368 research outputs found
X-ray behaviour of Circinus X-1 - I: X-ray Dips as a diagnostic of periodic behaviour
We examine the periodic nature of detailed structure (particularly dips) in
the RXTE/ASM lightcurve of Circinus X-1. The significant phase wandering of the
X-ray maxima suggests their identification with the response on a viscous
timescale of the accretion disk to perturbation. We find that the X-ray dips
provide a more accurate system clock than the maxima, and thus use these as
indicators of the times of periastron passage. We fit a quadratic ephemeris to
these dips, and find its predictive power for the X-ray lightcurve to be
superior to ephemerides based on the radio flares and the full archival X-ray
lightcurve. Under the hypothesis that the dips are tracers of the mass transfer
rate from the donor, we use their occurrence rate as a function of orbital
phase to explore the (as yet unconstrained) nature of the donor. The high
term in the ephemeris provides another piece of evidence that Cir X-1
is in a state of dynamical evolution, and thus is a very young post-supernova
system. We further suggest that the radio ``synchrotron nebula'' immediately
surrounding Cir X-1 is in fact the remnant of the event that created the
compact object, and discuss briefly the evidence for and against such an
interpretation.Comment: 11 pages, 11 figures, accepted for publication in MNRA
The Samurai Project: verifying the consistency of black-hole-binary waveforms for gravitational-wave detection
We quantify the consistency of numerical-relativity black-hole-binary
waveforms for use in gravitational-wave (GW) searches with current and planned
ground-based detectors. We compare previously published results for the
mode of the gravitational waves from an equal-mass
nonspinning binary, calculated by five numerical codes. We focus on the 1000M
(about six orbits, or 12 GW cycles) before the peak of the GW amplitude and the
subsequent ringdown. We find that the phase and amplitude agree within each
code's uncertainty estimates. The mismatch between the modes
is better than for binary masses above with respect to
the Enhanced LIGO detector noise curve, and for masses above
with respect to Advanced LIGO, Virgo and Advanced Virgo. Between the waveforms
with the best agreement, the mismatch is below . We find that
the waveforms would be indistinguishable in all ground-based detectors (and for
the masses we consider) if detected with a signal-to-noise ratio of less than
, or less than in the best cases.Comment: 17 pages, 9 figures. Version accepted by PR
Selection at a single locus leads to widespread expansion of toxoplasma gondii lineages that are virulent in mice
The determinants of virulence are rarely defined for eukaryotic parasites such as T. gondii, a widespread parasite of mammals that also infects humans, sometimes with serious consequences. Recent laboratory studies have established that variation in a single secreted protein, a serine/threonine kinase known as ROPO18, controls whether or not mice survive infection. Here, we establish the extent and nature of variation in ROP18among a collection of parasite strains from geographically diverse regions. Compared to other genes, ROP18 showed extremely high levels of diversification and changes in expression level, which correlated with severity of infection in mice. Comparison with an out-group demonstrated that changes in the upstream region that regulates expression of ROP18 led to an historical increase in the expression and exposed the protein to diversifying selective pressure. Surprisingly, only three atypically distinct protein variants exist despite marked genetic divergence elsewhere in the genome. These three forms of ROP18 are likely adaptations for different niches in nature, and they confer markedly different virulence to mice. The widespread distribution of a single mouse-virulent allele among geographically and genetically disparate parasites may have consequences for transmission and disease in other hosts, including humans
Optical variability properties of high luminosity AGN classes
We present the results of a comparative study of the intra-night optical
variability (INOV) characteristics of radio-loud and radio-quiet quasars, which
involves a systematic intra-night optical monitoring of seven sets of high
luminosity AGNs covering the redshift range {\it z} to {\it z}
. The sample, matched in the optical luminosity -- redshift (M
-- z) plane, consists of seven radio-quiet quasars (RQQs), eight radio
lobe-dominated quasars (LDQs), six radio core-dominated quasars (CDQs) and five
BL Lac objects (BLs). Systematic CCD observations, aided by a careful data
analysis procedure, have allowed us to detect INOV with amplitudes as low as
1%. Present observations cover a total of 113 nights (720 hours) with only a
