829 research outputs found
Magnetically-induced outflows from binary neutron star merger remnants
Recent observations by the Swift satellite have revealed long-lasting (), "plateau-like" X-ray afterglows in the vast majority
of short gamma-ray bursts events. This has put forward the idea of a long-lived
millisecond magnetar central engine being generated in a binary neutron star
(BNS) merger and being responsible for the sustained energy injection over
these timescales ("magnetar model"). We elaborate here on recent simulations
that investigate the early evolution of such a merger remnant in
general-relativistic magnetohydrodynamics. These simulations reveal very
different conditions than those usually assumed for dipole spin-down emission
in the magnetar model. In particular, the surrounding of the newly formed NS is
polluted by baryons due to a dense, highly magnetized and isotropic wind from
the stellar surface that is induced by magnetic field amplification in the
interior of the star. The timescales and luminosities of this wind are
compatible with early X-ray afterglows, such as the "extended emission". These
isotropic winds are a generic feature of BNS merger remnants and thus represent
an attractive alternative to current models of early X-ray afterglows. Further
implications to BNS mergers and short gamma-ray bursts are discussed.Comment: 4 pages, 2 figures. To appear in proceedings of "Swift: 10 Years of
Discovery
Electromagnetic emission from long-lived binary neutron star merger remnants II: lightcurves and spectra
Recent observations indicate that in a large fraction of binary neutron star
(BNS) mergers a long-lived neutron star (NS) may be formed rather than a black
hole. Unambiguous electromagnetic (EM) signatures of such a scenario would
strongly impact our knowledge on how short gamma-ray bursts (SGRBs) and their
afterglow radiation are generated. Furthermore, such EM signals would have
profound implications for multimessenger astronomy with joint EM and
gravitational-wave (GW) observations of BNS mergers, which will soon become
reality with the ground-based advanced LIGO/Virgo GW detector network starting
its first science run this year. Here we explore such EM signatures based on
the model presented in a companion paper, which provides a self-consistent
evolution of the post-merger system and its EM emission starting from an early
baryonic wind phase and resulting in a final pulsar wind nebula that is
confined by the previously ejected material. Lightcurves and spectra are
computed for a wide range of post-merger physical properties and particular
attention is paid to the emission in the X-ray band. In the context of SGRB
afterglow modeling, we present X-ray lightcurves corresponding to the
'standard' and the recently proposed 'time-reversal' scenario (SGRB prompt
emission produced at the time of merger or at the time of collapse of the
long-lived NS). The resulting afterglow lightcurve morphologies include, in
particular, single and two-plateau features with timescales and luminosities
that are in good agreement with the observations by the Swift satellite.
Furthermore, we compute the X-ray signal that should precede the SGRB in the
time-reversal scenario. If found, such a signal would represent smoking-gun
evidence for this scenario. Finally, we find a bright, highly isotropic EM
transient signal peaking in the X-ray band ...Comment: 20 pages, 16 figure
Magnetic field amplification in hypermassive neutron stars via the magnetorotational instability
Mergers of binary neutron stars likely lead to the formation of a
hypermassive neutron star (HMNS), which is metastable and eventually collapses
to a black hole. This merger scenario is thought to explain the phenomenology
of short gamma-ray bursts (SGRBs). The very high energies observed in SGRBs
have been suggested to stem from neutrino-antineutrino annihilation and/or from
very strong magnetic fields created during or after the merger by mechanisms
like the magnetorotational instability (MRI). Here, we report on results that
show for the first time the development of the MRI in HMNSs in
three-dimensional, fully general-relativistic magnetohydrodynamic simulations.
This instability amplifies magnetic fields exponentially and could be a vital
ingredient in solving the SGRB puzzle.Comment: 6 pages, 3 figures. Proceedings of the Karl Schwarzschild Meeting
201
Electromagnetic emission from long-lived binary neutron star merger remnants I: formulation of the problem
Binary neutron star (BNS) mergers are the leading model to explain the
phenomenology of short gamma-ray bursts (SGRBs), which are among the most
luminous explosions in the universe. Recent observations of long-lasting X-ray
afterglows of SGRBs challenge standard paradigms and indicate that in a large
fraction of events a long-lived neutron star (NS) may be formed rather than a
black hole. Understanding the mechanisms underlying these afterglows is
necessary in order to address the open questions concerning the nature of SGRB
central engines. However, recent theoretical progress has been hampered by the
fact that the timescales of interest for the afterglow emission are
inaccessible to numerical relativity simulations. Here we present a detailed
model to bridge the gap between numerical simulations of the merger process and
the relevant timescales for the afterglows, assuming that the merger results in
a long-lived NS. This model is formulated in terms of a set of coupled
differential equations that follow the evolution of the post-merger system and
predict its electromagnetic (EM) emission in a self-consistent way, starting
from initial data that can be extracted from BNS merger simulations and taking
into account the most relevant radiative processes. Moreover, the model can
accomodate the collapse of the remnant NS at any time during the evolution as
well as different scenarios for the prompt SGRB emission. A second major reason
of interest for BNS mergers is that they are considered the most promising
source of gravitational waves (GWs) for detection with the advanced
ground-based detector network LIGO/Virgo coming online this year.
