7,692 research outputs found
NIR and optical observations of the failed outbursts of black hole binary XTE J1550-564
A number of low mass X-ray binaries (LMXBs) undergo "failed outbursts" in
which, instead of evolving through the canonical states, they remain in a hard
state throughout the outburst. While the sources of X-ray and radio emission in
the hard state are relatively well understood, the origin of the near infrared
(NIR) and optical emission is more complex though it likely stems from an
amalgam of different emission processes, occurring as it does, at the
intersecting wavelengths of those processes. We aim to identify the NIR/optical
emission region(s) during a number of failed outbursts of one such low mass
X-ray binary and black hole candidate, XTE J1550-564, in order to confirm or
refute their classification as hard-state, failed outbursts. We present unique
NIR/optical images and spectra, obtained with the ESO-New Technology Telescope,
during the failed outbursts of 2001 and 2000. We compare the NIR/optical
photometric, timing, and spectral properties with those expected for the
different emission mechanisms in the various LMXB states. The NIR/optical data
are consistent with having come from reprocessing of X-rays in the accretion
disk, with no evidence of direct thermal emission from the disk itself.
However, the observed variability in high-cadence NIR light curves suggest that
the radio jet extends and contributes to the NIR wavelengths. We find that
these failed outbursts did not transition to an intermediate state but remained
in a true, hard state where there was no sign of jet quenching or deviation
from the observed hard state correlations.Comment: Accepted to Astronomy & Astrophysics (8 pages
A search for near infrared counterparts of 3 pulsar wind nebulae
While pulsar wind nebulae (PWNe) and their associated isolated pulsars are
commonly detected at X-ray energies, they are much rarer at near infrared (nIR)
and optical wavelengths. Here we examine three PWN systems in the Galactic
plane - IGR J14003-6326, HESS J1632-478 and IGR J18490-0000 - in a bid to
identify optical/nIR emission associated with either the extended PWNe or their
previously detected X-ray point sources. We obtain optical/nIR images of the
three fields with the ESO - New Technology Telescope and apply standard
photometric and astrometric calibrations. We find no evidence of any extended
emission associated with the PWNe in any of the fields; neither do we find any
new counterparts to the X-ray point sources, except to confirm the magnitude of
the previously identified counterpart candidate of IGR J18490-0000. Further
observations are required to confirm the association of the nIR source to IGR
J18490-0000 and to detect counterparts to IGR J14003-6326 and HESS J1632-478,
while a more accurate X-ray position is required to reduce the probability of a
chance superposition in the field of the latter.Comment: Accepted to A&A (4 pages, 1 figure
Observations of particle acceleration in the blast waves of Gamma-ray Bursts
The electron energy distribution index, p, is a fundamental parameter of the
process by which electrons at shock fronts are accelerated to relativistic
speeds and by which they radiate, via synchrotron emission. This acceleration
process is applicable to a myriad of astronomical jet sources such as AGN,
X-ray binaries and gamma-ray bursts (GRBs), as well as to particle acceleration
in the solar wind and supernovae, and the acceleration of cosmic rays. The
accurate measurement of the distribution of p is of fundamental importance to
differentiate between the possible theories of electron acceleration at any
relativistic shock front; there is division as to whether p has a universal
value or whether it has a distribution, and if so, what that distribution is.
