121 research outputs found
The cosmic X-ray experiment aboard HEAO-1
The HEAO-1 A-2 experiment, designed to study the large scale structure of the galaxy and the universe at X-ray energies is described. The instrument consists of six gas proportional counters of three types nominally covering the energy ranges of 0.15-3 keV, 1.2-20 keV, and 2.5-60 keV. The two low energy detectors have about 400 sq cm open area each while the four others have about 800 sq cm each. Dual field of view collimators allow the unambiguous determination of instrument internal background and diffuse X-ray brightness. Instrument characteristics and early performance are discussed
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
Radiative Transfer Analysis of Far-UV Background Observations Obtained with the Far-Ultraviolet Space Telescope (FAUST)
In 1992 the Far-Ultraviolet Space Telescope (FAUST) provided measurements of
the ultraviolet (140-180nm) diffuse sky background at high, medium, and low
Galactic latitudes. A significant fraction of the detected radiation was found
to be of Galactic origin, resulting from scattering by dust in the diffuse
interstellar medium. To simulate the radiative transfer in the Galaxy, we
employed a Monte Carlo model which utilized a realistic, non-isotropic
radiation field based on the measured fluxes (at 156nm) and positions of 58,000
TD-1 stars, and a cloud structure for the interstellar medium. The comparison
of the model predictions with the observations led to a separation of the
Galactic scattered radiation from an approximately constant background,
attributed to airglow and extragalactic radiation, and to a well constrained
determination of the dust scattering properties. The derived dust albedo a =
0.45 +/- 0.05 is substantially lower than albedos derived for dust in dense
reflection nebulae and star-forming regions, while the phase function asymmetry
g = 0.68 +/- 0.10 is indicative of a strongly forward directed phase function.
We show the highly non-isotropic phase function to be responsible, in
conjunction with the non-isotropic UV radiation field, for the wide range of
observed correlations between the diffusely scattered Galactic radiation and
the column densities of neutral atomic hydrogen. The low dust albedo is
attributed to a size distribution of grains in the diffuse medium with average
sizes smaller than those in dense reflection nebulae.Comment: 35 pages, 10 figures included, to be published in the Ap
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
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
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
Catching the radio flare in CTA 102 I. Light curve analysis
Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst
in April 2006. This event offered a unique chance to study the physical
properties of the jet. Aims: We used multifrequency radio and mm observations
to analyze the evolution of the spectral parameters during the flare as a test
of the shock-in-jet model under these extreme conditions. Methods: For the
analysis of the flare we took into account that the flaring spectrum is
superimposed on a quiescent spectrum. We reconstructed the latter from archival
data and fitted a synchrotron self-absorbed distribution of emission. The
uncertainties of the derived spectral parameters were calculated using Monte
Carlo simulations. The spectral evolution is modeled by the shock-in-jet model,
and the derived results are discussed in the context of a geometrical model
(varying viewing angle) and shock-shock interaction. Results: The evolution of
the flare in the turnover frequency-turnover flux density plane shows a double
peak structure. The nature of this evolution is dicussed in the frame of
shock-in-jet models. We discard the generation of the double peak structure in
the turnover frequency-turnover flux density plane purely based on geometrical
changes (variation of the Doppler factor). The detailed modeling of the
spectral evolution favors a shock-shock interaction as a possible physical
mechanism behind the deviations from the standard shock-in-jet model.Comment: 15 pages, 12 figure
Revealing a Ring-like Cluster Complex in a Tidal Tail of the Starburst Galaxy NGC 2146
We report the discovery of a ring-like cluster complex in the starburst
galaxy NGC 2146. The Ruby Ring, so named due to its appearance, shows a clear
ring-like distribution of star clusters around a central object. It is located
in one of the tidal streams which surround the galaxy. NGC 2146 is part of the
Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data has added
critical information to the available archival Hubble Space Telescope imaging
set of NGC 2146, allowing us to determine ages, masses, and extinctions of the
clusters in the Ruby Ring. These properties have then been used to investigate
the formation of this extraordinary system. We find evidence of a spatial and
temporal correlation between the central cluster and the clusters in the ring.
The latter are about 4 Myr younger than the central cluster, which has an age
of 7 Myr. This result is supported by the H alpha emission which is strongly
coincident with the ring, and weaker at the position of the central cluster.
From the derived total H alpha luminosity of the system we constrain the star
formation rate density to be quite high, e.g. ~ 0.47 Msun/yr/kpc^2. The Ruby
Ring is the product of an intense and localised burst of star formation,
similar to the extended cluster complexes observed in M51 and the Antennae, but
more impressive because is quite isolated. The central cluster contains only 5
% of the total stellar mass in the clusters that are determined within the
complex. The ring-like morphology, the age spread, and the mass ratio support a
triggering formation scenario for this complex. We discuss the formation of the
Ruby Ring in a "collect & collapse" framework. The predictions made by this
model agree quite well with the estimated bubble radius and expansion velocity
produced by the feedback from the central cluster, making the Ruby Ring an
interesting case of triggered star formation.Comment: 11 pages, 7 figures, 1 table; Accepted for publication in MNRA
The HELLAS2XMM survey. XIII. Multi-component analysis of the spectral energy distribution of obscured AGN
We combine near-to-mid-IR Spitzer data with shorter wavelength observations
(optical to X-rays) to get insights on the properties of a sample of luminous,
obscured Active Galactic Nuclei (AGN). We aim at modeling their broad-band
Spectral Energy Distributions (SEDs) in order to estimate the main parameters
related to the dusty torus. The sample comprises 16 obscured high-redshift
(0.9<z<2.1) xray luminous quasars (L_2-10 ~ 10^44 erg s-1) selected from the
HELLAS2XMM survey. The SEDs are described by a multi-component model including
a stellar component, an AGN component and a starburst. The majority (~80%) of
the sources show moderate optical depth (tau_9.7um<3) and the derived column
densities N_H are consistent with the xray inferred values (10^22 <N_H< 3x10^23
cm-2) for most of the objects, confirming that the sources are moderately
obscured Compton-thin AGN. Accretion luminosities in the range 5x10^44 < Lbol <
4x10^46 erg s-1 are inferred. We compare model luminosities with those obtained
by integrating the observed SED, finding that the latter are lower by a factor
of ~2 in the median. The discrepancy can be as high as an order of magnitude
for models with high optical depth (tau_9.7um=10). The ratio between the
luminosities obtained by the fitting procedure and from the observed SED
suggest that, at least for Type~2 AGN, observed bolometric luminosities are
likely to underestimate intrinsic ones and the effect is more severe for highly
obscured sources. Bolometric corrections from the hard X-ray band are computed
and have a median value of k_2-10kev ~ 20. The obscured AGN in our sample are
characterized by relatively low Eddington ratios (median lambda_Edd~0.08). On
average, they are consistent with the Eddington ratio increasing at increasing
bolometric correction (e.g. Vasudevan & Fabiam 2009).Comment: 16 pages, 10 figures. Accepted for pubblication in Astronomy and
Astrophysics
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