2,962 research outputs found
Intermittency and Lifetime of the 625 Hz QPO in the 2004 Hyperflare from the Magnetar SGR 1806-20 as evidence for magnetic coupling between the crust and the core
Quasi-periodic oscillations (QPOs) detected in the 2004 giant flare from SGR
1806-20 are often interpreted as global magneto-elastic oscillations of the
neutron star. There is, however, a large discrepancy between theoretical
models, which predict that the highest frequency oscillations should die out
rapidly, and the observations, which suggested that the highest-frequency
signals persisted for ~100s in X-ray data from two different spacecraft. This
discrepancy is particularly important for the high-frequency QPO at ~625 Hz.
However, previous analyses did not systematically test whether the signal could
also be there in much shorter data segments, more consistent with the
theoretical predictions. Here, we test for the presence of the high-frequency
QPO at 625 Hz in data from both the Rossi X-ray Timing Explorer (RXTE) and the
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) systematically both in
individual rotational cycles of the neutron star, as well as averaged over
multiple successive rotational cycles at the same phase. We find that the QPO
in the RXTE data is consistent with being only present in a single cycle, for a
short duration of ~0.5s, whereas the RHESSI data are as consistent with a
short-lived signal that appears and disappears as with a long-lived QPO. Taken
together, this data provides evidence for strong magnetic interaction between
the crust and the core.Comment: Accepted for publication in ApJ. The data and simulations are
available at
http://figshare.com/articles/SGR_1806_20_Giant_Flare_Data_and_Simulations/1126082
, the code can be downloaded from
https://github.com/dhuppenkothen/giantflare-paper , some documentation is
under
http://nbviewer.ipython.org/github/dhuppenkothen/giantflare-paper/blob/master/documents/giantflare-analysis.ipyn
Flame Propagation on the Surfaces of Rapidly Rotating Neutron Stars during Type I X-ray Bursts
We present the first vertically resolved hydrodynamic simulations of a
laterally propagating, deflagrating flame in the thin helium ocean of a
rotating accreting neutron star. We use a new hydrodynamics solver tailored to
deal with the large discrepancy in horizontal and vertical length scales
typical of neutron star oceans, and which filters out sound waves that would
otherwise limit our timesteps. We find that the flame moves horizontally with
velocities of order cm s, crossing the ocean in few seconds,
broadly consistent with the rise times of Type I X-ray bursts. We address the
open question of what drives flame propagation, and find that heat is
transported from burning to unburnt fuel by a combination of top-to-bottom
conduction and mixing driven by a baroclinic instability. The speed of the
flame propagation is therefore a sensitive function of the ocean conductivity
and spin: we explore this dependence for an astrophysically relevant range of
parameters and find that in general flame propagation is faster for slower
rotation and higher conductivity.Comment: Accepted for publication by MNRA
Rotational effects in thermonuclear Type I Bursts: equatorial crossing and directionality of flame spreading
In a previous study on thermonuclear (type I) nursts on accreting neutron
stars we addressed and demonstrated the importance of the effects of rotation,
through the Coriolis force, on the propagation of the burning flame. However,
that study only analysed cases of longitudinal propagation, where the Coriolis
force coefficient was constant. In this paper, we study the
effects of rotation on propagation in the meridional (latitudinal) direction,
where the Coriolis force changes from its maximum at the poles to zero at the
equator. We find that the zero Coriolis force at the equator, while affecting
the structure of the flame, does not prevent its propagation from one
hemisphere to another. We also observe structural differences between the flame
propagating towards the equator and that propagating towards the pole, the
second being faster. In the light of the recent discovery of the low spin
frequency of burster IGR~J17480-2446 rotating at 11 Hz (for which Coriolis
effects should be negligible) we also extend our simulations to slow rotation.Comment: Accepted for publication by MNRA
Fast and slow magnetic deflagration fronts in Type I X-ray bursts
Type I X-ray bursts are produced by thermonuclear runaways that develop on
accreting neutron stars. Once one location ignites, the flame propagates across
the surface of the star. Flame propagation is fundamental in order to
understand burst properties like rise time and burst oscillations. Previous
work quantified the effects of rotation on the front, showing that the flame
propagates as a deflagration and that the front strongly resembles a hurricane.
However the effect of magnetic fields was not investigated, despite the fact
that magnetic fields strong enough to have an effect on the propagating flame
are expected to be present on many bursters. In this paper we show how the
coupling between fluid layers introduced by an initially vertical magnetic
field plays a decisive role in determining the character of the fronts that are
responsible for the Type I bursts. In particular, on a star spinning at 450 Hz
(typical among the bursters) we test seed magnetic fields of
G and find that for the medium fields the magnetic stresses that develop during
the burst can speed up the velocity of the burning front, bringing the
simulated burst rise time close to the observed values. By contrast, in a
magnetic slow rotator like IGR J17480--2446, spinning at 11 Hz, a seed field
G is required to allow localized ignition and the magnetic field
plays an integral role in generating the burst oscillations observed during the
bursts.Comment: Pubblished on MNRA
AIOps for a Cloud Object Storage Service
With the growing reliance on the ubiquitous availability of IT systems and
services, these systems become more global, scaled, and complex to operate. To
maintain business viability, IT service providers must put in place reliable
and cost efficient operations support. Artificial Intelligence for IT
Operations (AIOps) is a promising technology for alleviating operational
complexity of IT systems and services. AIOps platforms utilize big data,
machine learning and other advanced analytics technologies to enhance IT
operations with proactive actionable dynamic insight.
In this paper we share our experience applying the AIOps approach to a
production cloud object storage service to get actionable insights into
system's behavior and health. We describe a real-life production cloud scale
service and its operational data, present the AIOps platform we have created,
and show how it has helped us resolving operational pain points.Comment: 5 page
Building Utopia: Performance and the Fantasy of Urban Renewal in Contemporary Toronto
Toronto markets itself as a city in renewal, a “creative city” of the future full of arts and culture. Alongside the official pitch, a number of street-level underground initiatives reimagine Toronto\u27s utopic future in a different way by means of site-specific performances
The Relocation Tool Kit project; Policies to enhance residents’ health, wellbeing and social inclusion
Copyright 2011, Brotherhood of St Laurence. Published version of the paper reproduced here with permission from the publisher (www.bsl.org.au
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