231 research outputs found
2XMMi J225036.9+573154 - a new eclipsing AM Her binary discovered using XMM-Newton
We report the discovery of an eclipsing polar, 2XMMi J225036.9+573154, using
XMM-Newton. It was discovered by searching the light curves in the 2XMMi
catalogue for objects showing X-ray variability. Its X-ray light curve shows a
total eclipse of the white dwarf by the secondary star every 174 mins. An
extended pre-eclipse absorption dip is observed in soft X-rays at phi=0.8-0.9,
with evidence for a further dip in the soft X-ray light curve at phi~0.4.
Further, X-rays are seen from all orbital phases (apart from the eclipse) which
makes it unusual amongst eclipsing polars. We have identified the optical
counterpart, which is faint (r=21), and shows a deep eclipse (>3.5 mag in white
light). Its X-ray spectrum does not show a distinct soft X-ray component which
is seen in many, but not all, polars. Its optical spectrum shows Halpha in
emission for a fraction of the orbital period.Comment: Accepted for publication in MNRA
Climate Change Across The Macaronesian Geographical Region, 1850 - 2100
The Macaronesian geographical zone extends from 10-40°N, 325-355°E and primarily includes the island chains of the Azores, Madeira, the Canary Islands and Cape Verde. This thesis presents a wide-ranging analysis of the physical climate and oceanography of the region back to 1850, in order to place recent climate change within a historical context. Subsequently, this thesis presents the most complete documentation of the physical climate of Macaronesia in the English language literature. One of the main outputs of this thesis is the creation of a long-term, monthly surface air temperature record for each island chain (from 1865 for the Azores and Madeira, 1885 for the Canary Islands and 1895 for Cape Verde). These temperature records exhibit generally coherent patterns of variability, and a post-1976 increase in temperature - most probably reflecting an anthropogenic climate signal - is the most ubiquitous, significant rise (or fall) in the record. Precipitation variability is also analysed, although only trends from Cape Verde are particularly significant, where a slight precipitation recovery - after the turn of the Twenty-first Century since drought conditions in the mid-late Twentieth Century - is apparent. Climatological extreme indices, based on calculations that assimilate daily temperature and precipitation data, were also analysed for the recent past (1979-2011) and point towards warmer conditions. An assessment of potential future changes in the mean state and extreme indices of climate across the islands by the end of the Twenty-first Century is provided. Warming magnitudes for the 2071-2100 period range between 0.8-3.0°C above the 1976-2005 mean temperature. Precipitation is expected to decrease across the Canary Islands and Madeira, whereas the Azores is expected to experience more extreme precipitation events and precipitation changes across Cape Verde are uncertain.
In addition to the analysis of temperature and precipitation changes, a daily North Atlantic Oscillation index extending back to 1850 using historical sea-level pressure data from the Azores was constructed. The temporal length of this newly created index exceeds the length of any previously available long-running, daily-resolution series by a hundred years and should be of great value to researchers across multiple disciplines. The spatial and temporal variability of the North Atlantic Oscillation was analysed, finding an increase in post-2004 winter variability, alongside a post-1991 negative summer trend. A novel method to characterise the strength of the Trade Winds by using data from the Azores and Cape Verde was also developed. The newly-defined Trade Wind index has been steadily increasing since 1973.
An additional analysis was a comprehensive overview and reconciliation of multiple data sources to answer the question of whether coastal upwelling has been increasing across the Canary Upwelling Ecosystem along the northwest African coastline. This analysis determined that the Bakun upwelling intensification hypothesis developed in 1990 appears to be realised in the summertime coastal upwelling indices. The North Atlantic Oscillation was discovered to be strongly related to upwelling magnitudes for all seasons except summer, in addition to exerting a strong control on temperatures and precipitation across the three northernmost Macaronesian island chains. The small-scale features affecting island climates and the large-scale modes of variability that influence the Macaronesian region are also discussed
First Spitzer Space Telescope Observations of Magnetic Cataclysmic Variables: Evidence for Excess Emission at 3--8 microns
We present the first observations of magnetic cataclysmic variables with the
Spitzer Space Telescope. We used the Infrared Array Camera to obtain photometry
of the polars EF Eri, GG Leo, V347 Pav, and RX J0154.0-5947 at 3.6, 4.5, 5.8,
and 8.0 m. In all of our targets, we detect excess mid-infrared emission
over that expected from the component stars alone. We explore the origin of
this IR excess by examining bremsstrahlung, cyclotron emission, circumbinary
dust, and L/T brown dwarf secondary stars. Bremsstrahlung and cyclotron
emission appear unlikely to be significant contributors to the observed fluxes.
