1,123 research outputs found
An X-ray view of the very faint black hole X-ray transient Swift J1357.2-0933 during its 2011 outburst
We report on the X-ray spectral (using XMM-Newton data) and timing behavior
(using XMM-Newton and Rossi X-ray Timing Explorer [RXTE] data) of the very
faint X-ray transient and black hole system Swift J1357.2-0933 during its 2011
outburst. The XMM-Newton X-ray spectrum of this source can be adequately fitted
with a soft thermal component with a temperature of ~0.22 keV (using a disc
model) and a hard, non-thermal component with a photon index of ~1.6 when using
a simple power-law model. In addition, an edge at ~ 0.73 keV is needed likely
due to interstellar absorption. During the first RXTE observation we find a 6
mHz quasi-periodic oscillation (QPO) which is not present during any of the
later RXTE observations or during the XMM-Newton observation which was taken 3
days after the first RXTE observation. The nature of this QPO is not clear but
it could be related to a similar QPO seen in the black hole system H 1743-322
and to the so-called 1 Hz QPO seen in the dipping neutron-star X-ray binaries
(although this later identification is quite speculative). The observed QPO has
similar frequencies as the optical dips seen previously in this source during
its 2011 outburst but we cannot conclusively determine that they are due to the
same underlying physical mechanism. Besides the QPO, we detect strong
band-limited noise in the power-density spectra of the source (as calculated
from both the RXTE and the XMM-Newton data) with characteristic frequencies and
strengths very similar to other black hole X-ray transients when they are at
low X-ray luminosities. We discuss the spectral and timing properties of the
source in the context of the proposed very high inclination of this source. We
conclude that all the phenomena seen from the source cannot, as yet, be
straightforwardly explained neither by an edge-on configuration nor by any
other inclination configuration of the orbit.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in MNRA
Parametric study on the impact of thermal bridges on the heat loss of internally insulated buildings
Internal wall insulation as energy efficiency retrofit measure could considerably help to reduce the greenhouse gas emissions of more than 6 million solid wall buildings in the UK. However during retrofit, junctions that are hard to reach are often left uninsulated, increasing heat loss and surface mould growth risk at thermal bridges. This paper presents a parametric study on the impact of thermal bridges on the total heat loss of an internally insulated mid-terrace house. Findings showed that heat flux through junctions occurred mainly at reveals and that the total heat flux at junctions per unit of exposed area was often higher than the default value used in the UK
Very hard states in neutron star low-mass X-ray binaries
We report on unusually very hard spectral states in three confirmed
neutron-star low-mass X-ray binaries (1RXS J180408.9-342058, EXO 1745-248, and
IGR J18245-2452) at a luminosity between ~ 10^{36-37} erg s^{-1}. When fitting
the Swift X-ray spectra (0.5 - 10 keV) in those states with an absorbed
power-law model, we found photon indices of \Gamma ~ 1, significantly lower
than the \Gamma = 1.5 - 2.0 typically seen when such systems are in their so
called hard state. For individual sources very hard spectra were already
previously identified but here we show for the first time that likely our
sources were in a distinct spectral state (i.e., different from the hard state)
when they exhibited such very hard spectra. It is unclear how such very hard
spectra can be formed; if the emission mechanism is similar to that operating
in their hard states (i.e., up-scattering of soft photons due to hot electrons)
then the electrons should have higher temperatures or a higher optical depth in
the very hard state compared to those observed in the hard state. By using our
obtained \Gamma as a tracer for the spectral evolution with luminosity, we have
compared our results with those obtained by Wijnands et al. (2015). We confirm
their general results in that also our sample of sources follow the same track
as the other neutron star systems, although we do not find that the accreting
millisecond pulsars are systematically harder than the non-pulsating systems.Comment: Accepted for publication in MNRA
The peculiar Galactic center neutron star X-ray binary XMM J174457-2850.3
The recent discovery of a milli-second radio pulsar experiencing an accretion
outburst similar to those seen in low mass X-ray binaries, has opened up a new
opportunity to investigate the evolutionary link between these two different
neutron star manifestations. The remarkable X-ray variability and hard X-ray
spectrum of this object can potentially serve as a template to search for other
X-ray binary/radio pulsar transitional objects. Here we demonstrate that the
transient X-ray source XMM J174457-2850.3 near the Galactic center displays
similar X-ray properties. We report on the detection of an energetic
thermonuclear burst with an estimated duration of ~2 hr and a radiated energy
output of ~5E40 erg, which unambiguously demonstrates that the source harbors
an accreting neutron star. It has a quiescent X-ray luminosity of Lx~5E32 erg/s
and exhibits occasional accretion outbursts during which it brightens to
Lx~1E35-1E36 erg/s for a few weeks (2-10 keV). However, the source often
lingers in between outburst and quiescence at Lx~1E33-1E34 erg/s. This unusual
X-ray flux behavior and its relatively hard X-ray spectrum, a power law with an
index of ~1.4, could possibly be explained in terms of the interaction between
the accretion flow and the magnetic field of the neutron star.Comment: 10 pages, 3 figures, 2 tables, accepted to ApJ after minor revision
(provided a more detailed description of the long-term X-ray behavior in
Section 3.1 and Figure 1
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
Assessing fungal risk criteria via simulated scenarios to address disparity between method and outcomes
Internal wall insulation is one of the most promising methods of improving the energy efficiency of buildings while maintaining their original facades and construction. However, porous materials or poor construction quality can allow moisture ingress, creating favourable conditions for mould development within building fabric. Currently, there is no established guidance on moisture criteria in the construction industry for the assessment of interstitial mould growth. Some studies have considered relative humidity (RH) criteria that account for the duration of exposure but potentially overestimate risk at interfaces. This study implemented more specific RH criteria, based not only on the duration of exposure but also on temperature and substrate material. Results demonstrated a dramatic decrease in predicted mould risk, with minimal risk to health or structural integrity, in comparison to the present more stringent standards
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