75 research outputs found
Experimental demonstration of a W-band gyroklystron amplifier
The experimental demonstration of a four cavity W-band (93 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 67 kW peak output power and 28% efficiency in the TE011 mode using a 55 kV, 4.3 A electron beam. The full width at half maximum instantaneous bandwidth is greater than 460 MHz, a significant increase over the bandwidth demonstrated in previous W-band gyroklystron amplifier experiments. The amplifier is unconditionally stable at this operating point. Experimental results are in good agreement with theoretical predictions
Geometrodynamical Distances to the Galaxy's Hydrogen Streams
We present a geometrodynamical method for determining distances to orbital
streams of HI gas in the Galaxy. The method makes use of our offset from the
Galactic centre and assumes that the gas comprising the stream nearly follows a
planar orbit about the Galactic centre. We apply this technique to the
Magellanic Stream and determine the distances to all points along it; a
consistency check shows that the angular momentum is approximately constant.
Applying this technique to the Large Magellanic Cloud itself gives an
independent distance which agrees within its accuracy of around 10%. Relaxing
the demand for exact conservation of energy and angular momentum at all points
along the stream allows for an increase in orbital period between the lagging
end and the front end led by the Magellanic Clouds. Similar methods are
applicable to other long streams of high-velocity clouds, provided they also
nearly follow planar orbits; these would allow otherwise unknown distances to
be determined.Comment: 9 pages, 7 figures; typos corrected after being accepted by MNRA
A kinematic study of Open Clusters: implications for their origin
The Galactic population of open clusters provides an insight into star
formation in the Galaxy. The open cluster catalogue by Dias et al.(2002b) is a
rich source of data, including kinematic information. This large sample made it
possible to carry out a systematic analysis of 481 open cluster orbits, using
parameters based on orbit eccentricity and separation from the Galactic plane.
These two parameters may be indicative of origin, and we find them to be
correlated. We also find them to be correlated with metallicity, another
parameter suggested elsewhere to be a marker for origin in that high values of
any of these two parameters generally indicates a low metallicity ([Fe/H]
Solar0.2 dex). The resulting analysis points to four open clusters in the
catalogue being of extra-Galactic origin by impact of high velocity cloud on
the disk: Berkeley21, 32, 99, and Melotte66, with a possible further four due
to this origin (NGC2158, 2420, 7789, IC1311). A further three may be due to
Galactic globular cluster impact on the disk i.e of internal Galactic origin
(NGC6791, 1817, and 7044).Comment: 14 pages, 816 figures, accepted for publication in MNRAS 14-May-201
High Velocity Rain: The Terminal Velocity of Model of Galactic Infall
A model is proposed for determining the distances to falling interstellar
clouds in the galactic halo by measuring the cloud velocity and column density
and assuming a model for the vertical density distribution of the Galactic
interstellar medium. It is shown that falling clouds with may be decelerated to a terminal velocity which increases with
increasing height above the Galactic plane. This terminal velocity model
correctly predicts the distance to high velocity cloud Complex M and several
other interstellar structures of previously determined distance. It is
demonstrated how interstellar absorption spectra alone may be used to predict
the distances of the clouds producing the absorption. If the distances to the
clouds are already known, we demonstrate how the model may be used to determine
the vertical density structure of the ISM. The derived density distribution is
consistent with the expected density distribution of the warm ionized medium,
characterized by Reynolds. There is also evidence that for
one or more of the following occurs: (1) the neutral fraction of the cloud
decreases to , (2) the density drops off faster than
characterized by Reynolds, or (3) there is a systematic decrease in drag
coefficient with increasing z.Comment: ApJ, in pres
Hard X-ray properties of blazars
We have considered all blazars observed in the X-ray band and for which the
slope of the X-ray spectrum is available. We have collected 421 spectra of 268
blazars, including 12 archival unpublished ASCA spectra of 7 blazars whose
analysisis presented here. The X-ray spectra of blazars show trends as a
function of their power, confirming that the blazar overall energy distribution
can be parameterized on the basis of one parameter only, i.e. the bolometric
luminosity. This is confirmed by the relatively new hard (2-10 keV) X-ray data.
