588 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
The Weak Blue Bump of H2106-099 and AGN De-Reddening
We present multi-frequency spectra of the Seyfert 1 galaxy H2106-099, from
radio to hard X-rays, spanning over a decade of observations. The hard X-ray
(2-20 keV) spectrum measured with Ginga had a Log slope of -0.80 +/- 0.02 on
1988 May 18 and -1.02 +/- 0.10 on 1988 May 22 / 23 UT, with no observed flux
changes. Other measurements showed variability and unusual spectral features:
The V band flux changed by a factor of 1.8 (> 10 sigma) in six weeks. Only
moderate optical Fe II emission is present, but strong [FeVII] and [Fe X] lines
are present in some epochs. The Balmer lines show > 25% variations in flux
relative to the mean, and He I changed by more than 100% relative to the mean
in <~ six years. The most surprising finds came from the composite UV through
near-IR spectrum: If the spectrum is de-reddened by the galactic extinction
value (from 21 cm observations), a residual 2175 Angstrom absorption feature is
present. Additional de-reddening to correct the feature yields E(B-V)=0.07 mag
due to material outside our galaxy, most probably associated with the AGN or
its host galaxy. No other clear indications of reddening are observed in this
object, suggesting that blue bump strength measurements in low and intermediate
red-shift AGN could be incorrect if derived without UV observations of the
region near 2175 Angstrom in the AGN frame. After all reddening corrections are
performed, the log slope of H2106-099 from the near IR (~12500 Angstrom) to the
UV (~1400 Angstrom), -0.94 +/- 0.05, is steep compared to other AGN, suggesting
that the blue bump in this object is intrinsically weak. Weak blue bumps are,
therefore, not always an artifact caused by reddening.Comment: Three parts: A. 33 pages text, B. one landscape table, C. 8 figure
Orbital Parameters for the Black Hole Binary XTE J1650-500
(Shortened) We present R-band photometry of the X-ray transient and candidate
black hole binary XTE J1650-500 obtained between 2003 May and August with the
6.5m Clay Telescope. A timing analysis of these data reveals a photometric
period of 0.3205 +/- 0.0007 days (i.e. 7.63 hr) with a possible alias at 0.3785
days (9.12 hr). Our photometry completely rules out the previously published
spectroscopic period of 0.212 days (5.09 hr). Consequently, we reanalyzed the
15 archival ESO/VLT spectra (obtained 2002 June by Sanchez-Fernandez et al.)
that were the basis of the previously published spectroscopic period. We used a
``restframe search'' technique that is well suited for cases when the
signal-to-noise ratio of individual spectra is low. The results confirmed the
photometric period of 0.3205 days, and rule out the alias period near 0.38
days. The best value for the velocity semiamplitude of the companion star is
K_2 = 435 +/- 30 km/sec, and the corresponding optical mass function is f(M) =
2.73 +/- 0.56 solar masses. The amplitude of the phased R-band light curve is
0.2 magnitudes, which gives a lower limit to the inclination of 50 +/- 3
degrees in the limiting case of no contribution to the R-band light curve from
the accretion disk. If the mass ratio of XTE J1650-500 is similar to the mass
ratios of other black hole binaries like A0620-00 or GRS 1124-683 (e.g. Q >~
10), then our lower limit to the inclination gives an upper limit to the mass
of the black hole in XTE J1650-500 of M_1 <~ 7.3 solar masses. However, the
mass can be considerably lower if the R-band flux is dominated by the accretion
disk. For example, if the accretion disk does contribute 80% of the flux, as
our preliminary results suggest, then the black hole mass would be only about 4
solar masses.Comment: Accepted to ApJ. 15 pages, 5 figures (two of degraded quality).
Revised after referee's Comments, conclusions are unchange
Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40
We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the
black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi
X-ray Timing Explorer. All LFQPOs could be identified as either type B or type
C using previously proposed classification schemes. In the soft state of the
outburst the type-C LFQPOs reached frequencies that are among the highest ever
seen for LFQPOs in black holes. At the peak of the outburst, in the
ultra-luminous state, the power spectrum showed two simultaneous,
non-harmonically related peaks which we identified as a type-B and a type-C
QPO. The simultaneous presence of a type-C and type-B QPO shows that at least
two of the three known LFQPO types are intrinsically different and likely the
result of distinct physical mechanisms. We also studied the properties of a
broad peaked noise component in the power spectra of the ultra-luminous state.
