1,630 research outputs found
Kinematics of Black Hole X-ray Binary GRS 1915+105
The space velocity of a stellar black hole encodes the history of its
formation and evolution. Here we measure the 3-dimensional motion of the
microquasar GRS 1915+105, using a decade of astrometry with the NRAO Very Long
Baseline Array, together with the published radial velocity. The velocity in
the Galactic Plane deviates from circular rotation by 53-80 +_ 8 km/s, where
the range covers any specific distance from 6-12 kpc. Perpendicular to the
plane, the velocity is only 10 +_ 4 km/s. The peculiar velocity is minimized at
a distance 9-10 kpc, and is then nearly in the radial direction towards the
Galactic Center. We discuss mechanisms for the origin of the peculiar velocity,
and conclude that it is most likely a consequence of Galactic velocity
diffusion on this old binary, rather than the result of a supernova kick during
the formation of the 14 Mo black hole. Finally, a brief comparison is made with
4 other BH binaries whose kinematics are well determined.Comment: 16 pages, 4 figures. ApJ accepte
Growth of InAs Quantum Dots on Germanium Substrate Using Metal Organic Chemical Vapor Deposition Technique
Self-assembled InAs quantum dots (QDs) were grown on germanium substrates by metal organic chemical vapor deposition technique. Effects of growth temperature and InAs coverage on the size, density, and height of quantum dots were investigated. Growth temperature was varied from 400 to 450 °C and InAs coverage was varied between 1.40 and 2.35 monolayers (MLs). The surface morphology and structural characteristics of the quantum dots analyzed by atomic force microscope revealed that the density of the InAs quantum dots first increased and then decreased with the amount of InAs coverage; whereas density decreased with increase in growth temperature. It was observed that the size and height of InAs quantum dots increased with increase in both temperature and InAs coverage. The density of QDs was effectively controlled by growth temperature and InAs coverage on GaAs buffer layer
A weak compact jet in a soft state of Cygnus X-1
We present evidence for the presence of a weak compact jet during a soft
X-ray state of Cygnus X-1. Very-high-resolution radio observations were taken
with the VLBA, EVN and MERLIN during a hard-to-soft spectral state change,
showing the hard state jet to be suppressed by a factor of about 3-5 in radio
flux and unresolved to direct imaging observations (i.e. < 1 mas at 4 cm). High
time-resolution X-ray observations with the RXTE-PCA were also taken during the
radio monitoring period, showing the source to make the transition from the
hard state to a softer state (via an intermediate state), although the source
may never have reached the canonical soft state. Using astrometric VLBI
analysis and removing proper motion, parallax and orbital motion signatures,
the residual positions show a scatter of ~0.2 mas (at 4 cm) and ~3 mas (at 13
cm) along the position angle of the known jet axis; these residuals suggest
there is a weak unresolved outflow, with varying size or opacity, during
intermediate and soft X-ray states. Furthermore, no evidence was found for
extended knots or shocks forming within the jet during the state transition,
suggesting the change in outflow rate may not be sufficiently high to produce
superluminal knots.Comment: Accepted in MNRAS; 4 figures and 1 tabl
Multiple relativistic outbursts of GRS 1915+105: radio emission and internal shocks
We present 5-GHz MERLIN radio images of the microquasar GRS 1915+105 during
two separate outbursts in 2001 March and 2001 July, following the evolution of
the jet components as they move outwards from the core of the system. Proper
motions constrain the intrinsic jet speed to be >0.57c, but the uncertainty in
the source distance prevents an accurate determination of the jet speed. No
deceleration is observed in the jet components out to an angular separation of
about 300mas. Linear polarisation is observed in the approaching jet component,
with a gradual rotation in position angle and a decreasing fractional
polarisation with time. Our data lend support to the internal shock model
whereby the jet velocity increases leading to internal shocks in the
pre-existing outflow before the jet switches off. The compact nuclear jet is
seen to re-establish itself within two days, and is visible as core emission at
all epochs. The energetics of the source are calculated for the possible range
of distances; a minimum power of 1-10 per cent of the Eddington luminosity is
required to launch the jet.Comment: 18 pages, 14 figures, accepted for publication in MNRAS. For
higher-resolution versions of Figures 3, 5, and 12, see
http://remote.science.uva.nl/~jmiller/grs1915/figures.htm
Measurement of the analyzing power in pp elastic scattering in the peak CNI region at RHIC
We report the first measurements of the A_N absolute value and shape in the
-t range from 0.0015 to 0.010GeV/c^2 with a precision better than 0.005 for
each A_N data point using a polarized atomic hydrogen gas jet target and the
100 GeV RHIC proton beam.Comment: 4 pages, 5 figure
Radio and X-ray observations during the outburst decay of the Black Hole Candidate XTE J1908+094
Obtaining simultaneous radio and X-ray data during the outburst decay of soft
X-ray transients is a potentially important tool to study the disc - jet
connection. Here we report results of the analysis of (nearly) simultaneous
radio (VLA or WSRT) and Chandra X-ray observations of XTE J1908+094 during the
last part of the decay of the source after an outburst. The limit on the index
of a radio - X-ray correlation we find is consistent with the value of 0.7
which was found for other black hole candidates in the low/hard state.
