165 research outputs found
Resistive anode image converter
The invention of an apparatus for imaging soft X-ray and ultraviolet electromagnetic radiation and charged particles was described. The apparatus includes a pair of microchannel electron multiplier plates connected in a cascaded chevron configuration which intercepts an incident beam of radiation or charged particles. Incident photons or charged particles strike the front surface of the chevron configuration causing emission of electrons. The electrons are accelerated by a voltage gradient and strike the inner side walls of the individual channels, causing emission of secondary electrons. Accelerated and multiplied secondary electrons impinge upon a resistive anode after they transverse the chevron configuration. A pulse position circuit converts the magnitude or transit time of the currents flowing from the point of impact of the electrons on the resistive anode to four contact electrodes mounted on their periphery of the resistive anode into the spatial coordinates of electron impact
Far Ultraviolet Absolute Flux of alpha Virginis
We present the far ultraviolet spectrum of alpha Virginis taken with EURD
spectrograph on-board MINISAT-01. The spectral range covered is from ~900 to
1080 A with 5 A spectral resolution. We have fitted Kurucz models to IUE
spectra of alpha Vir and compared the extension of the model to our wavelengths
with EURD data. This comparison shows that EURD fluxes are consistent with the
prediction of the model within 20-30%, depending on the reddening assumed. EURD
fluxes are consistent with Voyager observations but are ~60% higher than most
previous rocket observations of alpha Vir.Comment: 13 pages, 4 figures. Submitted to The Astrophysical Journa
The cooling rate of neutron stars after thermonuclear shell flashes
Thermonuclear shell flashes on neutron stars are detected as bright X-ray
bursts. Traditionally, their decay is modeled with an exponential function.
However, this is not what theory predicts. The expected functional form for
luminosities below the Eddington limit, at times when there is no significant
nuclear burning, is a power law. We tested the exponential and power-law
functional forms against the best data available: bursts measured with the
high-throughput Proportional Counter Array (PCA) on board the Rossi X-ray
Timing Explorer. We selected a sample of 35 'clean' and ordinary (i.e., shorter
than a few minutes) bursts from 14 different neutron stars that 1) show a large
dynamic range in luminosity, 2) are the least affected by disturbances by the
accretion disk and 3) lack prolonged nuclear burning through the rp-process. We
find indeed that for every burst a power law is a better description than an
exponential function. We also find that the decay index is steep, 1.8 on
average, and different for every burst. This may be explained by contributions
from degenerate electrons and photons to the specific heat capacity of the
ignited layer and by deviations from the Stefan-Boltzmann law due to changes in
the opacity with density and temperature. Detailed verification of this
explanation yields inconclusive results. While the values for the decay index
are consistent, changes of it with the burst time scale, as a proxy of ignition
depth, and with time are not supported by model calculations.Comment: 10 pages, 7 figures, recommended for publication in A&
Far Ultraviolet Spectra of B Stars near the Ecliptic
Spectra of B stars in the wavelength range of 911-1100 A have been obtained
with the EURD spectrograph onboard the Spanish satellite MINISAT-01 with ~5 A
spectral resolution. IUE spectra of the same stars have been used to normalize
Kurucz models to the distance, reddening and spectral type of the corresponding
star. The comparison of 8 main-sequence stars studied in detail (alpha Vir,
epsilon Tau, lambda Tau, tau Tau, alpha Leo, zeta Lib, theta Oph, and sigma
Sgr) shows agreement with Kurucz models, but observed fluxes are 10-40% higher
than the models in most cases. The difference in flux between observations and
models is higher in the wavelength range between Lyman alpha and Lyman beta. We
suggest that Kurucz models underestimate the FUV flux of main-sequence B stars
between these two Lyman lines. Computation of flux distributions of
line-blanketed model atmospheres including non-LTE effects suggests that this
flux underestimate could be due to departures from LTE, although other causes
cannot be ruled out. We found the common assumption of solar metallicity for
young disk stars should be made with care, since small deviations can have a
significant impact on FUV model fluxes. Two peculiar stars (rho Leo and epsilon
Aqr), and two emission line stars (epsilon Cap and pi Aqr) were also studied.
Of these, only epsilon Aqr has a flux in agreement with the models. The rest
have strong variability in the IUE range and/or uncertain reddening, which
makes the comparison with models difficult.Comment: 25 pages, 6 figures, to be published in The Astrophysical Journa
ProtoDESI: First On-Sky Technology Demonstration for the Dark Energy Spectroscopic Instrument
The Dark Energy Spectroscopic Instrument (DESI) is under construction to
measure the expansion history of the universe using the baryon acoustic
oscillations technique. The spectra of 35 million galaxies and quasars over
14,000 square degrees will be measured during a 5-year survey. A new prime
focus corrector for the Mayall telescope at Kitt Peak National Observatory will
deliver light to 5,000 individually targeted fiber-fed robotic positioners. The
fibers in turn feed ten broadband multi-object spectrographs. We describe the
ProtoDESI experiment, that was installed and commissioned on the 4-m Mayall
telescope from August 14 to September 30, 2016. ProtoDESI was an on-sky
technology demonstration with the goal to reduce technical risks associated
with aligning optical fibers with targets using robotic fiber positioners and
maintaining the stability required to operate DESI. The ProtoDESI prime focus
instrument, consisting of three fiber positioners, illuminated fiducials, and a
guide camera, was installed behind the existing Mosaic corrector on the Mayall
telescope. A Fiber View Camera was mounted in the Cassegrain cage of the
telescope and provided feedback metrology for positioning the fibers. ProtoDESI
also provided a platform for early integration of hardware with the DESI
Instrument Control System that controls the subsystems, provides communication
with the Telescope Control System, and collects instrument telemetry data.
