have magnetic fields. The
exact properties and origin of these magnetic fields are still uncertain even
though these fields play a key role in many astrophysical processes. Various
attempts have been made to derive the magnetic field strength and structure of
nearby galaxy clusters using Faraday rotation of extended cluster radio
sources. This approach needs to make various assumptions that could be
circumvented when using background radio sources. However, because the number
of polarised radio sources behind clusters is low, at the moment such a study
can only be done statistically. In this paper, we investigate the
depolarisation of radio sources inside and behind clusters in a sample of 124
massive clusters at observed with the Karl G. Jansky Very Large Array.
We detect a clear depolarisation trend with the cluster impact parameter, with
sources at smaller projected distances to the cluster centre showing more
depolarisation. By combining the radio observations with ancillary X-ray data
from Chandra, we compare the observed depolarisation with expectations from
cluster magnetic field models using individual cluster density profiles. The
best-fitting models have a central magnetic field strength of G with
power-law indices between and . We find no strong difference in the
depolarisation trend between sources embedded in clusters and background
sources located at similar projected radii, although the central region of
clusters is still poorly probed by background sources. We also examine the
depolarisation trend as a function of cluster properties such as the dynamical
state, mass, and redshift. Our findings show that the statistical
depolarisation of radio sources is a good probe of cluster magnetic field
parameters. [abridged]Comment: Replaced with Published version (A&A). 32 pages, 34 figure
Cytochrome oxidase subunit VI of Trypanosoma brucei is imported without a cleaved presequence and is developmentally regulated at both RNA and protein levels
Mitochondrial respiration in the African trypanosome undergoes dramatic developmental stage regulation. This requires co-ordinated control of components encoded by both the nuclear genome and the kinetoplast, the unusual mitochondrial genome of these parasites. As a model for understanding the co-ordination of these genomes, we have examined the regulation and mitochondrial import of a nuclear-encoded component of the cytochrome oxidase complex, cytochrome oxidase subunit VI (COXVI). By generating transgenic trypanosomes expressing intact or mutant forms of this protein, we demonstrate that COXVI is not imported using a conventional cleaved presequence and show that sequences at the N-terminus of the protein are necessary for correct mitochondrial sorting. Analyses of endogenous and transgenic COXVI mRNA and protein expression in parasites undergoing developmental stage differentiation demonstrates a temporal order of control involving regulation in the abundance of, first, mRNA and then protein. This represents the first dissection of the regulation and import of a nuclear-encoded protein into the cytochrome oxidase complex in these organisms, which were among the earliest eukaryotes to possess a mitochondrion
Time dependent density functional theory calculation of van der Waals coefficient C of alkali-metal atoms Li, Na, K, alkali dimers Li, Na, K and sodium clusters Na
In this paper we employ all-electron time dependent density functional theory
(TDDFT) to calculate the long range dipole-dipole dispersion coefficient (van
der Waals coefficient) of alkali-metal atoms Li, Na, K, alkali-metal
atom dimers Li, Na, K and sodium clusters containing even
number of atoms ranging from 2 to 20 atoms. The dispersion coefficients are
obtained via Casimir-Polder expression which relates it to the frequency
dependent linear polarizabilty at imaginary frequencies. The frequency
dependent polarizabilities are calculated by employing TDDFT--based complete
sum-over-states expressions for the atoms, and direct TDDFT linear response
theory for the closed shell dimers and clusters.Comment: 14 pages of text and 4 figure
A MeerKAT-meets-LOFAR study of Abell 1413: a moderately disturbed non-cool-core cluster hosting a kpc 'mini'-halo
Many relaxed cool-core clusters host diffuse radio emission on scales of
hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for
generating them, as well as their connection with central active galactic
nuclei, is elusive and many questions related to their physical properties and
origins remain unanswered. This paper presents new radio observations of the
galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and
LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of
mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a
moderately-disturbed non-cool-core cluster. Our study reveals an asymmetric
mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported
at similar frequencies. The spectral index is flatter than previously reported,
with an integrated value of , shows significant
spatial variation, and a tentative radial steepening. We studied the
point-to-point X-ray/radio surface brightness correlation to investigate the
thermal/non-thermal connection: our results show a strong connection between
these components, with a super-linear slope of at 1283 MHz
and at 145 MHz. We also explore the X-ray surface
brightness/radio spectral index correlation, finding a slope of . Both investigations support the evidence of spectral steepening.
