803 research outputs found
Multi-frequency Study of the LMC Supernova Remnant (SNR) B0513-692 and New SNR Candidate J051327-6911
We present a new multi-wavelength study of supernova remnant (SNR) B0513-692
in the Large Magellanic Cloud (LMC). The remnant also has a strong, superposed,
essentially unresolved, but unrelated radio source at its north-western edge,
J051324-691049. This is identified as a likely compact HII region based on
related optical imaging and spectroscopy. We use the Australia Telescope
Compact Array (ATCA) at 4790 and 8640 MHz to determine the large scale
morphology, spectral index and polarization characteristics of B0513-692 for
the first time. We detect a strongly polarized region (49%) in the remnant's
southern edge. Interestingly we also detect a small (~40 arcsec) moderately
bright, but distinct optical, circular shell in our Halpha imagery which is
adjacent to the compact HII region and just within the borders of the NE edge
of B0513-692. We suggest this is a separate new SNR candidate based on its
apparently distinct character in terms of optical morphology in 3 imaged
emission lines and indicative SNR optical spectroscopy (including enhanced
optical [SII] emission relative to Halpha).Comment: 12 page
The Physical Parameters of the Micro-quasar S26 in the Sculptor Group Galaxy NGC 7793
NGC 7793 - S26 is an extended source (350 pc 185 pc) previously
studied in the radio, optical and x-ray domains. It has been identified as a
micro-quasar which has inflated a super bubble. We used Integral Field Spectra
from the Wide Field Spectrograph on the ANU 2.3 m telescope to analyse spectra
between 3600--7000 \AA. This allowed us to derive fluxes and line ratios for
selected nebular lines. Applying radiative shock model diagnostics, we estimate
shock velocities, densities, radiative ages and pressures across the object. We
show that S26 is just entering its radiative phase, and that the northern and
western regions are dominated by partially-radiative shocks due to a lower
density ISM in these directions. We determine a velocity of expansion along the
jet of 330 km s, and a velocity of expansion of the bubble in the minor
axis direction of 132 km s. We determine the age of the structure to be
yr, and the jet energy flux to be erg
s The jet appears to be collimated within deg, and to undergo
very little precession. If the relativistic , then some 4
M of relativistic matter has already been processed through the jet.
We conclude that the central object in S26 is probably a Black Hole with a mass
typical of the ultra-luminous X-ray source population which is currently
consuming a fairly massive companion through Roche Lobe accretion.Comment: Accepted for publication in MNRAS; 12 pages, 7 figures and 3 table
Polarization of Broad Absorption Line QSOs I. A Spectropolarimetric Atlas
We present a spectropolarimetric survey of 36 broad absorption line
quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and
emission line polarization of BAL QSOs yield clues about their structure. We
confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs,
consistent with a more equatorial viewing direction for the former than the
latter. We have identified two new highly-polarized QSOs in our sample
(1232+1325 and 1333+2840). The polarization rises weakly to the blue in most
objects, perhaps due to scattering and absorption by dust particles. We find
that a polarization increase in the BAL troughs is a general property of
polarized BAL QSOs, indicating an excess of scattered light relative to direct
light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We
have also discovered evidence of resonantly scattered photons in the red wing
of the C IV broad emission lines of a few objects. In most cases, the broad
emission lines have lower polarization and a different position angle than the
continuum. The polarization characteristics of low-ionization BAL QSOs are
similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind
geometry.Comment: 39 pages, 6 figures (20 .gif files), accepted for publication in The
Astrophysical Journal Supplement
Triggered Star Formation in a Massive Galaxy at z=3.8: 4C41.17
Spectropolarimetric observations obtained with the W. M. Keck Telescope of
the z=3.8 radio galaxy 4C41.17 show that the UV continuum emission from this
galaxy, which is aligned with the radio axis, is unpolarized (P[2sigma] <
2.4%). This implies that scattered AGN light, which is generally the dominant
contributor to the rest-frame UV emission in z~1 radio galaxies, is unlikely to
be a major component of the UV flux from 4C41.17. The spectrum shows absorption
lines that are similar to those detected in the spectra of the recently
discovered population of star forming galaxies at z~2-3. A galaxian outflow may
contribute partially to the low ionization absorption lines; however, the high
velocity wings of the high ionization lines are unlikely to be dominated by a
galaxian wind since the implied outflow mass is very large. The detection of
stellar absorption lines, the shape of the SiIV profile, the unpolarized
continuum, the inability of any AGN-related processes to account for the UV
flux, and the similarity of the UV continuum spectra of 4C41.17 and the nearby
starburst region NGC 1741B1 suggest that the UV light in 4C41.17 is dominated
by young stars. If so, the implied star-formation rate is roughly
140-1100Msun/yr. We discuss the possibility that star formation in 4C41.17 was
triggered by the radio source. Our data are consistent with the hypothesis that
4C41.17 is undergoing its major epoch of star formation at z~4, and that by z~1
it will have evolved to have spectral and morphological properties similar to
those observed in known z~1 powerful radio galaxies.Comment: 28 pages (Latex text + figures); Accepted for publication in The
Astrophysical Journal (Dec 1, 1997 issue
Metal Abundances in the Magellanic Stream
We report on the first metallicity determination for gas in the Magellanic
Stream, using archival HST GHRS data for the background targets Fairall 9, III
Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new
Parkes Multibeam Narrowband observations, we have unequivocally detected the
MSI HI component of the Stream (near its head) in SII1250,1253 yielding a
metallicity of [SII/H]=-0.55+/-0.06(r)+/-0.2(s), consistent with either an SMC
or LMC origin and with the earlier upper limit set by Lu et al. (1994). We also
detect the saturated SiII1260 line, but set only a lower limit of
[SiII/H]>-1.5. We present serendipitous detections of the Stream, seen in
MgII2796,2803 absorption with column densities of (0.5-1)x10^13 cm^-2 toward
the Seyfert galaxies III Zw 2 and NGC 7469. These latter sightlines probe gas
near the tip of the Stream (80 deg down-Stream of Fairall 9). For III Zw 2, the
lack of an accurate HI column density and the uncertain MgIII ionization
correction limits the degree to which we can constrain [Mg/H]; a lower limit of
[MgII/HI]>-1.3 was found. For NGC 7469, an accurate HI column density
determination exists, but the extant FOS spectrum limits the quality of the
MgII column density determination, and we conclude that [MgII/HI]>-1.5.
