928 research outputs found
The Ethics of Providing Access
Archivists today make some of the most difficult ethical and legal decisions at the public service desk. It has always been a difficult process to balance the archivist\u27s legal and ethical obligations to the researcher, to the donors of collections, and to the institution served and, furthermore, to factor in obligations to those who often are not even aware that archives hold materials that impact on their lives. Balancing equality of access for all patrons with institutional needs and requirements is at least as difficult. Despite sincere efforts to limit the acquisition of restricted material, many important and potentially useful collections are restricted. Indeed, some have never been used. Naturally, archivists would like to encourage the use of 9ollections that reveal a wealth of information documenting social, economic, literary, and educational history
Diversity and Traditional Collections at Rutgers University
With the establishment of both regional and national ethnic, gender, cultural, and religious collections, the concept of diversity in archives encourages a fragmented world. This is one in which a researcher has a few clear-cut options--go to a women\u27s repository to do research on women, a labor collection for labor history, an African-American archives for African-Americans, and so forth. However, time has demonstrated that no repository has a comer on the market for diversity which can also be found in holdings of what are often referred to as traditional collections. Such repositories, whose primary objective is to document state and local history, already reflect the entire scope of, the society which they seek to chronicle including diverse ethnjc, religious, and cultural communities
Crustal heterogeneity of the moon viewed from the Galileo SSI camera: Lunar sample calibrations and compositional implications
Summaries are given of the spectral calibration, compositional parameters, nearside color, and limb and farside color of the Moon. The farside of the Moon, a large area of lunar crust, is dominated by heavily cratered terrain and basin deposits that represent the products of the first half billion years of crustal evolution. Continuing analysis of the returned lunar samples suggest a magma ocean and/or serial magmatism model for evolution of the primordial lunar crust. However, testing either hypothesis requires compositional information about the crustal stratigraphy and lateral heterogeneity. Resolution of this important planetary science issue is dependent on additional data. New Galileo multispectral images indicate previously unknown local and regional compositional diversity of the farside crust. Future analysis will focus on individual features and a more detailed assessment of crustal stratigraphy and heterogeneity
An ortholog of the Vasa intronic gene is required for small RNA-mediated translation repression in \u3ci\u3eChlamydomonas reinhardtii\u3c/i\u3e
Small RNAs (sRNAs) associate with Argonaute (AGO) proteins in effector complexes, termed RNA-induced silencing complexes (RISCs), which regulate complementary transcripts by translation inhibition and/or RNA degradation. In the unicellular alga Chlamydomonas, several metazoans, and land plants, emerging evidence indicates that polyribosome-associated transcripts can be translationally repressed by RISCs without substantial messenger RNA (mRNA) destabilization. However, the mechanism of translation inhibition in a polyribosomal context is not understood. Here we show that Chlamydomonas VIG1, an ortholog of the Drosophila melanogaster Vasa intronic gene (VIG), is required for this process. VIG1 localizes predominantly in the cytosol and comigrates with monoribosomes and polyribosomes by sucrose density gradient sedimentation. A VIG1- deleted mutant shows hypersensitivity to the translation elongation inhibitor cycloheximide, suggesting that VIG1 may have a nonessential role in ribosome function/structure. Additionally, FLAG-tagged VIG1 copurifies with AGO3 and Dicer-like 3 (DCL3), consistent with it also being a component of the RISC. Indeed, VIG1 is necessary for the repression of sRNA-targeted transcripts at the translational level but is dispensable for cleavagemediated RNA interference and for the association of the AGO3 effector with polyribosomes or target transcripts. Our results suggest that VIG1 is an ancillary ribosomal component and plays a role in sRNA-mediated translation repression of polyribosomal transcripts
Chandra Phase-Resolved Spectroscopy of the Crab Pulsar
We present the first phase-resolved study of the X-ray spectral properties of
