265 research outputs found
The impulsive phase of magnetar giant flares: assessing linear tearing as the trigger mechanism
Giant -ray flares comprise the most extreme radiation events observed
from magnetars. Developing on (sub)millisecond timescales and generating vast
amounts of energy within a fraction of a second, the initial phase of these
extraordinary bursts present a significant challenge for candidate trigger
mechanisms. Here we assess and critically analyse the linear growth of the
relativistic tearing instability in a globally twisted magnetosphere as the
trigger mechanism for giant -ray flares. Our main constraints are given
by the observed emission timescales, the energy output of the giant flare
spike, and inferred dipolar magnetic field strengths. We find that the minimum
growth time of the linear mode is comparable to the -folding rise time, i.e.
ms. With this result we constrain basic geometric parameters of
the current sheet. We also discuss the validity of the presumption that the
-folding emission timescale may be equated with the growth time of an MHD
instability.Comment: 15 pages, 4 figures, MNRAS in pres
Magnetar giant flare high-energy emission
High energy ( keV) emission has been detected persisting for several
tens of seconds after the initial spike of magnetar giant flares. It has been
conjectured that this emission might arise via inverse Compton scattering in a
highly extended corona generated by super-Eddington outflows high up in the
magnetosphere. In this paper we undertake a detailed examination of this model.
We investigate the properties of the required scatterers, and whether the
mechanism is consistent with the degree of pulsed emission observed in the tail
of the giant flare. We conclude that the mechanism is consistent with current
data, although the origin of the scattering population remains an open
question. We propose an alternative picture in which the emission is closer to
that star and is dominated by synchrotron radiation. The observations
of the December 2004 flare modestly favor this latter picture. We assess the
prospects for the Fermi Gamma-Ray Space Telescope to detect and characterize a
similar high energy component in a future giant flare. Such a detection should
help to resolve some of the outstanding issues.Comment: 20 pages, 14 figure
Rotational phase dependence of magnetar bursts
The trigger for the short bursts observed in -rays from many magnetar
sources remains unknown. One particular open question in this context is the
localization of burst emission to a singular active region or a larger area
across the neutron star. While several observational studies have attempted to
investigate this question by looking at the phase dependence of burst
properties, results have been mixed. At the same time, it is not obvious a
priori that bursts from a localized active region would actually give rise to a
detectable phase-dependence, taking into account issues such as geometry,
relativistic effects, and intrinsic burst properties such brightness and
duration. In this paper, we build a simple theoretical model to investigate the
circumstances under which the latter effects could affect detectability of a
dependence of burst emission on rotational phase. We find that even for
strongly phase-dependent emission, inferred burst properties may not show a
rotational phase dependence depending on the geometry of the system and the
observer. Furthermore, the observed properties of bursts with durations short
as 10-20% of the spin period can vary strongly depending on the rotational
phase at which the burst was emitted. We also show that detectability of a
rotational phase dependence depends strongly on the minimum number of bursts
observed, and find that existing burst samples may simply be too small to rule
out a phase dependence.Comment: 15 pages, 17 figure
Dissecting magnetar variability with Bayesian hierarchical models
Neutron stars are a prime laboratory for testing physical processes under
conditions of strong gravity, high density, and extreme magnetic fields. Among
the zoo of neutron star phenomena, magnetars stand out for their bursting
behaviour, ranging from extremely bright, rare giant flares to numerous, less
energetic recurrent bursts. The exact trigger and emission mechanisms for these
bursts are not known; favoured models involve either a crust fracture and
subsequent energy release into the magnetosphere, or explosive reconnection of
magnetic field lines. In the absence of a predictive model, understanding the
physical processes responsible for magnetar burst variability is difficult.
Here, we develop an empirical model that decomposes magnetar bursts into a
superposition of small spike-like features with a simple functional form, where
the number of model components is itself part of the inference problem. The
cascades of spikes that we model might be formed by avalanches of reconnection,
or crust rupture aftershocks. Using Markov Chain Monte Carlo (MCMC) sampling
augmented with reversible jumps between models with different numbers of
parameters, we characterise the posterior distributions of the model parameters
and the number of components per burst. We relate these model parameters to
physical quantities in the system, and show for the first time that the
variability within a burst does not conform to predictions from ideas of
self-organised criticality. We also examine how well the properties of the
spikes fit the predictions of simplified cascade models for the different
trigger mechanisms.Comment: accepted for publication in The Astrophysical Journal; code available
at https://bitbucket.org/dhuppenkothen/magnetron, data products at
http://figshare.com/articles/SGR_J1550_5418_magnetron_data/129242
Nitraat, nitriet en N-nitrosaminen in voedingsmiddelen
Dit verslag geeft een inventarisatie van de problematiek betreffende nitraat, nitriet en N-nitrosamines. Aan de hand van gegevens over groenten, vleesprodukten en rnelkprodukten wordt waar mogelijk de Nederlandse situatie geschetst. De moeilijkheden bij de analyse van N-nitrosamines worden kort aangegeven
MEK inhibition leads to BRCA2 downregulation and sensitization to DNA damaging agents in pancreas and ovarian cancer models
Targeting the DNA damage response (DDR) in tumors with defective DNA repair
is a clinically successful strategy. The RAS/RAF/MEK/ERK signalling pathway is
frequently deregulated in human cancers. In this study, we explored the effects of
MEK inhibition on the homologous recombination pathway and explored the potential
for combination therapy of MEK inhibitors with DDR inhibitors and a hypoxia-activated
prodrug.
