348 research outputs found
The ruff of equatorial emission around the SS433 jets: its spectral index and origin
We present unique radio observations of SS433, using MERLIN, the VLBA, and
the VLA, which allow us to, for the first time, properly image and derive a
meaningful spectral index for the `ruff' of equatorial emission which surrounds
SS433's jet. We interpret this smooth ruff as a wind-like outflow from the
binary.Comment: 4 pages, 4 figures, to appear in Proceedings of the 4th Microquasar
Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodrigue
Exploring a New Population of Compact Objects: X-ray and IR Observations of the Galactic Centre
I describe the IR and X-ray observational campaign we have undertaken for the
purpose of determining the nature of the faint discrete X-ray source population
discovered by Chandra in the Galactic Center (GC). Data obtained for this
project includes a deep Chandra survey of the Galactic Bulge; deep, high
resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane
Survey (GPS); and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By
cross-correlating the GC X-ray imaging from Chandra with our IR surveys, we
identify candidate counterparts to the X-ray sources via astrometry. Using a
detailed IR extinction map, we are deriving magnitudes and colors for all the
candidates. Having thus established a target list, we will use the multi-object
IR spectrograph FLAMINGOS-2 on Gemini-South to carry out a spectroscopic survey
of the candidate counterparts, to search for emission line signatures which are
a hallmark of accreting binaries. By determining the nature of these X-ray
sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on
our knowledge of the Galactic accreting binary population.Comment: 4 pages, 2 figures, to appear in the Proceedings of The Second
Kolkata Conference on Observational Evidence for Black Holes in the
Universe'', ed. S. Charkrabarti, Kolkata, India; AIP Conf. Serie
Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare
Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a
major radio outburst of Cygnus X-3 in 2001 September are presented, measuring
the evolution of the spectrum of the source over three decades in frequency,
over a period of six days. Following the peak of the flare, as the intensity
declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to
nu^{-0.6}, after which the spectral index remains at this latter terminal
value; a trend previously observed but hitherto not satisfactorily explained.
VLBA observations, for the first time, track over several days the expansion of
a sequence of knots whose initial diameters are approximately 8
milliarcseconds. The light-crossing time within these plasmons is of the same
order as the time-scale over which the spectrum is observed to evolve. We
contend that properly accounting for light-travel time effects in and between
plasmons which are initially optically thick, but which after expansion become
optically thin, explains the key features of the spectral evolution, for
example the observed timescale. Using the VLBA images, we have directly
measured for the first time the proper motions of individual knots, analysis of
which shows a two-sided jet whose axis is precessing. The best-fit jet speed is
roughly beta = 0.63 and the precession period is about 5 days, significantly
lower than fitted for a previous flare. Extrapolation of the positions of the
knots measured by the VLBA back to zero-separation shows this to occur
approximately 2.5 days after the detection of the rise in flux density of
Cygnus X-3.Comment: 23 pages, 10 figures, accepted by Ap
Control via electron count of the competition between magnetism and superconductivity in cobalt and nickel doped NaFeAs
Using a combination of neutron, muon and synchrotron techniques we show how
the magnetic state in NaFeAs can be tuned into superconductivity by replacing
Fe by either Co or Ni. Electron count is the dominant factor, since Ni-doping
has double the effect of Co-doping for the same doping level. We follow the
structural, magnetic and superconducting properties as a function of doping to
show how the superconducting state evolves, concluding that the addition of 0.1
electrons per Fe atom is sufficient to traverse the superconducting domain, and
that magnetic order coexists with superconductivity at doping levels less than
0.025 electrons per Fe atom.Comment: 4 pages, 6 figure
Jet velocity in SS433: its anti-correlation with precession-cone angle and dependence on orbital phase
We present a re-analysis of the optical spectroscopic data on SS433 from the
last quarter-century and demonstrate that these data alone contain systematic
and identifiable deviations from the traditional kinematic model for the jets:
variations in speed, which agree with our analysis of recent radio data; in
precession-cone angle and in phase. We present a simple technique for
separating out the jet speed from the angular properties of the jet axis,
assuming only that the jets are symmetric. With this technique, the archival
optical data reveal that the variations in jet speed and in precession-cone
angle are anti-correlated in the sense that when faster jet bolides are ejected
the cone opening angle is smaller. We also find speed oscillations as a
function of orbital phase.Comment: accepted by ApJ Letter
A map of OMC-1 in CO 9-8
The distribution of 12C16O J=9-8 (1.037 THz) emission has been mapped in
OMC-1 at 35 points with 84" resolution. This is the first map of this source in
this transition and only the second velocity-resolved ground-based observation
of a line in the terahertz frequency band. There is emission present at all
points in the map, a region roughly 4' by 6' in size, with peak antenna
temperature dropping only near the edges. Away from the Orion KL outflow, the
velocity structure suggests that most of the emission comes from the OMC-1
photon-dominated region, with a typical linewidthof 3-6 km/s. Large velocity
gradient modeling of the emission in J=9-8 and six lower transitions suggests
that the lines originate in regions with temperatures around 120 K and
densities of at least 10^(3.5) cm^(-3) near theta^(1) C Ori and at the Orion
Bar, and from 70 K gas at around 10^(4) cm^(-3) southeast and west of the bar.
