1,481 research outputs found
Fluctons
From the perspective of topological field theory we explore the physics
beyond instantons. We propose the fluctons as nonperturbative topological
fluctuations of vacuum, from which the self-dual domain of instantons is
attained as a particular case. Invoking the Atiyah-Singer index theorem, we
determine the dimension of the corresponding flucton moduli space, which gives
the number of degrees of freedom of the fluctons. An important consequence of
these results is that the topological phases of vacuum in non-Abelian gauge
theories are not necessarily associated with self-dual fields, but only with
smooth fields. Fluctons in different scenarios are considered, the basic
aspects of the quantum mechanical amplitude for fluctons are discussed, and the
case of gravity is discussed briefly
Discovery of coherent millisecond X-ray pulsations in Aql X-1
We report the discovery of an episode of coherent millisecond X-ray pulsation
in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for
slightly more than 150 seconds, during which the pulse frequency is consistent
with being constant. No X-ray burst or other evidence of thermonuclear burning
activity is seen in correspondence with the pulsation, which can thus be
identified as occurring in the persistent emission. The pulsation frequency is
550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from
burst oscillations in this source. Hence we identify this frequency with the
neutron star spin frequency. The pulsed fraction is strongly energy dependent,
ranging from 10% (16-30 keV). We discuss possible physical
interpretations and their consequences for our understanding of the lack of
pulsation in most neutron star low-mass X-ray binaries. If interpreted as
accretion-powered pulsation, Aql X-1 might play a key role in understanding the
differences between pulsating and non-pulsating sources.Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions.
Slightly extended discussion. One author added. Uses emulateapj.cl
Generating optimal comfort improving design solution with occupancy survey and Multi-Objective Optimization (MOO) technique: a case study for façade retrofit in a post-war office in London
In a UK post-war highly glazed open-plan office, occupants experienced intolerable discomfort from poorly designed and performing façade. However, the comfort-oriented solution was limited to internal of the window only by rental restriction. No direct performance data, e.g. monitoring data, was available for analysing potential discomfort issues. Such case presented a challenge for a balanced optimal solution for undefined but potentially compilated causes for problems.
This essay developed a practical and systematic framework to identify key comfort issues with occupancy survey and generate an optimized design option with multi-objective optimization with genetic algorithm for the case office. Benchmarking, correlation and ANOVA study were cross-referred and integrated with non-linear satisfaction theory to determine the key comfort factors. In this case, the key comfort factors were identified as direct sunlight glare, temperature and stability in winter and summer, and noise from colleagues.
These key comfort factors were parameterized with building simulation programs and set as objectives in optimization program thus integrating the qualitative survey into quantitate optimization algorithm for design options optimization. Glazing ratio, shading device length, and secondary glazing were set as changeable parameters (genes) controlling the façade characteristics.
The optimization program generated and compared 480 distinctive design options through 8 generations and discovered 101 Pareto front options. Filtered with a criteria-based filtering method from all pareto front, an optimal solution was recognized as fully opaque insulation for South and West façade, high glazing ratio on North and East façade, no extra shading device, and with secondary glazing. This optimal option effectively reduced glare under acceptable threshold while keeping large view out but only slightly improve summer thermal condition. Further improvements in ventilation system and building envelope performance was demonstrated essential for significant thermal comfort improvement
Forward Flux Sampling for rare event simulations
Rare events are ubiquitous in many different fields, yet they are notoriously
difficult to simulate because few, if any, events are observed in a conventiona
l simulation run. Over the past several decades, specialised simulation methods
have been developed to overcome this problem. We review one recently-developed
class of such methods, known as Forward Flux Sampling. Forward Flux Sampling
uses a series of interfaces between the initial and final states to calculate
rate constants and generate transition paths, for rare events in equilibrium or
nonequilibrium systems with stochastic dynamics. This review draws together a
number of recent advances, summarizes several applications of the method and
highlights challenges that remain to be overcome.Comment: minor typos in the manuscript. J.Phys.:Condensed Matter (accepted for
publication
Revealing a new symbiotic X-ray binary with Gemini NIFS
We use K-band spectroscopy of the counterpart to the rapidly variable X-ray
transient XMMU J174445.5-295044 to identify it as a new symbiotic X-ray binary.