Interestingly, the limit we find seems more consistent with a value of 1.4
which was recently shown to be typical for radiatively efficient accretion flow
models. We further show that when the correlation-index is the same for two
sources one can use the differences in normalisation in the radio - X-ray flux
correlation to estimate the distance towards the sources if the distance of one
of them is accurately known (assuming black hole spin and mass and jet Lorentz
factor differences are unimportant or minimal). Finally, we observed a strong
increase in the rate of decay of the X-ray flux. Between March 23, 2003 and
April 19, 2003 the X-ray flux decayed with a factor ~5 whereas between April
19, 2003 and May 13, 2003, the X-ray flux decreased by a factor ~750. The
source (0.5-10 keV) luminosity at the last Chandra observation was L~3x10^32
(d/8.5 kpc)^2 erg s^-1.Comment: 7 pages, 3 figures, accepted for publication by MNRA
Microwave shielding properties of Co/Ni attached to single walled carbon nanotubes
Cobalt/nickel nanoparticles attached to single-walled carbon nanotubes (Co/Ni@SWCNTs) were prepared by dc-arc discharge technique. Co/Ni@SWCNTs were characterized by scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and energy dispersive X-ray analysis techniques. HRTEM results confirmed attachment of magnetic nanoparticles onto SWCNTs having 1.2 nm diameter. A microwave shielding effectiveness value of 24 dB (blocking >99% radiation) by a 1.5 mm thick sample in the frequency range of 12.4-18 GHz was observed. In order to understand the mechanism of shielding, dielectric andmagnetic attributes of the shielding effectiveness of Co/Ni@SWCNTs have been evaluated. Eddy currents and natural resonances due to the presence of magnetic nanoparticles, electronic polarization and their relaxation, interfacial polarization and unique composition of the shield contributed significantly in achieving good shielding effectiveness. The observed microwave shielding crossed the limit required for commercial applications which suggests that these nanocomposites are promising microwave shielding materials in the Ku band
Study of the Largest Multiwavelength Campaign of the Microquasar GRS 1915+105
We present the results from a multiwavelength campaign of GRS 1915+105
performed from 2000 April 16 to 25. This is one of the largest coordinated set
of observations ever performed for this source, covering the wide energy band
in radio (13.3-0.3 cm), near-infrared (J-H-K), X-rays and Gamma-rays (from 1
keV to 10 MeV). During the campaign GRS 1915+105 was predominantly in the
"plateau" (or low/hard) state but sometimes showed soft X-ray oscillations:
before April 20.3, rapid, quasi-periodic (~= 45 min) flare-dip cycles were
observed. The radio flares observed on April 17 shows frequency- dependent peak
delay, consistent with an expansion of synchrotron-emitting region starting at
the transition from the hard-dip to the soft-flare states in X-rays. On the
other hand, infrared flares on April 20 appear to follow (or precede) the
beginning of X-ray oscillations with an inconstant time delay of ~= 5-30 min.
This implies that the infrared emitting region is located far from the black
hole by >~ 10E13 cm, while its size is <~ 10E12 cm constrained from the time
variability. We find a good correlation between the quasi-steady flux level in
the near-infrared band and in the X-ray band. From this we estimate that the
reprocessing of X-rays, probably occurring in the outer parts of the accretion
disk, accounts for about 20-30% of the observed K magnitude in the plateau
state. The OSSE spectrum in the 0.05-10 MeV band is represented by a single
power law with a photon index of 3.1 extending to ~1 MeV with no cutoff. The
power-law slope above ~30 keV is found to be very similar between different
states in spite of large flux variations in soft X-rays, implying that the
electron energy distribution is not affected by the change of the state in the
accretion disk.Comment: 31 pages, 11 figures. Accepted for publication in ApJ, vol. 571,
2002. Minor corrections. Figure 2 is revised (numbers on the top axis are
corrected). References are update
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