39 research outputs found
Comments on the superluminal motion in Cygnus X-3
Following the recent discovery that Cyg X-3 exhibits superluminal motion, the
implications of superluminal expansion and contraction are investigated. We
propose that the effect is due to either a propagating photon pattern or to
outwardly moving shells illuminated by an intense beam of radiation.Comment: accepted for publication in MNRAS; 5 pages, 3 figure
Excess sub-millimetre emission from GRS 1915+105
We present the first detections of the black hole X-ray binary GRS 1915+105
at sub-millimetre wavelengths. We clearly detect the source at 350 GHz on two
epochs, with significant variability over the 24 hr between epochs.
Quasi-simultaneous radio monitoring indicates an approximately flat spectrum
from 2 - 350 GHz, although there is marginal evidence for a minimum in the
spectrum between 15 - 350 GHz. The flat spectrum and correlated variability
imply that the sub-mm emission arises from the same synchrotron source as the
radio emission. This source is likely to be a quasi-steady partially
self-absorbed jet, in which case these sub-mm observations probe significantly
closer to the base of the jet than do radio observations and may be used in
future as a valuable diagnostic of the disc:jet connection in this source.Comment: 5 pages, 3 figures, accepted for publication in MNRA
Cygnus X-3 in outburst : quenched radio emission, radiation losses and variable local opacity
We present multiwavelength observations of Cygnus X-3 during an extended
outburst in 1994 February - March. Intensive radio monitoring at 13.3, 3.6 &
2.0 cm is complemented by observations at (sub)millimetre and infrared
wavelengths, which find Cyg X-3 to be unusually bright and variable, and
include the first reported detection of the source at 0.45 mm. We report the
first confirmation of quenched radio emission prior to radio flaring
independent of observations at Green Bank. The observations reveal evidence for
wavelength-dependent radiation losses and gradually decreasing opacity in the
environment of the radio jet. We find that the radiation losses are likely to
be predominantly inverse Compton losses experienced by the radio-emitting
electrons in the strong radiation field of a luminous companion to the compact
object. We interpret the decreasing opacity during the flare sequence as
resulting from a decreasing proportion of thermal electrons entrained in the
jet, reflecting a decreasing density in the region of jet formation. We
present, drawing in part on the work of other authors, a model based upon
mass-transfer rate instability predicting gamma-ray, X-ray, infrared and radio
trends during a radio flaring sequence.Comment: LaTeX, 11 pages, 6 figures. Submitted to MNRA
0-pi oscillations in nanostructured Nb/Fe/Nb Josephson junctions
The physics of the phase shift in ferromagnetic Josephson junctions may
enable a range of applications for spin-electronic devices and quantum
computing. We investigate transitions from ``0'' to ``'' states in
Nb/Fe/Nb Josephson junctions by varying the Fe barrier thickness from 0.5 nm to
5.5 nm. From magnetic measurements we estimate for Fe a magnetic dead layer of
about 1.1 nm. By fitting the characteristic voltage oscillations with existing
theoretical models we extrapolate an exchange energy of 256 meV, a Fermi
velocity of m/s and an electron mean free path of 6.2 nm, in
agreement with other reported values. From the temperature dependence of the
product we show that its decay rate exhibits a nonmonotonic
oscillatory behavior with the Fe barrier thickness.Comment: 7 pages, 5 figures, accepted for publication in Eur. Phys. J.
Critical Current Oscillations in Strong Ferromagnetic Pi-Junctions
We report magnetic and electrical measurements of Nb Josephson junctions with
strongly ferromagnetic barriers of Co, Ni and Ni80Fe20 (Py). All these
materials show multiple oscillations of critical current with barrier thickness
implying repeated 0-pi phase-transitions in the superconducting order
parameter. We show in particular that the Co barrier devices can be accurately
modelled using existing clean limit theories and so that, despite the high
exchange energy (309 meV), the large IcRN value in the pi-state means Co
barriers are ideally suited to the practical development of superconducting
pi-shift devices.Comment: 4 pages 3 figures 1 table. Revised version as accepted for
publication. To appear in Physical Review Letter
X 1908+075: An X-ray Binary with a 4.4 day Period
X 1908+075 is an optically unidentified and highly absorbed X-ray source that
appears in early surveys such as Uhuru, OSO-7, Ariel V, HEAO-1, and the EXOSAT
Galactic Plane Survey. These surveys measured a source intensity in the range
of 2-12 mCrab at 2-10 keV, and the position was localized to ~ 0.5 degrees. We
use the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) to confirm our
expectation that a particular Einstein IPC detection (1E 1908.4+0730) provides
the correct position for X 1908+075. The analysis of the coded mask shadows
from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ~
8 mCrab (1.5-12 keV) over a 3 year interval beginning in 1996 February.
Furthermore, we detect a period of 4.400 +- 0.001 days with a false alarm
probability < 1.0e-7 . The folded light curve is roughly sinusoidal, with an
amplitude that is 22 % of the mean flux. The X-ray period may be attributed to
the scattering and absorption of X-rays through a stellar wind combined with
the orbital motion in a binary system. We suggest that X 1908+075 is an X-ray
binary with a high mass companion star.Comment: 6 pages, two-column,"emulateapj" style, submitted to Ap
Radio flares and plasmon size in Cygnus X-3
We have observed a number of minor radio flares in Cyg X-3 using the MERLIN
array. Photometric observations show the system to be highly active with
multiple flares on hourly timescales over the one month observing programme.
Analysis of the source's power spectrum show no persistent periodicities in
these data, and no evidence of the 4.8 hr orbital period. An upper limit of 15
mJy can be placed on the amplitude of any sinusoidal varaition of source flux
at the orbital period. The brightness temperature of a flare is typically T >
10^{9}- 10^{10} K, with a number of small flares of 5 minute duration having
brightness temperatures of T > few x 10^{11} K. For such a change in flux to
occur within a typical 10 minute timescale, the radiation must originate from
plasmons with a size < 1.22 AU. This emission is unlikely to originate close to
the centre of the system as both the jets and compact object are buried deep
within an optically thick stellar wind. Assuming a spherically symmetric wind,
plasmons would become visible at distances ~ 13 AU from the core.Comment: Accepted for publication in MNRA
Properties of a Gamma Ray Burst Host Galaxy at z ~ 5
We describe the properties of the host galaxy of the gamma-ray burst
GRB060510B based on a spectrum of the burst afterglow obtained with the Gemini
North 8m telescope. The galaxy lies at a redshift of z = 4.941 making it the
fourth highest spectroscopically identified burst host. However, it is the
second highest redshift galaxy for which the quality of the spectrum permits a
detailed metallicity analysis. The neutral hydrogen column density has a
logarithmic value of 21.0--21.2 cm^-2 and the weak metal lines of Ni, S and Fe
show that the metallicity is in excess of a tenth of solar which is far above
the metallicities in damped Lyman alpha absorbers at high redshift. The
tightest constraint is from the Fe lines which place [Fe/H] in excess of -0.8.
We argue that the results suggest that metallicity bias could be a serious
problem with inferring star formation from the GRB population and consider how
future higher quality measurements could be used to resolve this question.Comment: Accepted for publication in ApJ Letter