172 research outputs found
The orbital modulation in the radio emission of Cygnus X-1
We present model lightcurves which have been created in order to explain the
orbital modulation observed in the radio emission of Cyg X-1. We invoke
variable absorption by the stellar wind as the black hole jet orbits around the
OB companion star and find that a very simple model is able to reproduce the
amplitudes and frequency dependence of the observed lightcurves.Comment: Accepted for publication in MNRA
Orbital modulation and longer-term variability in the radio emission from Cygnus X-1
20 months of observations of the radio emission at 15 GHz from Cygnus X-1,
starting in 1996 October, show variations at the binary period of 5.6 days, but
with a phase offset from those at X-ray wavelengths. There are also longer-term
variations on a time-scale of 150 days which are only loosely related to the
soft X-ray flux. The source was in the hard/low X-ray state throughout this
period. The mean 15-GHz flux density is 13 mJy, the radio spectrum is flat, and
the semi-amplitude of the orbital modulation about 2 mJy. We discuss the
possible origins of the modulation and the relationship to the soft X-ray
emission.Comment: Accepted for publication in MNRA
"Soft X-ray transient" outbursts which aren't soft
We have accumulated multiwavelength (X-ray, optical, radio) lightcurves for
the eight black hole X-ray binaries which have been observed to enter a
supposed `soft X-ray transient' outburst, but remained in the low/hard state
throughout the outburst. Comparison of the lightcurve morphologies, spectral
behaviour, properties of the quasi-periodic oscillations and the radio jet
provides the first study of such objects as a sub-class of X-ray transients.
However rather than assuming that these hard state X-ray transients are
different from the `canonical' soft X-ray transient, we prefer to consider the
possibility that new analysis of both soft and hard state X-ray transients in a
spectral context will provide a model capable of explaining the outburst
mechanisms of (almost) all black hole X-ray binaries.Comment: Accepted for publication in New Astronom
The Hard Truth about Some "Soft" X-ray Transients
We have accumulated multiwavelength lightcurves for eight black hole X-ray
binaries which have been observed to enter a supposed ``soft X-ray transient''
outburst, but which in fact remained in the low/hard state throughout the
outburst. Comparison of the lightcurve morphologies, spectral behaviour,
properties of the QPOs and the radio jet provides the first study of such
objects as a subclass of X-ray transients (XRTs). However, rather than assuming
that these hard state XRTs are different from ``canonical'' soft XRTs, we
prefer to consider the possibility that a new analysis of both soft and hard
state XRTs in a spectral context will provide a model capable of explaining the
outburst mechanisms for the majority of black hole X-ray binaries.Comment: 3 pages, 2 figures; to appear in the Proceedings of the 4th
Microquasar Workshop, 2002, eds. Durouchoux, Fuchs & Rodriguez, published by
the Center for Space Physics: Kolkat
A decelerating jet observed by the EVN and VLBA in the X-ray transient XTE J1752-223
The recently discovered Galactic X-ray transient XTE J1752-223 entered its
first known outburst in 2010, emitting from the X-ray to the radio regimes. Its
general X-ray properties were consistent with those of a black hole candidate
in various spectral states, when ejection of jet components is expected. To
verify this, we carried out very long baseline interferometry (VLBI)
observations. The measurements were carried out with the European VLBI Network
(EVN) and the Very Long Baseline Array (VLBA) at four epochs in 2010 February.
