254 research outputs found
Unusual magneto-transport of YBa2Cu3O7-d films due to the interplay of anisotropy, random disorder and nanoscale periodic pinning
We study the general problem of a manifold of interacting elastic lines whose
spatial correlations are strongly affected by the competition between random
and ordered pinning. This is done through magneto-transport experiments with
YBa2Cu3O7-d thin films that contain a periodic vortex pinning array created via
masked ion irradiation, in addition to the native random pinning. The strong
field-matching effects we observe suggest the prevalence of periodic pinning,
and indicate that at the matching field each vortex line is bound to an
artificial pinning site. However, the vortex-glass transition dimensionality,
quasi-2D instead of the usual 3D, evidences reduced vortex-glass correlations
along the vortex line. This is also supported by an unusual angular dependence
of the magneto-resistance, which greatly differs from that of Bose-glass
systems. A quantitative analysis of the angular magnetoresistance allows us to
link this behaviour to the enhancement of the system anisotropy, a collateral
effect of the ion irradiation
Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1
We present our Swift monitoring campaign of the slowly rotating neutron star
Be/X-ray transient GX 304-1 (spin period of ~275 s) when the source was not in
outburst. We found that between its type-I outbursts the source recurrently
exhibits a slowly decaying low-luminosity state (with luminosities of
10^(34-35) erg/s). This behaviour is very similar to what has been observed for
another slowly rotating system, GRO J1008-57. For that source, this
low-luminosity state has been explained in terms of accretion from a
non-ionised ('cold') accretion disk. Due to the many similarities between both
systems, we suggest that GX 304-1 enters a similar accretion regime between its
outbursts. The outburst activity of GX 304-1 ceased in 2016. Our continued
monitoring campaign shows that the source is in a quasi-stable low-luminosity
state (with luminosities a few factors lower than previously seen) for at least
one year now. Using our NuSTAR observation in this state, we found pulsations
at the spin period, demonstrating that the X-ray emission is due to accretion
of matter onto the neutron star surface. If the accretion geometry during this
quasi-stable state is the same as during the cold-disk state, then matter
indeed reaches the surface (as predicted) during this latter state. We discuss
our results in the context of the cold-disk accretion model.Comment: 5 pages, 2 figures, accepted for A&A Letter
Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27
The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in
2015. After the source transited to quiescence, we triggered our multi-year
Chandra monitoring programme to study its quiescent behaviour. The programme
was designed to follow the cooling of a potentially heated neutron-star crust
due to accretion of matter during the preceding outburst, similar to what we
potentially have observed before in two other Be/X-ray transients, namely 4U
0115+63 and V 0332+53. However, unlike for these other two systems, we do not
find any strong evidence that the neutron-star crust in GRO J1750-27 was indeed
heated during the accretion phase. We detected the source at a rather low X-ray
luminosity (~10^33 erg/s) during only three of our five observations. When the
source was not detected it had very low-luminosity upper limits (<10^32 erg/s;
depending on assumed spectral model). We interpret these detections and the
variability observed as emission likely due to very low-level accretion onto
the neutron star. We also discuss why the neutron-star crust in GRO J1750-27
might not have been heated while the ones in 4U 0115+63 and V 0332+53 possibly
were.Comment: 13 pages, 6 figures, 5 tables. Accepted for A&
Consistent accretion-induced heating of the neutron-star crust in MXB 1659-29 during two different outbursts
Monitoring the cooling of neutron-star crusts heated during accretion
outbursts allows us to infer the physics of the dense matter present in the
crust. We examine the crust cooling evolution of the low-mass X-ray binary MXB
1659-29 up to ~505 days after the end of its 2015 outburst (hereafter outburst
II) and compare it with what we observed after its previous 1999 outburst
(hereafter outburst I) using data obtained from the Swift, XMM-Newton, and
Chandra observatories. The observed effective surface temperature of the
neutron star in MXB 1659-29 dropped from ~92 eV to ~56 eV from ~12 days to ~505
days after the end of outburst II. The most recently performed observation
after outburst II suggests that the crust is close to returning to thermal
equilibrium with the core. We model the crust heating and cooling for both its
outbursts collectively to understand the effect of parameters that may change
for every outburst (e.g., the average accretion rate, the length of outburst,
the envelope composition of the neutron star at the end of the outburst) and