Finally, in the context of understanding the particle acceleration mechanism,
we present a simple theoretical model which demonstrates that hybrid scenarios
- secondary electrons (re-)accelerated by turbulence - reproduce a super-linear
correlation slope.Comment: 19 pages, 11 figures, 4 tables. Accepted for publication in MNRA
Heterotrophic feeding as a newly identified survival strategy of the dinoflagellate Symbiodinium
Survival of free-living and symbiotic dinoflagellates (Symbiodinium spp.) in coral reefs is critical to the maintenance of a healthy coral community. Most coral reefs exist in oligotrophic waters, and their survival strategy in such nutrient-depleted waters remains largely unknown. In this study, we found that two strains of Symbiodinium spp. cultured from the environment and acquired from the tissues of the coral Alveopora japonica had the ability to feed heterotrophically. Symbiodinium spp. fed on heterotrophic bacteria, cyanobacteria (Synechococcus spp.), and small microalgae in both nutrient-replete and nutrient-depleted conditions. Cultured free-living Symbiodinium spp. displayed no autotrophic growth under nitrogen-depleted conditions, but grew when provided with prey. Our results indicate that Symbiodinium spp.’s mixotrophic activity greatly increases their chance of survival and their population growth under nitrogen-depleted conditions, which tend to prevail in coral habitats. In particular, free-living Symbiodinium cells acquired considerable nitrogen from algal prey, comparable to or greater than the direct uptake of ammonium, nitrate, nitrite, or urea. In addition, free-living Symbiodinium spp. can be a sink for planktonic cyanobacteria (Synechococcus spp.) and remove substantial portions of Synechococcus populations from coral reef waters. Our discovery of Symbiodinium’s feeding alters our conventional views of the survival strategies of photosynthetic Symbiodinium and corals
Deep Low-frequency Radio Observations of A2256. I. the Filamentary Radio Relic
We present deep and high-fidelity images of the merging galaxy cluster A2256 at low frequencies using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6″ resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of -1.07 ± 0.02 between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re)acceleration of electrons that are responsible for the observed radio emission
Alignment in the orientation of LOFAR radio sources
©ESO 2020. The original publication is available at https://doi.org/10.1051/0004-6361/202037680Various studies have laid claim to finding an alignment of the polarization vectors or radio jets of active galactic nuclei (AGN) over large distances, but these results have proven controversial and so far, there is no clear explanation for this observed alignment. To investigate this case further, we tested the hypothesis that the position angles of radio galaxies are randomly oriented in the sky by using data from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS). A sample of 7,555 double-lobed radio galaxies was extracted from the list of 318,520 radio sources in the first data release of LoTSS at 150 MHz. We performed statistical tests for uniformity of the two-dimensional (2D) orientations for the complete 7,555 source sample. We also tested the orientation uniformity in three dimensions (3D) for the 4,212 source sub-sample with photometric or spectroscopic redshifts. Our sample shows a significant deviation from uniformity (p-value <) in the 2D analysis at angular scales of about four degrees, mainly caused by sources with the largest flux densities. No significant alignment was found in the 3D analysis. Although the 3D analysis has access to fewer sources and suffers from uncertainties in the photometric redshift, the lack of alignment in 3D points towards the cause of the observed effect being unknown systematics or biases that predominantly affect the brightest sources, although this has yet to be demonstrated irrefutably and should be the subject of subsequent studies.Peer reviewe
Efficacy of alpha-Blockers on Hemodynamic Control during Pheochromocytoma Resection:A Randomized Controlled Trial