We studied effects of combining pimasertib, a selective allosteric inhibitor of
MEK1/2, with olaparib, a small molecule inhibitor of poly (adenosine diphosphate
[ADP]-ribose) polymerases (PARP), and with the hypoxia-activated prodrug
evofosfamide in ovarian and pancreatic cancer cell lines. Apoptosis was assessed
by Caspase 3/7 assay and protein expression was detected by immunoblotting.
DNA damage response was monitored with ÎłH2AX and RAD51 immunofluorescence
staining. In vivo antitumor activity of pimasertib with evofosfamide were assessed
in pancreatic cancer xenografts.
We found that BRCA2 protein expression was downregulated following pimasertib
treatment under hypoxic conditions. This translated into reduced homologous
recombination repair demonstrated by levels of RAD51 foci. MEK inhibition was
sufficient to induce formation of ÎłH2AX foci, suggesting that inhibition of this pathway
would impair DNA repair. When combined with olaparib or evofosfamide, pimasertib
treatment enhanced DNA damage and increased apoptosis. The combination of
pimasertib with evofosfamide demonstrated increased anti-tumor activity in BRCA
wild-type Mia-PaCa-2 xenograft model, but not in the BRCA mutated BxPC3 model.
Our data suggest that targeted MEK inhibition leads to impaired homologous
recombination DNA damage repair and increased PARP inhibition sensitivity in BRCA-
2 proficient cancers
Clonogenic growth of human breast cancer cells co-cultured in direct contact with serum-activated fibroblasts
INTRODUCTION: Accumulating evidence suggests that fibroblasts play a pivotal role in promoting the growth of breast cancer cells. The objective of the present study was to characterize and validate an in vitro model of the interaction between small numbers of human breast cancer cells and human fibroblasts. METHODS: We measured the clonogenic growth of small numbers of human breast cancer cells co-cultured in direct contact with serum-activated, normal human fibroblasts. Using DNA microarrays, we also characterized the gene expression profile of the serum-activated fibroblasts. In order to validate the in vivo relevance of our experiments, we then analyzed clinical samples of metastatic breast cancer for the presence of myofibroblasts expressing α-smooth muscle actin. RESULTS: Clonogenic growth of human breast cancer cells obtained directly from in situ and invasive tumors was dramatically and consistently enhanced when the tumor cells were co-cultured in direct contact with serum-activated fibroblasts. This effect was abolished when the cells were co-cultured in transwells separated by permeable inserts. The fibroblasts in our experimental model exhibited a gene expression signature characteristic of 'serum response' (i.e. myofibroblasts). Immunostaining of human samples of metastatic breast cancer tissue confirmed that myofibroblasts are in direct contact with breast cancer cells. CONCLUSION: Serum-activated fibroblasts promote the clonogenic growth of human breast cancer cells in vitro through a mechanism that involves direct physical contact between the cells. This model shares many important molecular and phenotypic similarities with the fibroblasts that are naturally found in breast cancers
Human breast cancer stem cell markers CD44 and CD24: enriching for cells with functional properties in mice or in man?
Identification of breast cancer stem cells as the cells within breast tumors that have the ability to give rise to cells that make up the bulk of the tumor mass has shifted the focus of cancer research. However, there is still much debate concerning the unique nature of the markers that distinguish cancer stem cells in the breast. As such, understanding whether CD44+/CD24- breast cancer cells are merely more successful in overcoming an engraftment incompatibility that exists when injecting human cells into the mouse adipose tissue or are indeed bona fide cancer stem cells is of great importance
Dynamic Regulation of Tgf-B Signaling by Tif1Îł: A Computational Approach
TIF1Îł (Transcriptional Intermediary Factor 1 Îł) has been implicated in
Smad-dependent signaling by Transforming Growth Factor beta (TGF-ÎČ).
Paradoxically, TIF1Îł functions both as a transcriptional repressor or as an
alternative transcription factor that promotes TGF-ÎČ signaling. Using
ordinary differential-equation models, we have investigated the effect of
TIF1Îł on the dynamics of TGF-ÎČ signaling. An integrative model that
includes the formation of transient TIF1Îł-Smad2-Smad4 ternary complexes is
the only one that can account for TGF-ÎČ signaling compatible with the
different observations reported for TIF1Îł. In addition, our model predicts
that varying TIF1Îł/Smad4 ratios play a critical role in the modulation of
the transcriptional signal induced by TGF-ÎČ, especially for short
stimulation times that mediate higher threshold responses. Chromatin
immunoprecipitation analyses and quantification of the expression of TGF-ÎČ
target genes as a function TIF1Îł/Smad4 ratios fully validate this
hypothesis. Our integrative model, which successfully unifies the seemingly
opposite roles of TIF1Îł, also reveals how changing TIF1Îł/Smad4 ratios
affect the cellular response to stimulation by TGF-ÎČ, accounting for a
highly graded determination of cell fate
Generation of Breast Cancer Stem Cells through Epithelial-Mesenchymal Transition
Recently, two novel concepts have emerged in cancer biology: the role of so-called âcancer stem cellsâ in tumor initiation, and the involvement of an epithelial-mesenchymal transition (EMT) in the metastatic dissemination of epithelial cancer cells. Using a mammary tumor progression model, we show that cells possessing both stem and tumorigenic characteristics of âcancer stem cellsâ can be derived from human mammary epithelial cells following the activation of the Ras-MAPK pathway. The acquisition of these stem and tumorigenic characters is driven by EMT induction
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