At present, the most likely candidate for the excess emission is dust that is
probably located in a circumbinary disk with an inner temperature near 800 K.
However, a simple dust disk plus any reasonable low mass or brown dwarf-like
secondary star is unable to fully explain the observed flux densities in the
3--8 m region.Comment: Accepted to ApJ Letter
Anomalous blocking over Greenland preceded the 2013 extreme early melt of local sea ice
The Arctic marine environment is undergoing a transition from thick multi-year to first-year sea ice cover with coincident lengthening of the melt season. Such changes are evident in the Baffin Bay-Davis Strait-Labrador Sea (BDL) region where melt onset has occurred ~8 days decade-1 earlier from 1979-2015. A series of anomalously early events has occurred since the mid-1990s, overlapping a period of increased upper-air ridging across Greenland and the northwestern North Atlantic. We investigate an extreme early melt event observed in spring 2013 below the 1981-2010 melt climatology), with respect to preceding sub-seasonal mid-tropospheric circulation conditions as described by a daily Greenland Blocking Index (GBI).
The 40-days prior to the 2013 BDL melt onset are characterized by a persistent, strong 500 hPa anticyclone over the region (GBI >+1 on >75% of days). This circulation pattern advected warm air from northeastern Canada and the northwestern Atlantic poleward onto the thin, first-year sea ice and caused melt about 50 days earlier than normal. The episodic increase in the ridging atmospheric pattern near western Greenland as in 2013, exemplified by large positive GBI values, is an important recent process impacting the atmospheric circulation over a North Atlantic cryosphere undergoing accelerated regional climate change
On the shear estimation bias induced by the spatial variation of colour across galaxy profiles
The spatial variation of the colour of a galaxy may introduce a bias in the
measurement of its shape if the PSF profile depends on wavelength. We study how
this bias depends on the properties of the PSF and the galaxies themselves. The
bias depends on the scales used to estimate the shape, which may be used to
optimise methods to reduce the bias. Here we develop a general approach to
quantify the bias. Although applicable to any weak lensing survey, we focus on
the implications for the ESA Euclid mission.
Based on our study of synthetic galaxies we find that the bias is a few times
10^-3 for a typical galaxy observed by Euclid. Consequently, it cannot be
neglected and needs to be accounted for. We demonstrate how one can do so using
spatially resolved observations of galaxies in two filters. We show that HST
observations in the F606W and F814W filters allow us to model and reduce the
bias by an order of magnitude, sufficient to meet Euclid's scientific
requirements. The precision of the correction is ultimately determined by the
number of galaxies for which spatially-resolved observations in at least two
filters are available. We use results from the Millennium Simulation to
demonstrate that archival HST data will be sufficient for the tomographic
cosmic shear analysis with the Euclid dataset.Comment: MNRAS submitted, 18 pages, 13 Figure
Low State, Phase-Resolved IR Spectroscopy of VV Puppis
We present phase-resolved low resolution and higher resolution -band
spectroscopy of the polar VV Pup. All observations were obtained when VV Pup
was in a low accretion state having a K magnitude near 15. The low resolution
observations reveal cyclotron emission in the band during some phases,
consistent with an origin near the active 30.5 MG pole on the white dwarf. The
secondary in VV Pup appears to be a normal M7V star and we find that the
and band fluxes are entirely due to this star at all orbital phases during
the low accretion state. We use our higher resolution Keck spectroscopy to
produce the first -band radial velocity curve for VV Pup. Our orbital
solution yields =414 km sec and leads to mass estimates of
M=0.730.05 M and M=0.100.02 M. We find
that the mass accretion rates during the normal low states of the polars VV
Pup, EF Eri, and EQ Cet are near 10 M yr. The fact
that \.M is not zero in low state polars indicates active secondary stars in
these binary systems, including the sub-stellar donor star present in EF Eri.Comment: Accepted in Astronomical Journal 5 figure
Zeeman tomography of magnetic white dwarfs. IV, The complex field structure of the polars EF Eridani, BL Hydri and CP Tucanae
Context. The magnetic fields of the accreting white dwarfs in magnetic cataclysmic variables (mCVs) determine the accretion geometries, the emission properties, and the secular evolution of these objects.