CONTEXT: Pretreatment with α-adrenergic receptor blockers is recommended to prevent hemodynamic instability during resection of a pheochromocytoma or sympathetic paraganglioma (PPGL). OBJECTIVE: To determine which type of α-adrenergic receptor blocker provides the best efficacy. DESIGN: Randomized controlled open-label trial (PRESCRIPT; ClinicalTrials.gov NCT01379898). SETTING: Multicenter study including 9 centers in The Netherlands. PATIENTS: 134 patients with non-metastatic PPGL. INTERVENTION: phenoxybenzamine or doxazosin starting 2-3 weeks before surgery using a blood pressure targeted titration schedule. Intraoperative hemodynamic management was standardized. MAIN OUTCOME MEASURES: Primary efficacy endpoint was the cumulative intraoperative time outside the blood pressure target range (i.e., SBP >160 mmHg or MAP <60 mmHg) expressed as a percentage of total surgical procedure time. Secondary efficacy endpoint was the value on a hemodynamic instability score. RESULTS: Median cumulative time outside blood pressure targets was 11.1% [IQR: 4.3-20.6] in the phenoxybenzamine group compared to 12.2% [5.3-20.2] in the doxazosin group (P=0.75, r=0.03). The hemodynamic instability score was 38.0 [28.8-58.0] and 50.0 [35.3-63.8] in the phenoxybenzamine and doxazosin group, respectively (P=0.02, r=0.20). The 30-day cardiovascular complication rate was 8.8% and 6.9% in the phenoxybenzamine and doxazosin group, respectively (P=0.68). There was no mortality after 30 days. CONCLUSIONS: The duration of blood pressure outside the target range during resection of a PPGL was not different after preoperative treatment with either phenoxybenzamine or doxazosin. Phenoxybenzamine was more effective in preventing intraoperative hemodynamic instability, but it could not be established whether this was associated with a better clinical outcome
Consumer Complaints and Company Market Value
Consumer complaints affect company market value and common sense suggests that a negative impact is
expected. However, do complaints always negatively impact company market value? We hypothesize in this
study that complaints may have a non-linear effect on market value. Positive (e.g. avoiding high costs to solve
complaints) and negative (e.g. speedy and intense diffusion) tradeoffs may occur given the level of complaints.
To test our non-linear hypothesis, a panel data was collected from cell phone service providers from 2005 to
2013. The results supported our tradeoff rationale. Low levels of complaints allow for companies to increase
market value, while high levels of complaints cause increasing harm to market value. The sample, model and
period considered in this study, indicates a level of 0.49 complaints per thousand consumers as the threshold for
a shift in tradeoffs. The effects on market value become increasingly negative when trying to make reductions to
move below this level, due to negative tradeoffs
The LOFAR Two-Metre Sky Survey (LoTSS): VI. Optical identifications for the second data release
The second data release of the LOFAR Two-Metre Sky Survey (LoTSS) covers 27%
of the northern sky, with a total area of deg. The high
angular resolution of LOFAR with Dutch baselines (6 arcsec) allows us to carry
out optical identifications of a large fraction of the detected radio sources
without further radio followup; however, the process is made more challenging
by the many extended radio sources found in LOFAR images as a result of its
excellent sensitivity to extended structure. In this paper we present source
associations and identifications for sources in the second data release based
on optical and near-infrared data, using a combination of a likelihood-ratio
cross-match method developed for our first data release, our citizen science
project Radio Galaxy Zoo: LOFAR, and new approaches to algorithmic optical
identification, together with extensive visual inspection by astronomers. We
also present spectroscopic or photometric redshifts for a large fraction of the
optical identifications. In total 4,116,934 radio sources lie in the area with
good optical data, of which 85% have an optical or infrared identification and
58% have a good redshift estimate. We demonstrate the quality of the dataset by
comparing it with earlier optically identified radio surveys. This is by far
the largest ever optically identified radio catalogue, and will permit robust
statistical studies of star-forming and radio-loud active galaxies.Comment: 29 pages. Accepted by A&A; data products available at
https://lofar-surveys.org/dr2_release.htm
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