164 research outputs found
Velocity and temperature distributions of coal-slag layers on magnetohydrodynamic generators walls
Approximate analytical expressions are derived for the velocity and temperature distributions in steady state coal slag deposits flowing over MHD generator walls. Effects of slag condensation and Joule heating are included in the analysis. The transport conditions and the slag temperature at the slag-gas interface are taken to be known parameters in the formulation. They are assumed to have been predetermined either experimentally or from the slag properties and the gas dynamic calculations of the free stream flow. The analysis assumes a power law velocity profile for the slag and accounts for the coupling between the energy and momentum conservation equations. Comparisons are made with the more exact numerical solutions to verify the accuracy of the results
Performance optimization of an MHD generator with physical constraints
A method to optimize the Faraday MHD generator performance under a prescribed set of electrical and magnet constraints is described. The results of generator performance calculations using this technique are presented for a very large MHD/steam plant. The differences between the maximum power and maximum net power generators are described. The sensitivity of the generator performance to the various operational parameters are presented
BeppoSAX Observations of Markarian 501 in June 1999
We present the preliminary results of a long BeppoSAX observation of the BL
Lac object Mkn501 carried out in June 1999. The source was fainter than found
during the BeppoSAX pointings of 1997 and 1998, but is still detected with a
good signal-to-noise ratio up to ~40 keV. The X-ray spectrum in the energy
range 0.1-40 keV, produced through synchrotron radiation, is steeper than in
the previous years, it is clearly curved, and peaks (in nu*F_nu) at ~0.5 keV.
This energy is much lower than those at which the synchrotron component was
found to peak in 1997 and 1998. Some intraday variability suggests that
activity of the source on small time scales accompanies the large long time
scale changes of brightness and spectrum.Comment: 4 pages, Latex, 2 PostScript figures, to appear in the Proceedings of
the Conference "X-ray Astronomy '999: Stellar Endpoints, AGNs, and the
Diffuse X-ray Background" (Bologna, 6-10 September 1999
Mid-Cycle Changes in Eta Carinae
In late 2006, ground-based photometry of Car plus the Homunculus
showed an unexpected decrease in its integrated apparent brightness, an
apparent reversal of its long-term brightening. Subsequent HST/WFPC2 photometry
of the central star in the near-UV showed that this was not a simple reversal.
This multi-wavelength photometry did not support increased extinction by dust
as the explanation for the decrease in brightness. A spectrum obtained with
GMOS on the Gemini-South telescope, revealed subtle changes mid-way in
Car's 5.5 yr spectroscopic cycle 0when compared with HST/STIS spectra at the
same phase in the cycle. At mid-cycle the secondary star is 20--30 AU from the
primary. We suggest that the spectroscopic changes are consistent with
fluctuations in the density and velocity of the primary star's wind, unrelated
to the 5.5 yr cycle but possibly related to its latitude-dependent morphology.
We also discuss subtle effects that must be taken into account when comparing
ground-based and HST/STIS spectra.Comment: 34 pages, 9 Figure
Gamma-Ray Burst 980329 and its X-Ray Afterglow
GRB 980329 is the brightest gamma-ray burst detected so far with the Wide
Field Cameras aboard BeppoSAX, both in gamma-rays and X-rays. With respect to
its fluence (2.6 X 10**-5 erg/s/cm**2 in 50 to 300 keV) it would be in the top
4% of gamma-ray bursts in the 4B catalog (Meegan et al. 1998). The
time-averaged burst spectrum from 2 to 20 and 70 to 650 keV can be well
described by the empirical model of Band et al. (1993). The resulting photon
index above the break energy is exceptionally hard at -1.32 +/- 0.03. An X-ray
afterglow was detected with the narrow-field instruments aboard BeppoSAX 7 h
after the event within the error box as determined with the Wide Field Cameras.
Its peak flux is (1.4 +/- 0.2) X 10**-12 erg/s/cm**2 (2 to 10 keV). The
afterglow decayed according to a power law function with an index of -1.35 +/-
0.03. GRB 980329 is characterized by being bright and hard, and lacking strong
spectral evolution.Comment: 13 pages with 3 figures. Accepted for publication in ApJ Let
Multi-colour optical monitoring of eight red blazars
We present the observational results of multi-colour optical monitoring of
eight red blazars from 2003 September to 2004 February. The aim of our
monitoring is to investigate the spectral variability as well as the flux
variations at short and long time scales. The observations were carried out
using the 1.0 m robotic telescope of Mt. Lemmon Optical Astronomy Observatory,
in Arizona, USA, the 0.6 m telescope of Sobaeksan Optical Astronomy Observatory
and the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory, in the
Republic of Korea. During the observations, all sources show strong flux
variations with amplitudes of larger than 0.5 mag. Variations with amplitudes
of over 1 mag are found in four sources. Intraday variations with amplitudes
larger than 0.15 mag, and a rapid brightness increase with a rate of ~0.2 mag
per day in four days, are detected in S5 0716+71. We investigate the
relationship between the colour index and source brightness for each source. We
find that two out of three FSRQs tend to be redder when they are brighter, and,
conversely, all BL Lac objects tend to be bluer. In particular, we find a
significant anti-correlation between the V-I colour index and R magnitude for
3C 454.3. This implies that the spectrum became steeper when the source was
brighter, which is opposite to the common trend for blazars. In contrast,
significant positive correlations are found in 3C 66A, S5 0716+71, and BL Lac.