The images at the first three epochs show a moving jet component that is
significantly decelerated by the last epoch, when a new jet component appears
that is likely to be associated with the receding jet side. The overall picture
is consistent with an initially mildly relativistic jet, interacting with the
interstellar medium or with swept-up material along the jet. The brightening of
the receding ejecta at the final epoch can be well explained by initial Doppler
deboosting of the emission in the decelerating jet.Comment: Accepted for publication in MNRAS Letters. 5 pages, 2 figure
Disentangling jet and disc emission from the 2005 outburst of XTE J1118+480
The black hole X-ray transient, XTE J1118+480, has now twice been observed in
outburst - 2000 and 2005 - and on both occasions remained in the low/hard X-ray
spectral state. Here we present radio, infrared, optical, soft X-ray and hard
X-ray observations of the more recent outburst. We find that the lightcurves
have very different morphologies compared with the 2000 event and the optical
decay is delayed relative to the X-ray/radio. We attribute this lesser degree
of correlation to contributions of emission from multiple components, in
particular the jet and accretion disc. Whereas the jet seemed to dominate the
broadband spectrum in 2000, in 2005 the accretion disc seems to be more
prominent and we use an analysis of the lightcurves and spectra to distinguish
between the jet and disc emission. There also appears to be an optically thin
component to the radio emission in the 2005 data, possibly associated with
multiple ejection events and decaying as the outburst proceeds. These results
add to the discussion that the term "low/hard state'" covers a wider range of
properties than previously thought, if it is to account for XTE J1118+480
during these two outbursts.Comment: Accepted for publication in MNRA
A search for electron cyclotron maser emission from compact binaries
Unipolar induction (UI) is a fundamental physical process, which occurs when
a conducting body transverses a magnetic field. It has been suggested that UI
is operating in RX J0806+15 and RX J1914+24, which are believed to be
ultra-compact binaries with orbital periods of 5.4 min and 9.6 min
respectively. The UI model predicts that those two sources may be electron
cyclotron maser sources at radio wavelengths. Other systems in which UI has
been predicted to occur are short period extra-solar terrestrial planets with
conducting cores. If UI is present, circularly polarised radio emission is
predicted to be emitted. We have searched for this predicted radio emission
from short period binaries using the VLA and ATCA. In one epoch we find
evidence for a radio source, coincident in position with the optical position
of RX J0806+15. Although we cannot completely exclude that this is a chance
alignment between the position of RX J0806+15 and an artifact in the data
reduction process, the fact that it was detected at a significance level of 5.8
sigma and found to be transient, suggests that it is more likely that RX
J0806+15 is a transient radio source. We find an upper limit on the degree of
circular polarisation to be ~50%. The inferred brightness temperature exceeds
10^18 K, which is too high for any known incoherent process, but is consistent
with maser emission and UI being the driving mechanism. We did not detect radio
emission from ES Cet, RX J1914+24 or Gliese 876.Comment: Accepted for publication MNRA
A highly polarised radio jet during the 1998 outburst of the black hole transient XTE J1748-288
XTE J1748-288 is a black hole X-ray transient which went into outburst in
1998 June. The X-ray lightcurves showed canonical morphologies, with minor
variations on the ``Fast Rise Exponential Decay'' profile. The radio source,
however, reached an unusually high flux density of over 600 mJy. This high
radio flux was accompanied by an exceptional (>20%) fractional linear
polarisation, the variability of which was anti-correlated with the flux
density. We use this variability to discuss possible depolarisation mechanisms
and to predict the underlying behaviour of the (unresolved) core/jet
components.Comment: Accepted for publication in MNRA
Three episodes of jet activity in the FRII radio galaxy B0925+420
We present Very Large Array images of a "Double-Double Radio Galaxy", a class
of objects in which two pairs of lobes are aligned either side of the nucleus.
In this object, B0925+420, we discover a third pair of lobes, close to the core
and again in alignment with the other lobes. This first-known "Triple-Double"
object strongly increases the likelihood that these lobes represent mutiple
episodes of jet activity, as opposed to knots in an underlying jet. We model
the lobes in terms of their dynamical evolution. We find that the inner pair of
lobes is consistent with the outer pair having been displaced buoyantly by the
ambient medium. The middle pair of lobes is more problematic - to the extent
where an alternative model interpreting the middle and inner "lobes" as
additional bow shocks within the outer lobes may be more appropriate - and we
discuss the implications of this on our understanding of the density of the
ambient medium.Comment: Accepted for publication in MNRAS. Figure 2 is best viewed in colou
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