Our results confirm the idea that in low power objects the X-ray emission
mechanism is the synchrotron process, dominating both the soft and the hard
X-ray emissions. Low energy peaked BL Lac objects are intermediate, often
showing harder spectra in the hard X-ray band, suggesting that the synchrotron
process dominates in the soft band, with the inverse Compton process dominating
at high energies. The most powerful objects have X-ray spectra that are flat
both in the soft and in the hard band, consistent with a dominating inverse
Compton component.Comment: 20 pages, 18 figures, accepted for publication in A&
Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption line measurements
for the high- and intermediate velocity clouds (HVCs and IVCs). New values are
derived for N(HI) in the direction of observed probes, in order to arrive at
reliable abundances and abundance limits (the HI data are described in Paper
II). Distances to stellar probes are revisited and calculated consistently, in
order to derive distance brackets or limits for many of the clouds, taking care
to properly interpret non-detections. The main conclusions are the following.
1) Absolute abundances have been measured using lines of SII, NI and OI, with
the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar
for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two
northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the
range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs
are like those in warm disk or halo gas. 3) Most distance limits are based on
strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major
HVCs are >5 kpc, while distance brackets for several IVCs are in the range
0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major
HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5
within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies
by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be
useful for determining both lower and upper distance limits, but NaI only
yields upper limits.Comment: To appear in the "Astrophysical Journal Supplement"; 82 pages;
figures 6, 9 and 10 are in color; degraded figures (astro-ph restriction) -
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High-resolution observations of interstellar Na I and Ca II towards the southern opening of the 'Local Interstellar Chimney': probing the disc—halo connection
We present high-resolution (R = 400 000) observations of interstellar Ca II and Na I absorption lines towards seven stars in the direction of the southern opening of the recently identified Local Interstellar Chimney. These lines of sight probe the lower Galactic halo (0.3 ≲∣z∣≲ 2.5 kpc), without the complication of sampling dense foreground interstellar material. In addition to components with velocities expected from Galactic rotation, these stars also exhibit components with negative local standard of rest velocities, which are contrary to the sense of Galactic rotation for the sightlines observed. After a discussion of possible origins for these peculiar velocities, we conclude that at least some of them result from gas falling towards the Galactic plane from distances of ∣z∣≳ 300 pc. The narrow linewidths are generally inconsistent with temperatures as high as the ∼6000 K generally assumed for the so-called Lockman layer. Rather, the picture that emerges is one of a scattered, generally infalling, population of high-∣z∣ diffuse clouds, seemingly not very different from those encountered in the local interstellar medium. Overall, we argue that our results are most consistent with a ‘Galactic fountain’ model
CIRCUIT ASPECTS OF THE NRL/INDUSTRIAL 94 GHz GYROKLYSTRON AMPLIFIER
Abstract A wide bandwidth, high average power W-band gyroklystron amplifier is currently under cooperative development by NRL, Litton Electron Devices, and Communication and Power Industries. The amplifier circuit is comprised of 4 stagger-tuned cavities operating in the fundamental TE 011 circular cavity mode. The input coupler is the first cavity of the circuit and must exhibit reasonable coupling strength between the TE 10 mode in rectangular waveguide and the desired TE 011 circular cavity mode over a better than 600 MHz bandwidth centered at 93.4 GHz, with high TE 01 mode purity. A single WR-8 rectangular waveguide drives a combined coaxial/cylindrical cavity system. The coaxial cavity resonating in the TE 411 mode is tightly coupled to the cylindrical cavity, excited to resonate in the TE 011 mode. The rf magnetic field couples the cavities through 4 azimuthally spaced apertures
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