682 research outputs found
Collision and Diffusion in Microwave Breakdown of Nitrogen Gas in and around Microgaps
The microwave induced breakdown of N2 gas in microgaps was modeled using the
collision frequency between electrons and neutral molecules and the effective
electric field concept. Low pressure breakdown at the threshold electric field
occurs outside the gap, but at high pressures it is found to occur inside the
microgap with a large threshold breakdown electric field corresponding to a
very large electron oscillation amplitude. Three distinct pressure regimes are
apparent in the microgap breakdown: a low pressure multipactor branch, a
mid-pressure Paschen branch, both of which occur in the space outside the
microgap, and a high pressure diffusion-drift branch, which occurs inside the
microgap. The Paschen and diffusion-drift branches are divided by a sharp
transition and each separately fits the collision frequency model. There is
evidence that considerable electron loss to the microgap faces accompanies the
diffusion-drift branch in microgaps.Comment: 4 figure
LNRF-velocity hump-induced oscillations of a Keplerian disc orbiting near-extreme Kerr black hole: A possible explanation of high-frequency QPOs in GRS 1915+105
At least four high-frequency quasiperiodic oscillations (QPOs) at frequencies
41Hz, 67Hz, 113Hz, and 167Hz were reported in a binary system GRS 1915+105
hosting near-extreme Kerr black hole with a dimensionless spin a>0.98. We use
the idea of oscillations induced by the hump of the orbital velocity profile
(related to locally non-rotating frames - LNRF) in discs orbiting near-extreme
Kerr black holes, which are characterized by a "humpy frequency" f_h, that
could excite the radial and vertical epicyclic oscillations with frequencies
f_r, f_v. Due to non-linear resonant phenomena the combinational frequencies
are allowed as well. Assuming mass M=14.8M_sun and spin a=0.9998 for the GRS
1915+105 Kerr black hole, the model predicts frequencies f_h=41Hz, f_r=67Hz,
(f_h+f_r)=108Hz, (f_v-f_r)=170Hz corresponding quite well to the observed ones.
For black-hole parameters being in good agreement with those given
observationally, the forced resonant phenomena in non-linear oscillations,
excited by the "hump-induced" oscillations in a Keplerian disc, can explain
high-frequency QPOs in GRS 1915+105 within the range of observational errors.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics, added references, corrected typo
An Accretion-Jet Model for Black Hole Binaries: Interpreting the Spectral and Timing Features of XTE J1118+480
Multi-wavelength observations of the black hole X-ray binary XTE J1118+480
have offered abundant spectral and timing information about the source, and
have thus provided serious challenges to theoretical models. We propose a
coupled accretion-jet model to interpret the observations. We model the
accretion flow as an outer standard thin accretion disk truncated at a
transition radius by an inner hot accretion flow. The accretion flow accounts
for the observed UV and X-ray emission, but it substantially under-predicts the
radio and infrared fluxes, even after we allow for nonthermal electrons in the
hot flow. We attribute the latter components to a jet. We model the jet
emission by means of the internal shock scenario which is widely employed for
gamma-ray bursts. In our accretion-jet model of XTE J1118+480, the jet
dominates the radio and infrared emission, the thin disk dominates the UV
emission, and the hot flow produces most of the X-ray emission. The optical
emission has contributions from all three components: jet, thin disk, and hot
flow. The model qualitatively accounts for timing features, such as the
intriguing positive and negative time lags between the optical and X-ray
emission, and the wavelength-dependent variability amplitude.Comment: 27 pages, 4 figures (one in color); to appear in ApJ in Feb. 200
HST/NICMOS Observations of Fast Infrared Flickering in the Microquasar GRS 1915+105
We report infrared observations of the microquasar GRS 1915+105 using the
NICMOS instrument of the Hubble Space Telescope during 9 visits in April-June
2003. During epochs of high X-ray/radio activity near the beginning and end of
this period, we find that the \um infrared flux is generally low ( mJy) and relatively steady. However, during the X-ray/radio ``plateau''
state between these epochs, we find that the infrared flux is significantly
higher ( mJy), and strongly variable. In particular, we find events
with amplitudes % occurring on timescales of s
(e-folding timescales of s). These flickering timescales are several
times faster than any previously-observed infrared variability in GRS 1915+105
and the IR variations exceed corresponding X-ray variations at the same () timescale. These results suggest an entirely new type of infrared
variability from this object. Based on the properties of this flickering, we
conclude that it arises in the plateau-state jet outflow itself, at a distance
AU from the accretion disk. We discuss the implications of this work and
the potential of further flickering observations for understanding jet
formation around black holes.Comment: 19 pages, incl. 4 figures; accepted for publication in Ap
Near-infrared jet emission in the microquasar XTE J1550-564
Context: Microquasars are accreting Galactic sources that are also observed
to launch relativistic jets. A key signature of the ejection is non-thermal
radio emission. The level of this jet component at high frequencies is still
poorly constrained. Aims: The X-ray binary and microquasar black hole candidate
XTE J1550-564 exhibited a faint X-ray outburst in April 2003 during which it
stayed in the X-ray low/hard state. We took optical and near-infrared (NIR)
observations with the ESO/NTT telescope during this outburst to disentangle the
various contributions to the spectral energy distribution (SED) and investigate
the presence of a jet component. Methods: Photometric and spectroscopic
observations allowed us to construct an SED and also to produce a high
time-resolution lightcurve. Results: The SED shows an abrupt change of slope
from the NIR domain to the optical. The NIR emission is attributed to
non-thermal synchrotron emission from the compact, self-absorbed jet that is
known to be present in the low/hard state. This is corroborated by the fast
variability, colours, lack of prominent spectral features and evidence for
intrinsic polarisation. The SED suggests the jet break from the optically thick
to the thin regime occurs in the NIR. Conclusions: The simultaneous optical-NIR
data allow an independent confirmation of jet emission in the NIR. The
transition to optically thin synchrotron occurs at NIR frequencies or below,
which leads to an estimated characteristic size greater than 2x10^8cm and
magnetic field less than 5T for the jet base, assuming a homogeneous one-zone
synchrotron model.Comment: Accepted for publication in Astronomy and Astrophysics (Section 7.
Stellar structure and evolution; 8 pages, 6 figures
Production of intense, coherent, tunable narrowâband lymanâalpha radiation
Nearly transform limited pulses of 1216 Ă
radiation have been generated by sum frequency generation in 0.1 to 10 torr of mercury vapor. The summed input beams, consisting of photons at 3127 Ă
and 5454 Ă
originate in 1 MHz bandâwidth ringâdye laser oscillators. The beams are amplified in pulsedâdye amplifiers pumped by the frequency doubled output of a Nd:YAG laser. The 3127 Ă
photons are tuned to be resonant with the twoâphoton 61S to 71S mercury transition. The VUV radiation can be tuned by varying the frequency of the third nonâresonant photon. We have also observed difference frequency generation at 2193 Ă
and intense fluorescence from the 61P state at 1849 Ă
. We have studied the intensity and linewidth dependence of the 1849 Ă
fluorescence and 1216 Ă
sum frequency signals on input beam intensity, mercury density, and buffer gas pressure and composition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87716/2/49_1.pd
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
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