XMMU J174445.5-295044 has shown a hard X-ray spectrum (we verify its
association with an Integral/IBIS 18-40 keV detection in 2013 using a short
Swift/XRT observation), high and varying N, and rapid flares on timescales
down to minutes, suggesting wind accretion onto a compact star. We observed its
near-infrared counterpart using the Near-infrared Integral Field Spectrograph
(NIFS) at Gemini-North, and classify the companion as ~ M2 III. We infer a
distance of kpc (conservative 1-sigma errors), and
therefore calculate that the observed X-ray luminosity (2-10 keV) has reached
to at least 4 erg/s. We therefore conclude that the source is a
symbiotic X-ray binary containing a neutron star (or, less likely, black hole)
accreting from the wind of a giant.Comment: 7 pages, 3 figures, MNRAS in pres
Discovery of a Second Transient Low-Mass X-ray Binary in the Globular Cluster NGC 6440
We have identified a new transient luminous low-mass X-ray binary, NGC 6440
X-2, with Chandra/ACIS, RXTE/PCA, and Swift/XRT observations of the globular
cluster NGC 6440. The discovery outburst (July 28-31, 2009) peaked at
L_X~1.5*10^36 ergs/s, and lasted for <4 days above L_X=10^35 ergs/s. Four other
outbursts (May 29-June 4, Aug. 29-Sept. 1, Oct. 1-3, and Oct. 28-31 2009) have
been observed with RXTE/PCA (identifying millisecond pulsations, Altamirano et
al. 2009a) and Swift/XRT (confirming a positional association with NGC 6440
X-2), with similar peak luminosities and decay times. Optical and infrared
imaging did not detect a clear counterpart, with best limits of V>21, B>22 in
quiescence from archival HST imaging, g'>22 during the August outburst from
Gemini-South GMOS imaging, and J>~18.5$ and K>~17 during the July outburst from
CTIO 4-m ISPI imaging.
Archival Chandra X-ray images of the core do not detect the quiescent
counterpart, and place a bolometric luminosity limit of L_{NS}< 6*10^31 ergs/s
(one of the lowest measured) for a hydrogen atmosphere neutron star. A short
Chandra observation 10 days into quiescence found two photons at NGC 6440 X-2's
position, suggesting enhanced quiescent emission at L_X~6*10^31 ergs/s .
NGC 6440 X-2 currently shows the shortest recurrence time (~31 days) of any
known X-ray transient, although regular outbursts were not visible in the bulge
scans before early 2009. Fast, low-luminosity transients like NGC 6440 X-2 may
be easily missed by current X-ray monitoring.Comment: 13 pages (emulateapj), 8 (color) figures, ApJ in press. Revised
version adds 5th outburst (Oct./Nov. 2009), additional discussion of possible
causes of short outburst recurrence time
Indoor Air Quality and Thermal Comfort: is all well with the Well Standard?