Multimessenger astronomy with joint EM and GW observations of the merger and
post-merger phase can greatly enhance the scientific output of either type of
observation. However, the actual benefit depends on ...Comment: 27 pages, 3 figures, 4 appendice
Magnetically driven winds from differentially rotating neutron stars and X-ray afterglows of short gamma-ray bursts
Besides being among the most promising sources of gravitational waves,
merging neutron star binaries also represent a leading scenario to explain the
phenomenology of short gamma-ray bursts (SGRBs). Recent observations have
revealed a large subclass of SGRBs with roughly constant luminosity in their
X-ray afterglows, lasting s. These features are generally taken
as evidence of a long-lived central engine powered by the magnetic spin-down of
a uniformly rotating, magnetized object. We propose a different scenario in
which the central engine powering the X-ray emission is a differentially
rotating hypermassive neutron star (HMNS) that launches a quasi-isotropic and
baryon-loaded wind driven by the magnetic field, which is built-up through
differential rotation. Our model is supported by long-term, three-dimensional,
general-relativistic, and ideal magnetohydrodynamic simulations, showing that
this isotropic emission is a very robust feature. For a given HMNS, the
presence of a collimated component depends sensitively on the initial magnetic
field geometry, while the stationary electromagnetic luminosity depends only on
the magnetic energy initially stored in the system. We show that our model is
compatible with the observed timescales and luminosities and express the latter
in terms of a simple scaling relation.Comment: 6 pages, 5 figures. Updated to match published articl
Magnetorotational instability in relativistic hypermassive neutron stars
A differentially rotating hypermassive neutron star (HMNS) is a metastable
object which can be formed in the merger of neutron-star binaries. The eventual
collapse of the HMNS into a black hole is a key element in generating the
physical conditions expected to accompany the launch of a short gamma-ray
burst. We investigate the influence of magnetic fields on HMNSs by performing
three-dimensional simulations in general-relativistic magnetohydrodynamics. In
particular, we provide direct evidence for the occurrence of the
magnetorotational instability (MRI) in HMNS interiors. For the first time in
simulations of these systems, rapidly-growing and spatially-periodic structures
are observed to form with features like those of the channel flows produced by
the MRI in other systems. Moreover, the growth time and wavelength of the
fastest-growing mode are extracted and compared successfully with analytical
predictions. The MRI emerges as an important mechanism to amplify magnetic
fields over the lifetime of the HMNS, whose collapse to a black hole is
accelerated. The evidence provided here that the MRI can actually develop in
HMNSs could have a profound impact on the outcome of the merger of neutron-star
binaries and on its connection to short gamma-ray bursts.Comment: 5 pages, 4 figures. Updated to match published versio
Short gamma-ray bursts in the "time-reversal" scenario
Short gamma-ray bursts (SGRBs) are among the most luminous explosions in the
Universe and their origin still remains uncertain. Observational evidence
favors the association with binary neutron star or neutron star-black hole
(NS-BH) binary mergers. Leading models relate SGRBs to a relativistic jet
launched by the BH-torus system resulting from the merger. However, recent
observations have revealed a large fraction of SGRB events accompanied by X-ray
afterglows with durations , suggesting continuous
energy injection from a long-lived central engine, which is incompatible with
the short () accretion timescale of a BH-torus system.
The formation of a supramassive NS, resisting the collapse on much longer
spin-down timescales, can explain these afterglow durations, but leaves serious
doubts on whether a relativistic jet can be launched at merger. Here we present
a novel scenario accommodating both aspects, where the SGRB is produced after
the collapse of a supramassive NS. Early differential rotation and subsequent
spin-down emission generate an optically thick environment around the NS
consisting of a photon-pair nebula and an outer shell of baryon-loaded ejecta.
While the jet easily drills through this environment, spin-down radiation
diffuses outwards on much longer timescales and accumulates a delay that allows
the SGRB to be observed before (part of) the long-lasting X-ray signal. By
analyzing diffusion timescales for a wide range of physical parameters, we find
delays that can generally reach , compatible with
observations. The success of this fundamental test makes this "time-reversal"
scenario an attractive alternative to current SGRB models.Comment: 5 pages, 3 figures. Including proof corrections, matching published
versio
Evocative computing â creating meaningful lasting experiences in connecting with the past
We present an approach â evocative computing â that demonstrates how âat handâ technologies can be âpicked upâ and used by people to create meaningful and lasting experiences, through connecting and interacting with the past. The approach is instantiated here through a suite of interactive technologies configured for an indoor-outdoor setting that enables groups to explore, discover and research the history and background of a public cemetery. We report on a two-part study where different groups visited the cemetery and interacted with the digital tools and resources. During their activities serendipitous uses of the technology led to connections being made between personal memo-ries and ongoing activities. Furthermore, these experiences were found to be long-lasting; a follow-up study, one year later, showed them to be highly memorable, and in some cases leading participants to take up new directions in their work. We discuss the value of evocative computing for enriching user experiences and engagement with heritage practices
Envisioning futures of practice-centered computing
© Copyright 2019 held by Authors. In this panel, we will engage with the conference's membership and friends to consider directions for the possible futures of practice-centered computing. This panel is not targeting or aiming to result in a single, agreed "universalâ vision, nor to ask for a shared vision among the panelists and the audience. Rather, we offer several and diverse vision statements by distinguished and innovative ECSCW scholars, being experts in their specific domain or context of research. These statements will be necessarily incomplete until the ECSCW membership has joined the discussion, offering their own, additional visions of the futures of the field. With this, the panel aims to engage in a discussion that foresees exciting future research directions for the field of ECSCW but likewise also unveils potential hurdles the community might face
Cultural heritage communities: Technologies and challenges
This workshop will explore the role of technology support-ing and mediating cultural heritage practices for both pro-febional communities (cultural heritage profebionals, her-itage institutions, etc.) and civic communities (citizen-led heritage initiatives, heritage volunteers, personal and com-munity identified heritage, heritage crowdsourcing, etc.). The workshop-which aims to attract participants from her-itage studies and practice, community engagement, digital humanities and human-centred computing-will discub challenges and future opportunities for technology use and for design and participatory procebes in the context of var-ious heritage communities, and the role of different stake-holders in engaging with heritage in a technologically-mediated way
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