Lacking a spectrograph, ProtoDESI monitored the output of the fibers using a
Fiber Photometry Camera mounted on the prime focus instrument. ProtoDESI was
successful in acquiring targets with the robotically positioned fibers and
demonstrated that the DESI guiding requirements can be met.Comment: Accepted versio
The EUV Emission in the Coma Cluster of Galaxies and the Underlying Source of this Radiation
Observations with the Extreme Ultraviolet Explorer (EUVE) have shown the Coma
Cluster to be a source of EUV emission in excess of that produced by X-ray gas
in the cluster. We have re-examined the EUVE data on this cluster in an attempt
to obtain clues as to the origin of this emission. We find two important new
results. First, the ratio between the azimuthally averaged EUV excess emission
and the ROSAT hard X-ray flux is constant as a function of distance from the
cluster center outward. Second, a correlation analysis between the EUV excess
emission and the X-ray emission shows that on a detailed level the EUV excess
is spatially closely related to the X-ray emission. These findings contradict
previous suggestions as to the underlying source of the diffuse EUV emission in
Coma and provide important information in regards to the true source of this
emission. We propose a new explanation for the source of this emission: inverse
Compton scattering of microwave background photons by secondary electrons and
positrons. We explore this possibility in some detail and show that it is
consistent with all of the available observational evidence. The parent cosmic
ray protons may have been produced by any of a number of sources, including
supernovae, active galaxies, galactic winds, and cluster formation shocks, but
we believe that the most likely source is cluster formation shocks. If the EUV
emission in the Coma Cluster is, in fact, the result of secondary electrons,
this may be the only direct evidence for secondary electrons in the
intracluster medium of a cluster of galaxies, since recent work suggests that
secondary electrons may not be the cause of radio halos.Comment: 17 pages, 7 eps figures inline, submitted to Ap
Compton reflection and iron fluorescence in BeppoSAX observations of Seyfert type 1 galaxies
A sample of nine bright Seyfert 1 and NELG type galaxies, observed with
BeppoSAX, is analyzed to assess on a truly broad band basis (0.1-200 keV) the
issue of the spectral contributions of Compton reflection and iron line
fluorescence from circumnuclear gas. The empirical description adopted for the
direct continuum is the commonly used power law with an exponential cut-off.
The most direct test of the theoretical predictions, namely that the equivalent
width of the line, Walpha, and the strength R of the reflection relative to the
direct continuum are closely related to each other, gives a substantially
positive result, that is their mean ratio is very close to expectation, and
only a modest spread in the iron abundance seems implied. The existence of a
steep correlation between R and the slope Gamma of the power law is not
confirmed. A weak evidence is found that the existence of a very shallow trend
to increase on average with Gamma cannot be altogether excluded in both R and
Walpha, but needs to be tested with a larger sample. The energy Ef in the
exponential cut-off spans a range from about 80 to more than 300 keV. A
possible correlation is found, with Ef increasing on average with Gamma: if
ignored, for instance by keeping Ef at a fixed value in a sample study, it
could be cause of artificial steepening in a correlation between R and Gamma.Comment: accepted for publication in A&
Reconsidering the Barefoot Doctor Programme
This paper examines the widely acclaimed Barefoot Doctor campaign in China. The Barefoot Doctor Campaign has come to symbolize the success of Chinese health care to the extent that it has become a model for WHO public health strategy. Yet little has been done to understand how or whether it worked on the ground and what difficulties and contradictions emerged in its implementation. Using previously unexplored party archives as well as newly collected oral interviews, this paper moves away from a narrow focus on party politics and policy formulation by examining the reality of health care at the local level and the challenges faced by local authorities and individuals as the campaigns evolved
Multi-wavelength analysis of high energy electrons in solar flares: a case study of August 20, 2002 flare
A multi-wavelength spatial and temporal analysis of solar high energy
electrons is conducted using the August 20, 2002 flare of an unusually flat
(gamma=1.8) hard X-ray spectrum. The flare is studied using RHESSI, Halpha,
radio, TRACE, and MDI observations with advanced methods and techniques never
previously applied in the solar flare context. A new method to account for
X-ray Compton backscattering in the photosphere (photospheric albedo) has been
used to deduce the primary X-ray flare spectra. The mean electron flux
distribution has been analysed using both forward fitting and model independent
inversion methods of spectral analysis. We show that the contribution of the
photospheric albedo to the photon spectrum modifies the calculated mean
electron flux distribution, mainly at energies below 100 keV. The positions of
the Halpha emission and hard X-ray sources with respect to the current-free
extrapolation of the MDI photospheric magnetic field and the characteristics of
the radio emission provide evidence of the closed geometry of the magnetic
field structure and the flare process in low altitude magnetic loops. In
agreement with the predictions of some solar flare models, the hard X-ray
sources are located on the external edges of the Halpha emission and show
chromospheric plasma heated by the non-thermal electrons. The fast changes of
Halpha intensities are located not only inside the hard X-ray sources, as
expected if they are the signatures of the chromospheric response to the
electron bombardment, but also away from them.Comment: 26 pages, 9 figures, accepted to Solar Physic
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