7,793 research outputs found
Missing hard states and regular outbursts: the puzzling case of the black hole candidate 4U 1630-472
4U 1630-472 is a recurrent X-ray transient classified as a black-hole
candidate from its spectral and timing properties. One of the peculiarities of
this source is the presence of regular outbursts with a recurrence period
between 600 and 730 d that has been observed since the discovery of the source
in 1969. We report on a comparative study on the spectral and timing behaviour
of three consecutive outbursts occurred in 2006, 2008 and 2010. We analysed all
the data collected by the INTErnational Gamma-Ray Astrophysics Laboratory
(INTEGRAL) and the Rossi X-ray timing Explorer (RXTE) during these three years
of activity. We show that, in spite of having a similar spectral and timing
behaviour in the energy range between 3 and 30 keV, these three outbursts show
pronounced differences above 30 keV. In fact, the 2010 outburst extends at high
energies without any detectable cut-off until 150-200 keV, while the two
previous outbursts that occurred in 2006 and 2008 are not detected at all above
30 keV. Thus, in spite of a very similar accretion disk evolution, these three
outbursts exhibit totally different characteristics of the Compton electron
corona, showing a softening in their evolution rarely observed before in a low
mass X-ray binary hosting a black hole. We argue the possibility that the
unknown perturbation that causes the outbursts to be equally spaced in time
could be at the origin of this particular behaviour. Finally we describe
several possible scenarios that could explain the regularity of the outbursts,
identifying the most plausible, such as a third body orbiting around the binary
system.Comment: April 2015: accepted for publication in MNRAS. May 2015: in pres
Cosmological implications of interacting Group Field Theory models: cyclic Universe and accelerated expansion
We study the cosmological implications of interactions between spacetime
quanta in the Group Field Theory (GFT) approach to Quantum Gravity from a
phenomenological perspective. Our work represents a first step towards
understanding Early Universe Cosmology by studying the dynamics of the emergent
continuum spacetime, as obtained from a fundamentally discrete microscopic
theory. In particular, we show how GFT interactions lead to a recollapse of the
Universe while preserving the bounce replacing the initial singularity, which
has already been shown to occur in the free case. It is remarkable that cyclic
cosmologies are thus obtained in this framework without any a priori assumption
on the geometry of spatial sections of the emergent spacetime. Furthermore, we
show how interactions make it possible to have an early epoch of accelerated
expansion, which can be made to last for an arbitrarily large number of
e-folds, without the need to introduce an ad hoc potential for the scalar
field.Comment: 11 pages, 6 figure
Dynamics of anisotropies close to a cosmological bounce in quantum gravity
We study the dynamics of perturbations representing deviations from perfect
isotropy in the context of the emergent cosmology obtained from the group field
theory formalism for quantum gravity. Working in the mean field approximation
of the group field theory formulation of the Lorentzian EPRL model, we derive
the equations of motion for such perturbations to first order. We then study
these equations around a specific simple isotropic background, characterised by
the fundamental representation of \mbox{SU(2)}, and in the regime of the
effective cosmological dynamics corresponding to the bouncing region replacing
the classical singularity, well approximated by the free GFT dynamics. In this
particular example, we identify a region in the parameter space of the model
such that perturbations can be large at the bounce but become negligible away
from it, i.e. when the background enters the non-linear regime. We also study
the departures from perfect isotropy by introducing specific quantities, such
as the surface-area-to-volume ratio and the effective volume per quantum, which
make them quantitative.Comment: 45 pages, 4 figure
Contra generative AI detection in higher education assessments
This paper presents a critical analysis of generative Artificial Intelligence
(AI) detection tools in higher education assessments. The rapid advancement and
widespread adoption of generative AI, particularly in education, necessitates a
reevaluation of traditional academic integrity mechanisms. We explore the
effectiveness, vulnerabilities, and ethical implications of AI detection tools
in the context of preserving academic integrity. Our study synthesises insights
from various case studies, newspaper articles, and student testimonies to
scrutinise the practical and philosophical challenges associated with AI
detection. We argue that the reliance on detection mechanisms is misaligned
with the educational landscape, where AI plays an increasingly widespread role.
This paper advocates for a strategic shift towards robust assessment methods
and educational policies that embrace generative AI usage while ensuring
academic integrity and authenticity in assessments.Comment: v2: added references, fixed typos. 20 pages, 3 figure
Contra generative AI detection in higher education assessments
This paper presents a critical analysis of generative Artificial Intelligence (AI) detection tools in higher education assessments. The rapid advancement and widespread adoption of generative AI, particularly in education, necessitates a reevaluation of traditional academic integrity mechanisms. We explore the effectiveness, vulnerabilities, and ethical implications of AI detection tools in the context of preserving academic integrity. Our study synthesises insights from various case studies, newspaper articles, and student testimonies to scrutinise the practical and philosophical challenges associated with AI detection. We argue that the reliance on detection mechanisms is misaligned with the educational landscape, where AI plays an increasingly widespread role. This paper advocates for a strategic shift towards robust assessment methods and educational policies that embrace generative AI usage while ensuring academic integrity and authenticity in assessments
Efficiency of Xist-mediated silencing on autosomes is linked to chromosomal domain organisation
BACKGROUND: X chromosome inactivation, the mechanism used by mammals to equalise dosage of X-linked genes in XX females relative to XY males, is triggered by chromosome-wide localisation of a cis-acting non-coding RNA, Xist. The mechanism of Xist RNA spreading and Xist-dependent silencing is poorly understood. A large body of evidence indicates that silencing is more efficient on the X chromosome than on autosomes, leading to the idea that the X chromosome has acquired sequences that facilitate propagation of silencing. LINE-1 (L1) repeats are relatively enriched on the X chromosome and have been proposed as candidates for these sequences. To determine the requirements for efficient silencing we have analysed the relationship of chromosome features, including L1 repeats, and the extent of silencing in cell lines carrying inducible Xist transgenes located on one of three different autosomes. RESULTS: Our results show that the organisation of the chromosome into large gene-rich and L1-rich domains is a key determinant of silencing efficiency. Specifically genes located in large gene-rich domains with low L1 density are relatively resistant to Xist-mediated silencing whereas genes located in gene-poor domains with high L1 density are silenced more efficiently. These effects are observed shortly after induction of Xist RNA expression, suggesting that chromosomal domain organisation influences establishment rather than long-term maintenance of silencing. The X chromosome and some autosomes have only small gene-rich L1-depleted domains and we suggest that this could confer the capacity for relatively efficient chromosome-wide silencing. CONCLUSIONS: This study provides insight into the requirements for efficient Xist mediated silencing and specifically identifies organisation of the chromosome into gene-rich L1-depleted and gene-poor L1-dense domains as a major influence on the ability of Xist-mediated silencing to be propagated in a continuous manner in cis
Preliminary findings from a survey on the MD state of the practice
In the context of an Italian research project, this paper reports on an on-line survey, performed with 155 software professionals, with the aim of investigating about their opinions and experiences in modeling during software development and Model-driven engineering usage. The survey focused also on used modeling languages, processes and tools. A preliminary analysis of the results confirmed that Model-driven engineering, and more in general software modeling, are very relevant phenomena. Approximately 68% of the sample use models during software development. Among then, 44% generate code starting from models and 16% execute them directly. The preferred language for modeling is UML but DSLs are used as wel
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