Here one such source of synchrotron emission is examined: the blast waves of
GRB afterglows observed by the Swift satellite. Within the framework of the GRB
blast wave model, the constraints placed on the distribution of p by the
observed X-ray spectral and temporal indices are examined and the distribution
parametrized. The results show that the observed distribution of spectral
indices is inconsistent with an underlying distribution of p composed of a
single discrete value but consistent with a Gaussian distribution centred at p
= 2.4 and having a width of 0.6. This finding disagrees with theoretical work
that argues for a single, universal value of p, but also demonstrates that the
width of the distribution is not as wide as has been suggested by some authors.Comment: 6 pages, 2 figures; accepted to the proceedings of the meeting, HEPRO
III: High Energy Phenomena in Relativistic Outflows (Barcelona, June 2011
Encoding of Marginal Utility across Time in the Human Brain
Marginal utility theory prescribes the relationship between the objective property of the magnitude of rewards and their subjective value. Despite its pervasive influence, however, there is remarkably little direct empirical evidence for such a theory of value, let alone of its neurobiological basis. We show that human preferences in an intertemporal choice task are best described by a model that integrates marginally diminishing utility with temporal discounting. Using functional magnetic resonance imaging, we show that activity in the dorsal striatum encodes both the marginal utility of rewards, over and above that which can be described by their magnitude alone, and the discounting associated with increasing time. In addition, our data show that dorsal striatum may be involved in integrating subjective valuation systems inherent to time and magnitude, thereby providing an overall metric of value used to guide choice behavior. Furthermore, during choice, we show that anterior cingulate activity correlates with the degree of difficulty associated with dissonance between value and time. Our data support an integrative architecture for decision making, revealing the neural representation of distinct subcomponents of value that may contribute to impulsivity and decisiveness
Muon-spin rotation measurements of the vortex state in SrRuO: type-1.5 superconductivity, vortex clustering and a crossover from a triangular to a square vortex lattice
Muon-spin rotation has been used to probe vortex state in SrRuO. At
moderate fields and temperatures a lattice of triangular symmetry is observed,
crossing over to a lattice of square symmetry with increasing field and
temperature. At lower fields it is found that there are large regions of the
sample that are completely free from vortices which grow in volume as the
temperature falls. Importantly this is accompanied by {\it increasing} vortex
density and increasing disorder within the vortex-cluster containing regions.
Both effects are expected to result from the strongly temperature-dependent
long-range vortex attractive forces arising from the multi-band chiral-order
superconductivity.Comment: 13 pages, 4 figure
Equilibrium properties of the mixed state in superconducting niobium in a transverse magnetic field: Experiment and theoretical model
Equilibrium magnetic properties of the mixed state in type-II superconductors
were measured with high purity bulk and film niobium samples in parallel and
perpendicular magnetic fields using dc magnetometry and scanning Hall-probe
microscopy. Equilibrium magnetization data for the perpendicular geometry were
obtained for the first time. It was found that none of the existing theories is
consistent with these new data. To address this problem, a theoretical model is
developed and experimentally validated. The new model describes the mixed state
in an averaged limit, i.e. %without detailing the samples' magnetic structure
and therefore ignoring interactions between vortices. It is quantitatively
consistent with the data obtained in a perpendicular field and provides new
insights on properties of vortices. % and the entire mixed state. At low values
of the Ginzburg-Landau parameter, the model converts to that of Peierls and
London for the intermediate state in type-I superconductors. It is shown that
description of the vortex matter in superconductors in terms of a 2D gas is
more appropriate than the frequently used crystal- and glass-like scenarios.Comment: 8 pages, 9 figure
Initial data for black hole-neutron star binaries, with rotating stars
The coalescence of a neutron star with a black hole is a primary science
target of ground-based gravitational wave detectors. Constraining or measuring
the neutron star spin directly from gravitational wave observations requires
knowledge of the dependence of the emission properties of these systems on the
neutron star spin. This paper lays foundations for this task, by developing a
numerical method to construct initial data for black hole--neutron star
binaries with arbitrary spin on the neutron star. We demonstrate the robustness
of the code by constructing initial-data sets in large regions of the parameter
space. In addition to varying the neutron star spin-magnitude and
spin-direction, we also explore neutron star compactness, mass-ratio, black
hole spin, and black hole spin-direction. Specifically, we are able to
construct initial data sets with neutron stars spinning near centrifugal
break-up, and with black hole spins as large as .Comment: 25 pages, 12 figure
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