However, there are only very weak correlations for PKS 0735+17 and OJ 287. We
propose that the different relative contributions of the thermal versus
non-thermal radiation to the optical emission may be responsible for the
different trends of the colour index with brightness in FSRQs and BL Lac
objects.Comment: 15 pages, 12 figures. Accepted for publication in A&
Spiralling out of control: 3D hydrodynamical modelling of the colliding winds in Carinae
Three dimensional (3D) adaptive-mesh refinement (AMR) hydrodynamical
simulations of the wind-wind collision between the enigmatic super-massive star
\etacar and its mysterious companion star are presented which include radiative
driving of the stellar winds, gravity, optically-thin radiative cooling, and
orbital motion. Simulations with static stars with a periastron passage
separation reveal that the preshock companion star's wind speed is sufficiently
reduced that radiative cooling in the postshock gas becomes important,
permitting the runaway growth of non-linear thin shell (NTSI) instabilities
which massively distort the WCR. However, large-scale simulations which include
the orbital motion of the stars, show that orbital motion reduces the impact of
radiative inhibition, and thus increases the acquired preshock velocities. As
such, the postshock gas temperature and cooling time see a commensurate
increase, and sufficient gas pressure is preserved to stabilize the WCR against
catastrophic instability growth. We then compute synthetic X-ray spectra and
lightcurves and find that, compared to previous models, the X-ray spectra agree
much better with {\it XMM-Newton} observations just prior to periastron. The
narrow width of the 2009 X-ray minimum can also be reproduced. However, the
models fail to reproduce the extended X-ray mimimum from previous cycles. We
conclude that the key to explaining the extended X-ray minimum is the rate of
cooling of the companion star's postshock wind. If cooling is rapid then
powerful NTSIs will heavily disrupt the WCR. Radiative inhibition of the
companion star's preshock wind, albeit with a stronger radiation-wind coupling
than explored in this work, could be an effective trigger.Comment: 25 pages, 20 figures, accepted for publication in Ap
WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps
The blazar 3C 454.3 underwent an unprecedented optical outburst in spring
2005. This was first followed by a mm and then by a cm radio outburst, which
peaked in February 2006. We report on follow-up observations by the WEBT to
study the multiwavelength emission in the post-outburst phase. XMM-Newton
observations on July and December 2006 added information on the X-ray and UV
fluxes. The source was in a faint state. The radio flux at the higher
frequencies showed a fast decreasing trend, which represents the tail of the
big radio outburst. It was followed by a quiescent state, common at all radio
frequencies. In contrast, moderate activity characterized the NIR and optical
light curves, with a progressive increase of the variability amplitude with
increasing wavelength. We ascribe this redder-when-brighter behaviour to the
presence of a "little blue bump" due to line emission from the broad line
region, which is clearly visible in the source SED during faint states.
Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV,
suggesting the existence of a "big blue bump" due to thermal emission from the
accretion disc. The X-ray spectra are well fitted with a power-law model with
photoelectric absorption, possibly larger than the Galactic one. However, the
comparison with previous X-ray observations would imply that the amount of
absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum
presents a curvature, which may depend on the X-ray brightness. In this case,
two scenarios are possible.Comment: 9 pages, 7 figures, accepted for publication in A&
The early multicolor afterglow of GRB 050502a: Possible evidence for a uniform medium with density clumps
The 2 m robotic Liverpool Telescope reacted promptly to the gamma-ray burst GRB 050502a, discovered by INTEGRAL, and started observing 3 minutes after the onset of the burst. The automatic identification of a bright afterglow with r' ~ 15.8 mag triggered, for the first time, an observation sequence in the BVr'i' filters during the first hour after a GRB. Observations continued for ~1 day using the RoboNet-1.0 network of 2 m robotic telescopes. The light curve in all filters can be described by a simple power law with index of 1.2 ± 0.1. We find evidence for a bump rising at t ~ 0.02 days in all filters. From the spectrum and the light curve, we investigate different scenarios and find possible evidence for a uniform circumburst medium with clumps in density, as in the case of GRB 021004. Other interpretations of such bumps, such as the effect of energy injection through refreshed shocks or the result of a variable energy profile, are less favored. The optical afterglow of GRB 050502a is likely to be the result of slow electron cooling, with the optical bands lying between the synchrotron peak frequency and the cooling frequency
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