single quasar monitored as continuously as possible on a night. Considering
cases of only unambiguous detections of INOV we have estimated duty cycles
(DCs) of 17%, 12%, 20% and 72% respectively for RQQs, LDQs, CDQs, and BLs. The
low amplitude and low DC of INOV shown by RQQs compared to BLs can be
understood in terms of their having optical synchrotron jets which are modestly
misdirected from us. From our fairly extensive dataset, no unambiguous general
trend of a correlation between the INOV amplitude and the apparent optical
brightness of the quasar is noticed.Comment: 36 pages, 14 Figures, due to large size Fig. 5,6,11 and 12 are not
included. Intersted people contact to [email protected]. Submitted to
Journal of Astrophysics and Astronom
Testing gravitational-wave searches with numerical relativity waveforms: Results from the first Numerical INJection Analysis (NINJA) project
The Numerical INJection Analysis (NINJA) project is a collaborative effort
between members of the numerical relativity and gravitational-wave data
analysis communities. The purpose of NINJA is to study the sensitivity of
existing gravitational-wave search algorithms using numerically generated
waveforms and to foster closer collaboration between the numerical relativity
and data analysis communities. We describe the results of the first NINJA
analysis which focused on gravitational waveforms from binary black hole
coalescence. Ten numerical relativity groups contributed numerical data which
were used to generate a set of gravitational-wave signals. These signals were
injected into a simulated data set, designed to mimic the response of the
Initial LIGO and Virgo gravitational-wave detectors. Nine groups analysed this
data using search and parameter-estimation pipelines. Matched filter
algorithms, un-modelled-burst searches and Bayesian parameter-estimation and
model-selection algorithms were applied to the data. We report the efficiency
of these search methods in detecting the numerical waveforms and measuring
their parameters. We describe preliminary comparisons between the different
search methods and suggest improvements for future NINJA analyses.Comment: 56 pages, 25 figures; various clarifications; accepted to CQ
Expenditure Reform in Industrialised Countries: A Case Study Approach
This study examines reforms of public expenditure in industrialised countries over the past two decades. We distinguish ambitious and timid reformers and analyse in detail reform experiences in eight case studies of ambitious reform episodes. We find that ambitious reform countries reduce spending on transfers, subsidies and public consumption while largely sparing education spending. Such expenditure retrenchment is also typically part of a comprehensive reform package that includes improvements in fiscal institutions as well as structural and other macroeconomic reforms. The study finds that ambitious expenditure retrenchment and reform coincides with large improvements in fiscal and economic growth indicators
Importance of the Global Regulators Agr and SaeRS in the Pathogenesis of CA-MRSA USA300 Infection
CA-MRSA infection, driven by the emergence of the USA300 genetic background, has become epidemic in the United States. USA300 isolates are hypervirulent, compared with other CA- and HA-MRSA strains, in experimental models of necrotizing pneumonia and skin infection. Interestingly, USA300 isolates also have increased expression of core genomic global regulatory and virulence factor genes, including agr and saeRS. To test the hypothesis that agr and saeRS promote the observed hypervirulent phenotype of USA300, isogenic deletion mutants of each were constructed in USA300. The effects of gene deletion on expression and protein abundance of selected downstream virulence genes were assessed by semiquantitative real-time reverse-transcriptase PCR (qRT-PCR) and western blot, respectively. The effects of gene deletion were also assessed in mouse models of necrotizing pneumonia and skin infection. Deletion of saeRS, and, to a lesser extent, agr, resulted in attenuated expression of the genes encoding α-hemolysin (hla) and the Panton-Valentine leukocidin (lukSF-PV). Despite the differences in hla transcription, the toxin was undetectable in culture supernatants of either of the deletion mutants. Deletion of agr, but not saeRS, markedly increased the expression of the gene encoding protein A (spa), which correlated with increased protein abundance. Each deletion mutant demonstrated significant attenuation of virulence, compared with wild-type USA300, in mouse models of necrotizing pneumonia and skin infection. We conclude that agr and saeRS each independently contribute to the remarkable virulence of USA300, likely by means of their effects on expression of secreted toxins
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