These observations are among the first made with the 0.8 m Smithsonian
Astrophysical Observatory Receiver Lab Telescope, a new instrument designed to
observe at frequencies above 1 THz from an extremely high and dry site in
northern Chile.Comment: Minor changes to references, text to match ApJ versio
A Detailed Study of the Lobes of Eleven Powerful Radio Galaxies
Radio lobes of a sample of eleven very powerful classical double radio
galaxies were studied. Each source was rotated so that the symmetry axis of the
source was horizontal, and vertical cross-sectional cuts were taken across the
source at intervals of one beam size. These were used to study the
cross-sectional surface brightness profiles, the width of each slice, radio
emissivity as a function of position across each slice, the first and second
moments, and the average surface brightness, minimum energy magnetic field
strength, and pressure of each slice. A Gaussian provides a good description of
the surface brightness profile of cross-sectional slices. The Gaussian FWHM as
a function of distance from the hot spot first increases and then decreases
with distance from the hot spot. The width as a function of distance from the
hot spot is highly symmetric on each side of the source. The radio emissivity
is often close to flat across a slice, indicating a roughly constant emissivity
and pressure for that slice. Some slices show variations in radio emissivity
that indicate an ``edge-peaked'' pressure profile for that slice; these often
occur in slices near the local maxima of the bridge width. The emissivity does
not exhibit any signature of emission from a jet. The first moment is generally
quite close to zero indicating only small excursions of the ridge line from the
symmetry axis of the source. The second moment indicates the same source shape
as is found using the Gaussian FWHM. The average magnetic field strength and
pressure decrease with increasing distance from the hot spot, reaching a
roughly constant value at a location that is typically just before the location
of a local maximum of the bridge width. These results are interpreted in terms
of a heuristic model for the radio lobes.Comment: 102 pages, 136 figures, accepted for publication in ApJ Supplement
Serie
Opacity effects and shock-in-jet modelling of low-level activity in Cygnus X-3
We present simultaneous dual-frequency radio observations of Cygnus X-3 during a phase of low-level activity. We constrain the minimum variability time-scale to be 20 min at 43 GHz and 30 min at 15 GHz, implying source sizes of 2-4 au. We detect polarized emission at a level of a few per cent at 43 GHz which varies with the total intensity. The delay of ∼10 min between the peaks of the flares at the two frequencies is seen to decrease with time, and we find that synchrotron self-absorption and free-free absorption by entrained thermal material play a larger role in determining the opacity than absorption in the stellar wind of the companion. A shock-in-jet model gives a good fit to the light curves at all frequencies, demonstrating that this mechanism, which has previously been used to explain the brighter, longer lived giant outbursts in this source, is also applicable to these low-level flaring events. Assembling the data from outbursts spanning over two orders of magnitude in flux density shows evidence for a strong correlation between the peak brightness of an event, and the time-scale and frequency at which this is attained. Brighter flares evolve on longer time-scales and peak at lower frequencies. Analysis of the fitted model parameters suggests that brighter outbursts are due to shocks forming further downstream in the jet, with an increased electron normalization and magnetic field strength both playing a role in setting the strength of the outburs
Images of an equatorial outflow in SS433
We have imaged the X-ray binary SS433 with unprecedented Fourier-plane
coverage at 6cm using simultaneously the VLBA, MERLIN, and the VLA, and also at
20cm with the VLBA. At both wavelengths we have securely detected smooth,
low-surface brightness emission having the appearance of a `ruff' or collar
attached perpendicularly to the well-studied knotty jets in this system,
extending over at least a few hundred AU. We interpret this smooth emission as
a wind-like outflow from the binary, and discuss its implications for the
present evolutionary stage of this system.Comment: Accepted by ApJ Letter
Coordinated oncogenic transformation and inhibition of host immune responses by the PAX3-FKHR fusion oncoprotein
Tumors have evolved elaborate mechanisms for evading immune detection, such as production of immunoinhibitory cytokines and down-regulation of major histocompatibility complex (MHC) expression. We have studied PAX3-FKHR as an example of an oncogenic fusion protein associated with an aggressive metastatic cancer. We show that PAX3-FKHR alters expression of genes that are normally regulated by Janus kinase/signal transducer and activator of transcription (STAT) signaling pathways. This occurs as a result of a specific interaction between PAX3-FKHR and the STAT3 transcription factor, which results in a dramatic reduction in tumor MHC expression, and an alteration in local cytokine concentrations to inhibit surrounding inflammatory cells and immune detection. Collectively, these data show that an oncogenic transcription factor can promote tumor growth and tissue invasion while inhibiting local inflammatory and immune responses. This is the first time that an immunomodulatory role has been described for an oncogenic fusion protein
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