6,509 research outputs found
Radio-loudness in black hole transients: evidence for an inclination effect
Accreting stellar-mass black holes appear to populate two branches in a
radio:X-ray luminosity plane. We have investigated the X-ray variability
properties of a large number of black hole low-mass X-ray binaries, with the
aim of unveiling the physical reasons underlying the radio-loud/radio-quiet
nature of these sources, in the context of the known accretion-ejection
connection. A reconsideration of the available radio and X-ray data from a
sample of black hole X-ray binaries confirms that being radio-quiet is the more
normal mode of behaviour for black hole binaries. In the light of this we chose
to test, once more, the hypothesis that radio loudness could be a consequence
of the inclination of the X-ray binary. We compared the slope of the
`hard-line' (an approximately linear correlation between X-ray count rate and
rms variability, visible in the hard states of active black holes), the orbital
inclination, and the radio-nature of the sources of our sample. We found that
high-inclination objects show steeper hard-lines than low-inclination objects,
and tend to display a radio-quiet nature (with the only exception of V404 Cyg),
as opposed to low-inclination objects, which appear to be radio-loud(er). While
in need of further confirmation, our results suggest that - contrary to what
has been believed for years - the radio-loud/quiet nature of black-hole low
mass X-ray binaries might be an inclination effect, rather than an intrinsic
source property. This would solve an important issue in the context of the
inflow-outflow connection, thus providing significant constraints to the models
for the launch of hard-state compact jets.Comment: 16 pages, 6 figures, accepted for pubblication on MNRA
A connection between accretion states and the formation of ultra-relativistic outflows in a neutron star X-ray binary
The nearby accreting neutron star binary Sco X-1 is the closest example of
ongoing relativistic jet production at high Eddington ratios. Previous radio
studies have revealed that alongside mildly relativistic, radio-emitting
ejecta, there is at times a much faster transfer of energy from the region of
the accretion flow along the jet. The nature of this ultrarelativistic flow
remains unclear and while there is some evidence for a similar phenomenon in
other systems which might contain neutron stars, it has never been observed in
a confirmed black hole system. We have compared these previous radio
observations with a new analysis of simultaneous X-ray observations which were
performed with the RXTE mission. We find that the ejection of the
ultra-relativistic flow seems to be associated with the simultaneous appearance
of two particular types of quasi-periodic oscillations in the X-ray power
spectrum. In contrast, the mildly relativistic, radio-emitting outflows may be
associated with flat-topped broad band noise in the X-ray power spectrum. This
is the first time a link, albeit tentative, has been found between these
mysterious unseen flows and the accretion flow from which they are launched.Comment: 14 pages, 7 figures, accepted for publication in MNRA
Non-Gaussian features of primordial magnetic fields in power-law inflation
We show that a conformal-invariance violating coupling of the inflaton to
electromagnetism produces a cross correlation between curvature fluctuations
and a spectrum of primordial magnetic fields. According to this model, in the
case of power-law inflation, a primordial magnetic field is generated with a
nearly flat power spectrum and rms amplitude ranging from nG to pG. We study
the cross correlation, a three-point function of the curvature perturbation and
two powers of the magnetic field, in real and momentum space. The
cross-correlation coefficient, a dimensionless ratio of the three-point
function with the curvature perturbation and magnetic field power spectra, can
be several orders of magnitude larger than expected as based on the amplitude
of scalar metric perturbations from inflation. In momentum space, the
cross-correlation peaks for flattened triangle configurations, and is three
orders of magnitude larger than the squeezed triangle configuration. These
results suggest likely methods for distinguishing the observational signatures
of the model.Comment: 15 pages, 2 figure
Testing reflection features in 4U 1705-44 with XMM-Newton, BeppoSAX and RXTE in the hard and soft state
We use data of the bright atoll source 4U 1705-44 taken with XMM-Newton,
BeppoSAX and RXTE both in the hard and in the soft state to perform a
self-consistent study of the reflection component in this source. Although the
data from these X-ray observatories are not simultaneous, the spectral
decomposition is shown to be consistent among the different observations, when
the source flux is similar. We therefore select observations performed at
similar flux levels in the hard and soft state in order to study the spectral
shape in these two states in a broad band (0.1-200 keV) energy range, with good
energy resolution, and using self-consistent reflection models. These
reflection models provide a good fit for the X-ray spectrum both in the hard
and in the soft state in the whole spectral range. We discuss the differences
in the main spectral parameters we find in the hard and the soft state,
respectively, providing evidence that the inner radius of the optically thick
disk slightly recedes in the hard state.Comment: Accepted for publication in A&A, 20 pages, 12 figure
Stand structural complexity of mixed old-growth and adjacent selection forests in the Dinaric Mountains of Bosnia and Herzegovina
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