Aims. We determine the structure of the surface magnetic fields of the white dwarf primaries in magnetic CVs using Zeeman tomography.
Methods. Our study is based on orbital-phase resolved optical flux and circular polarization spectra of the polars EF Eri, BL Hyi, and CP Tuc obtained with FORS1 at the ESO VLT. An evolutionary algorithm is used to synthesize best fits to these spectra from an
extensive database of pre-computed Zeeman spectra. The general approach has been described in previous papers of this series.
Results. The results achieved with simple geometries as centered or offset dipoles are not satisfactory. Significantly improved fits are obtained for multipole expansions that are truncated at degree lmax = 3 or 5 and include all tesseral and sectoral components with
0 †m †l. The most frequent field strengths of 13, 18, and 10MG for EF Eri, BL Hyi, and CP Tuc, and the ranges of field strength covered are similar for the dipole and multipole models, but only the latter provide access to accreting matter at the right locations on the white dwarf. The results suggest that the field geometries of the white dwarfs in short-period mCVs are quite complex, with
strong contributions from multipoles higher than the dipole in spite of a typical age of the white dwarfs in CVs in excess of 1 Gyr.
Conclusions. It is feasible to derive the surface field structure of an accreting white dwarf from phase-resolved low-state circular spectropolarimetry of sufficiently high signal-to-noise ratio. The fact that independent information is available on the strength and
direction of the field in the accretion spot from high-state observations helps in unraveling the global field structure
Quasi-periodic oscillations in accreting magnetic white dwarfs II. The asset of numerical modelling for interpreting observations
Magnetic cataclysmic variables are close binary systems containing a strongly
magnetized white dwarf that accretes matter coming from an M-dwarf companion.
High-energy radiation coming from those objects is emitted from the accretion
column close to the white dwarf photosphere at the impact region. Its
properties depend on the characteristics of the white dwarf and an accurate
accretion column model allows the properties of the binary system to be
inferred, such as the white dwarf mass, its magnetic field, and the accretion
rate. We study the temporal and spectral behaviour of the accretion region and
use the tools we developed to accurately connect the simulation results to the
X-ray and optical astronomical observations. The radiation hydrodynamics code
Hades was adapted to simulate this specific accretion phenomena. Classical
approaches were used to model the radiative losses of the two main radiative
processes: bremsstrahlung and cyclotron. The oscillation frequencies and
amplitudes in the X-ray and optical domains are studied to compare those
numerical results to observational ones. Different dimensional formulae were
developed to complete the numerical evaluations. The complete characterization
of the emitting region is described for the two main radiative regimes: when
only the bremsstrahlung losses and when both cyclotron and bremsstrahlung
losses are considered. The effect of the non-linear cooling in- stability
regime on the accretion column behaviour is analysed. Variation in luminosity
on short timescales (~ 1 s quasi-periodic oscillations) is an expected
consequence of this specific dynamic. The importance of secondary shock
instability on the quasi-periodic oscillation phenomenon is discussed. The
stabilization effect of the cyclotron process is confirmed by our numerical
simulations, as well as the power distribution in the various modes of
oscillation.Comment: 13 pages, 13 figures, 2 tables. Accepted for publication in A&
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