Cundall’s London office is the First WELL certificated building in Europe. Whether indoor air quality (IAQ) and thermal comfort satisfies occupants’ requirements and meets the WELL standard requires investigation. Based on ASHRAE Performance Measurement Protocols for Commercial Buildings (PMP), including indoor air quality monitoring combined with a Building Utilization Survey (BUS) based survey, this study investigates this question. Monitoring using Cundall’s sensors (IEQube) measuring temperature, relative humidity, light, CO2, PM2.5, PM10 and TVOC were combined with sensors from UCL to provide objective independent data for comparison. The BUS survey was conducted in the main open-plan office area to obtain the occupant’s perceptions of the environment. Monitoring results show IAQ is within the standard whilst results from the BUS questionnaire indicates most occupants are satisfied. Comments from the survey are useful to help continuing to improve the working environment
Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that
exhibits an accretion outburst approximately once every year. Whether the
thermal X-rays detected in intervening quiescent episodes are the result of
cooling of the neutron star or due to continued low-level accretion remains
unclear. In this work we use Swift data obtained after the long and bright 2011
and 2013 outbursts, as well as the short and faint 2015 outburst, to
investigate the hypothesis that cooling of the accretion-heated neutron star
crust dominates the quiescent thermal emission in Aql X-1. We demonstrate that
the X-ray light curves and measured neutron star surface temperatures are
consistent with the expectations of the crust cooling paradigm. By using a
thermal evolution code, we find that ~1.2-3.2 MeV/nucleon of shallow heat
release describes the observational data well, depending on the assumed
mass-accretion rate and temperature of the stellar core. We find no evidence
for varying strengths of this shallow heating after different outbursts, but
this could be due to limitations of the data. We argue that monitoring Aql X-1
for up to ~1 year after future outbursts can be a powerful tool to break model
degeneracies and solve open questions about the magnitude, depth and origin of
shallow heating in neutron star crusts.Comment: 14 pages, 5 figures, 3 tables, accepted to MNRA
Low-level accretion in neutron-star X-ray binaries
We search the literature for reports on the spectral properties of
neutron-star low-mass X-ray binaries when they have accretion luminosities
between 1E34 and 1E36 ergs/s. We found that in this luminosity range the photon
index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV
range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the
spectrum softens). Such behaviour has been reported before for individual
sources, but here we demonstrate that very likely most (if not all)
neutron-star systems behave in a similar manner and possibly even follow a
universal relation. When comparing the neutron-star systems with black-hole
systems, it is clear that most black-hole binaries have significantly harder
spectra at luminosities of 1E34 - 1E35 erg/s. Despite a limited number of data
points, there are indications that these spectral differences also extend to
the 1E35 - 1E36 erg/s range. This observed difference between the neutron-star
binaries and black-hole ones suggests that the spectral properties (between
0.5-10 keV) at 1E34 - 1E35 erg/s can be used to tentatively determine the
nature of the accretor in unclassified X-ray binaries. We discuss our results
in the context of properties of the accretion flow at low luminosities and we
suggest that the observed spectral differences likely arise from the
neutron-star surface becoming dominantly visible in the X-ray spectra. We also
suggest that both the thermal component and the non-thermal component might be
caused by low-level accretion onto the neutron-star surface for luminosities
below a few times 1E34 erg/s.Comment: Accepted for publication in MNRA
Probing the effects of a thermonuclear X-ray burst on the neutron star accretion flow with NuSTAR
Observational evidence has been accumulating that thermonuclear X-ray bursts
ignited on the surface of neutron stars influence the surrounding accretion
flow. Here, we exploit the excellent sensitivity of NuSTAR up to 79 keV to
analyze the impact of an X-ray burst on the accretion emission of the neutron
star LMXB 4U 1608-52. The ~200 s long X-ray burst occurred during a hard X-ray
spectral state, and had a peak intensity of ~30-50 per cent of the Eddington
limit with no signs of photospheric radius expansion. Spectral analysis
suggests that the accretion emission was enhanced up to a factor of ~5 during
the X-ray burst. We also applied a linear unsupervised decomposition method,
namely non-negative matrix factorization (NMF), to study this X-ray burst. We
find that the NMF performs well in characterizing the evolution of the burst
emission and is a promising technique to study changes in the underlying
accretion emission in more detail than is possible through conventional
spectral fitting. For the burst of 4U 1608-52, the NMF suggests a possible
softening of the accretion spectrum during the X-ray burst, which could
potentially be ascribed to cooling of a corona. Finally, we report a small (~3
per cent) but significant rise in the accretion emission ~0.5 h before the
X-ray burst, although it is unclear whether this was related to the X-ray burst
ignition.Comment: 10 pages, 10 figures, 1 table, to appear in MNRA
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