9,391 research outputs found
Microlensing Searches for Exoplanets
Gravitational microlensing finds planets through their gravitational
influence on the light coming from a more distant background star. The presence
of the planet is then inferred from the tell-tale brightness variations of the
background star during the lensing event, even if no light is detectable from
the planet or the host foreground star. This review covers fundamental
theoretical concepts in microlensing, addresses how observations are performed
in practice, the~challenges of obtaining accurate measurements, and explains
how planets reveal themselves in the data. It~concludes with a presentation of
the most important findings to-date, a description of the method's strengths
and weaknesses, and a discussion of the future prospects of microlensing.Comment: 35 pages,9 figures, invited review for Geosciences Special Issue
"Detection and Characterization of Extrasolar Planets
Evidence for a Variable Ultrafast Outflow in the Newly Discovered Ultraluminous Pulsar NGC 300 ULX-1
Ultraluminous pulsars are a definite proof that persistent super-Eddington
accretion occurs in nature. They support the scenario according to which most
Ultraluminous X-ray Sources (ULXs) are super-Eddington accretors of stellar
mass rather than sub-Eddington intermediate mass black holes. An important
prediction of theories of supercritical accretion is the existence of powerful
outflows of moderately ionized gas at mildly relativistic speeds. In practice,
the spectral resolution of X-ray gratings such as RGS onboard XMM-Newton is
required to resolve their observational signatures in ULXs. Using RGS, outflows
have been discovered in the spectra of 3 ULXs (none of which are currently
known to be pulsars). Most recently, the fourth ultraluminous pulsar was
discovered in NGC 300. Here we report detection of an ultrafast outflow (UFO)
in the X-ray spectrum of the object, with a significance of more than
3{\sigma}, during one of the two simultaneous observations of the source by
XMM-Newton and NuSTAR in December 2016. The outflow has a projected velocity of
65000 km/s (0.22c) and a high ionisation factor with a log value of 3.9. This
is the first direct evidence for a UFO in a neutron star ULX and also the first
time that this its evidence in a ULX spectrum is seen in both soft and hard
X-ray data simultaneously. We find no evidence of the UFO during the other
observation of the object, which could be explained by either clumpy nature of
the absorber or a slight change in our viewing angle of the accretion flow.Comment: 10 pages, 4 figures. Accepted to MNRA
The X-ray emission of the gamma Cassiopeiae stars
Long considered as the "odd man out" among X-ray emitting Be stars, \gamma
Cas (B0.5e IV) is now recognized as the prototype of a class of stars that emit
hard thermal X-rays. Our classification differs from the historical use of the
term "gamma Cas stars" defined from optical properties alone. The luminosity
output of this class contributes significantly to the hard X-ray production in
massive stars in the Galaxy. The gamma Cas stars have light curves showing
variability on a few broadly-defined timescales and spectra indicative of an
optically thin plasma consisting of one or more hot thermal components. By now
9--13 Galactic \approx B0-1.5e main sequence stars are judged to be members or
candidate members of the \gamma Cas class. Conservative criteria for this
designation are for a \approxB0-1.5e III-V star to have an X-ray luminosity of
10^{32}--10^{33} ergs s^{-1}, a hot thermal spectrum containing the short
wavelength Ly \alpha FeXXV and FeXXVI lines and the fluorescence FeK feature
all in emission. If thermality cannot be demonstrated, for example from either
the presence of these Ly \alpha lines or curvature of the hard continuum; these
are the gamma Cas candidates. We discuss the history of the discovery of the
complicated characteristics of the variability in the optical, UV, and X-ray
domains, leading to suggestions for the physical cause of the production of
hard X-rays. These include scenarios in which matter from the Be star accretes
onto a degenerate secondary star and interactions between magnetic fields on
the Be star and its decretion disk. The greatest aid to the choice of the
causal mechanism is the temporal correlations of X-ray light curves and spectra
with diagnostics in the optical and UV wavebands. We show why the magnetic
star-disk interaction scenario is the most tenable explanation for the creation
of hard X-rays on these stars.Comment: Review paper for "X-ray Emissions from Hot Stars and their Winds"
compendium to be published by Advances in Space Research in mid-2016. Paper
is comprised of 66 pages, 15 figure
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