Ionization corrections associated with MgIII and HII suggest that the
corresponding [Mg/H] may range lower by 0.3-1.0 dex. However, an upward
revision of 0.5-1.0 dex would be expected under the assumption that the Stream
exhibits a dust depletion pattern similar to that seen in the Magellanic
Clouds. Remaining uncertainties do not allow us to differentiate between an LMC
versus SMC origin to the Stream gas.Comment: 30 pages, 8 figures, LaTeX (aaspp4), also available at
http://casa.colorado.edu/~bgibson/publications.html, accepted for publication
in The Astronomical Journa
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Setting targets for HIV/AIDS-What lessons can be learned from other disease control programmes?
Our analysis of experience from programmes targeting malaria, leprosy and TB shows the importance of drawing broadly on research and implementation expertise, and civil society more broadly, when setting targets for HIV control. The engagement of stakeholders from the highest burden settings, including affected populations, is crucial, to ensure that disease control efforts uphold human rights and tackle HIV-related stigma and discrimination.
An appropriate balance is needed between ambitious, galvanising global targets that drive funding and political/public engagement, and targets that reflect the complexities and local epidemiological variations in disease profile. Ethical issues and unintended consequences need to be considered when setting targets—particularly around local effects and opportunity costs of having foregone other areas of disease control and public health. Intermediate and adaptable targets are needed that allow for course corrections to programmes.
Overly burdensome reporting requirements for individual local programmes and countries should be avoided, as well as potential for overlapping and sometimes conflicting targets both within and across vertical disease programmes. Process targets should be distinguished from outcome targets, which should be measurable and based on high-quality data.
Retention of expert healthcare worker skills and specialist services is vital, while moving towards integrated health systems if effective disease control programmes are to be maintained. Target development should seek areas of programme delivery where an opportunity to codevelop targets and integrate services exists. Global efforts to move to universal health coverage (UHC), for example, could be factored in when developing targets.
Sustaining investment and continuing political interest in the end phase of any elimination or eradication strategy, once incidence and prevalence are low, are critical to achieve success. Equity- and access-based service delivery targets become increasingly important as the elimination strategy nears its end and should be factored into planning.
Achieving disease elimination and/or eradication is only possible with sufficient investment in research to develop new prevention tools such as vaccines, point-of-care diagnostics, and treatments to counteract the effects of increasing drug resistance and the challenging latency period of diseases; public health infrastructure upgrades that address wider determinants of health; and health and surveillance systems that allow for equitable delivery and access to services
Why royalties ? Evidence from French distribution networks
Working paper GATE 2011-02This empirical note deals with the contractual design of relationships in distribution networks. In the framework of agency theory, I study the royalty rate as an incentive device for the upstream firm in maintaining brand-name value, using recent French data to estimate probit models. The results are consistent with the analytical framework
Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2-216
We present 1420 MHz polarization images of a 2.5 X 2.5 degree region around
the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian
Galactic Plane Survey (CGPS). An arc of low polarized intensity appears
prominently in the north-east portion of the visible disk of Sh 2-216,
coincident with the optically identified interaction region between the PN and
the interstellar medium (ISM). The arc contains structural variations down to
the ~1 arcminute resolution limit in both polarized intensity and polarization
angle. Several polarization-angle "knots" appear along the arc. By comparison
of the polarization angles at the centers of the knots and the mean
polarization angle outside Sh 2-216, we estimate the rotation measure (RM)
through the knots to be -43 +/- 10 rad/m^2. Using this estimate for the RM and
an estimate of the electron density in the shell of Sh 2-216, we derive a
line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 microG.
We believe it more likely the observed magnetic field is interstellar than
stellar, though we cannot completely dismiss the latter possibility. We
interpret our observations via a simple model which describes the ISM magnetic
field around Sh 2-216, and comment on the potential use of old PNe as probes of
the magnetized ISM.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Adaptive patch foraging in deep reinforcement learning agents
Patch foraging is one of the most heavily studied behavioral optimization
challenges in biology. However, despite its importance to biological
intelligence, this behavioral optimization problem is understudied in
artificial intelligence research. Patch foraging is especially amenable to
study given that it has a known optimal solution, which may be difficult to
discover given current techniques in deep reinforcement learning. Here, we
investigate deep reinforcement learning agents in an ecological patch foraging
task. For the first time, we show that machine learning agents can learn to
patch forage adaptively in patterns similar to biological foragers, and
approach optimal patch foraging behavior when accounting for temporal
discounting. Finally, we show emergent internal dynamics in these agents that
resemble single-cell recordings from foraging non-human primates, which
complements experimental and theoretical work on the neural mechanisms of
biological foraging. This work suggests that agents interacting in complex
environments with ecologically valid pressures arrive at common solutions,
suggesting the emergence of foundational computations behind adaptive,
intelligent behavior in both biological and artificial agents.Comment: Published in Transactions on Machine Learning Research (TMLR). See:
https://openreview.net/pdf?id=a0T3nOP9s
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