This noise component becomes more coherent with count rate and there are strong
suggestions that it evolves into a type-B QPO at the highest observed count
rates.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in MNRA
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
Broad-band X-Ray Spectra of the Black Hole Candidate GRO J1655-40
We present broad-band (2 keV to 2 MeV) X-ray spectra of GRO J1655-40, a
luminous X-ray transient and occasional source of relativistic radio jets,
obtained with RXTE and OSSE. In one observation, the luminosity is found to be
18% of the Eddington limit, which is one of the highest luminosities ever
observed from GRO J1655-40. For this observation, we find that an adequate fit
is obtained when a broad iron line and a reflection component are added to a
model consisting of a power-law plus a soft excess component. The 95%
confidence lower limit on the rms line width is 0.86 keV. The power-law
component has a photon index of 2.72 and extends to at least 800 keV without a
cutoff. After this observation, a significant drop in the (5-12 keV)/(1.5-5
keV) hardness ratio occurred on a timescale less than 2 hours. From an RXTE
observation of GRO J1655-40 made after the hardness transition, we find that
the power-law index is harder (2.415 +/- 0.011), the flux of the power-law
component is lower, and the total luminosity is 10% of the Eddington limit. The
change in the power-law component is consistent with the correlation between
the spectral index and power-law flux previously reported for GRO J1655-40.Comment: 20 pages, 5 figures, Accepted for publication in the Astrophysical
Journa
CU and the CDC
Legionella is a gram-negative genus of bacteria that is the cause of Legionnaires disease. Currently, 50 species and 70 serogroups of Legionella have been identified. The Center for Disease Control (CDC) in Atlanta maintains a bank of identified and unidentified Legionella samples. The availability of sequencing technologies has increased since many samples were collected allowing for identification of many previously unidentifiable isolates. We received 68 unidentified samples from the CDC. A sequence based typing scheme was used for characterization. Genomic DNA was extracted from samples and polymerase chain reactionwas performed on the 16S and mip genes. These samples were then sequenced at Clemson University Genomics Institute. Currently, we have identified several samples which were previously undescribed. Once a sample is identified as novel, characterization through sequencing other genes along with morphological and biochemical assays will be conducted. As a collaborative project, regular meetings occur with scientists from the CDC. Characterization of novel strains expands this lab\u27s ability to conduct outbreak analysis and risk assessment along with expanding our knowledge of the pathogen
A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105
(abridged) In this paper we extend the idea suggested previously by Petri
(2005a,b) that the high frequency quasi-periodic oscillations observed in
low-mass X-ray binaries may be explained as a resonant oscillation of the
accretion disk with a rotating asymmetric background (gravitational or
magnetic) field imposed by the compact object. Here, we apply this general idea
to black hole binaries. It is assumed that a test particle experiences a
similar parametric resonance mechanism such as the one described in paper I and
II but now the resonance is induced by the interaction between a spiral density
wave in the accretion disk, excited close to the innermost stable circular
orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry
to deduce the characteristic frequencies of this test particle. The response of
the test particle is maximal when the frequency ratio of the two strongest
resonances is equal to 3:2 as observed in black hole candidates. Finally,
applying our model to the microquasar GRS 1915+105, we reproduce the correct
value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz
features in the power spectrum time analysis is predicted. Moreover, based only
on the two HF-QPO frequencies, our model is able to constrain the mass and angular momentum of the accreting black hole.Comment: Accepted for publication in Astrophysics & Space Scienc
A Unified Description of the Timing Features of Accreting X-ray Binaries
We study an empirical model for a unified description of the power spectra of
accreting neutron stars and black holes. This description is based on a
superposition of multiple Lorentzians and offers the advantage that all QPO and
noise components are dealt with in the same way, without the need of deciding
in advance the nature of each component. This approach also allows us to
compare frequencies of features with high and low coherences in a consistent
manner and greatly facilitates comparison of power spectra across a wide range
of source types and states. We apply the model to six sources, the
low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269 and GS 1826-24, the
high-latitude transient XTE J1118+480, the bright system Cir X-1, and the Z
source GX 17+2. We find that it provides a good description of the observed
spectra, without the need for a scale-free (1/f) component. We update
previously reported correlations between characteristic frequencies of timing
features in the light of this new approach and discuss similarities between
different types of systems which may point towards similar underlying physics.Comment: 13 pages, to appear in The Astrophysical Journa
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