those which can be assumed to remain the same during these two outbursts (e.g.,
the neutron star mass, its radius). Our modelling indicates that all parameters
were consistent between the two outbursts with no need for any significant
changes. In particular, the strength and the depth of the shallow heating
mechanism at work (in the crust) were inferred to be the same during both
outbursts, contrary to what has been found when modelling the cooling curves
after multiple outburst of another source, MAXI J0556-332. This difference in
source behaviour is not understood. We discuss our results in the context of
our current understanding of cooling of accretion-heated neutron-star crusts,
and in particular with respect to the unexplained shallow heating mechanism.Comment: Submitted to A&A. The supplementary video can be found at
https://www.youtube.com/watch?v=OpJ053zq9-
Warming will affect phytoplankton differently: evidence through a mechanistic approach
Although the consequences of global warming in aquatic ecosystems are only beginning to be revealed, a key to forecasting the impact on aquatic communities is an understanding of individual species' vulnerability to increased temperature. Despite their microscopic size, phytoplankton support about half of the global primary production, drive essential biogeochemical cycles and represent the basis of the aquatic food web. At present, it is known that phytoplankton are important targets and, consequently, harbingers of climate change in aquatic systems. Therefore, investigating the capacity of phytoplankton to adapt to the predicted warming has become a relevant issue. However, considering the polyphyletic complexity of the phytoplankton community, different responses to increased temperature are expected. We experimentally tested the effects of warming on 12 species of phytoplankton isolated from a variety of environments by using a mechanistic approach able to assess evolutionary adaptation (the so-called ratchet technique). We found different degrees of tolerance to temperature rises and an interspecific capacity for genetic adaptation. The thermal resistance level reached by each species is discussed in relation to their respective original habitats. Our study additionally provides evidence on the most resistant phytoplankton groups in a future warming scenario
Quality assurance of surface wind observations from automated weather stations
Meteorological data of good quality arc important for understanding both global and regional climates In this respect, great efforts have been made to evaluate temperature- and precipitation-related records This study summarizes the evaluations made to date of the quality of wind speed and direction records acquired at 41 automated weather stations in the northeast of the Iberian Peninsula Observations were acquired from 1992 to 2005 at a temporal resolution of 10 and 30 min A quality assurance system was imposed to select) the records for 1) manipulation errors associated with storage and management of the data. 2) consistency limits to to ensure that observations ale within their natural limits of variation, and 3) temporal consistency to assess abnormally low/high variations in the individual time series In addition. the most important biases of the dataset are analyzed and corrected wherever possible A total of 1 8% wind speed and 3 7% wind direction records was assumed invalid. pointing to specific problems in wind measurement The study not only tiles to contribute to the science with the creation of a wind damsel of unmoved quality. but it also reports on potential errors that could be plc:sent in other wind dataset
Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63
Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63
In 2017, the Be/X-ray transient 4U 0115+63 exhibited a new type-II outburst
that was two times fainter than its 2015 giant outburst (in the Swift/BAT count
rates). Despite this difference between the two bright events, the source
displayed similar X-ray behaviour after these periods. Once the outbursts
ceased, the source did not transit towards quiescence directly, but was
detected about a factor of 10 above its known quiescent level. It eventually
decayed back to quiescence over time scales of months. In this paper we present
the results of our Swift monitoring campaign, and an XMM-Newton observation of
4U 0115+63 during the decay of the 2017 type-II outburst, and its subsequent
low-luminosity behaviour. We discuss the possible origin of the decaying source
emission at this low-level luminosity, which has now been shown as a recurrent
phenomenon, in the framework of the two proposed scenarios to explain this
faint state: cooling from an accretion-heated neutron-star crust or continuous
low-level accretion. In addition, we compare the outcome of our study with the
results we obtained from the 2015/2016 monitoring campaign on this source.Comment: 12 pages, 5 figures, 3 tables. Accepted, Astronomy & Astrophysic
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