the Crab Pulsar that covers all pulse phases. The superb angular resolution of
the Chandra X-ray Observatory enables distinguishing the pulsar from the
surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged
spectrum measures interstellar X-ray extinction due primarily to photoelectric
absorption and secondarily to scattering by dust grains in the direction of the
Crab Nebula. We confirm previous findings that the line-of-sight to the Crab is
underabundant in oxygen, although more-so than recently measured. Using the
abundances and cross sections from Wilms, Allen & McCray (2000) we find [O/H] =
(3.33 +/-0.25) x 10**-4. Analysis of the spectrum as a function of pulse phase
measures the low-energy X-ray spectral index even at pulse minimum -- albeit
with large statistical uncertainty -- and we find marginal evidence for
variations of the spectral index. The data are also used to set a new (3-sigma)
upper limit to the temperature of the neutron star of log T(infinity) < 6.30.Comment: 20 Pages including 7 figures. Accepted for publication in the
Astrophysical Journa
A Millisecond Pulsar Optical Counterpart with Large-Amplitude Variability in the Globular Cluster 47 Tucanae
Using extensive HST imaging, combined with Chandra X-ray and Parkes radio
data, we have detected the optical binary companion to a second millisecond
pulsar (MSP) in the globular cluster 47 Tucanae. This faint (V=22.3) blue
(V-I=0.7) star shows a large amplitude (60-70%) sinusoidal variation in both V
and I. The period (3.19066 hr) and phase of the variation match those of the
MSP 47 Tuc W (which does not have an accurate radio timing position) to within
0.5 seconds and 1.2 minutes respectively, well within the ~1sigma errors. The
phase dependence of the intensity and color implies that heating of a tidally
locked companion causes the observed variations. The eclipsing nature of this
MSP in the radio, combined with the relatively large companion mass (> 0.13
solar masses) and the companion's position in the color-magnitude diagram,
suggest that the companion is a main sequence star, a rare circumstance for an
MSP companion. This system is likely to have had a complex evolution and
represents an interesting case study in MSP irradiation of a close companion.
We present evidence for another optical variable with similar properties to the
companion of 47 Tuc W. This variable may also be an MSP companion, although no
radio counterpart has yet been detected.Comment: 11 pages, 9 figures, accepted for publication in Ap
New Observations of the Crab Nebula and Pulsar
We present a phase-resolved study of the X-ray spectrum of the Crab Pulsar, using data obtained in a special mode with the Chandra X-ray Observatory. The superb angular resolution easily enables discerning the Pulsar from the surrounding nebulosity, even at pulse minimum. We find that the Pulsar's X-ray spectral index varies sinusoidally with phase---except over the same phase range for which rather abrupt changes in optical polarization magnitude and position angle have been reported. In addition, we use the X-ray data to constrain the surface temperature for various neutron-star equations of state and atmospheres. Finally, we present new data on dynamical variations of structure within the Nebula
Chandra Phase-Resolved X-ray Spectroscopy of the Crab Pulsar II
We present a new study of the X-ray spectral properties of the Crab Pulsar.
The superb angular resolution of the Chandra X-ray Observatory enables
distinguishing the pulsar from the surrounding nebulosity. Analysis of the
spectrum as a function of pulse phase allows the least-biased measure of
interstellar X-ray extinction due primarily to photoelectric absorption and
secondarily to scattering by dust grains in the direction of the Crab Nebula.
We modify previous findings that the line-of-sight to the Crab is
under-abundant in oxygen and provide measurements with improved accuracy and
less bias. Using the abundances and cross sections from Wilms, Allen & McCray
(2000) we find [O/H] = ( is
solar abundance). We also measure for the first time the impact of scattering
of flux out of the image by interstellar grains. We find . Analysis of the spectrum as a function of pulse phase also
measures the X-ray spectral index even at pulse minimum --- albeit with
increasing statistical uncertainty. The spectral variations are, by and large,
consistent with a sinusoidal variation. The only significant variation from the
sinusoid occurs over the same phase range as some rather abrupt behavior in the
optical polarization magnitude and position angle. We compare these spectral
variations to those observed in Gamma-rays and conclude that our measurements
are both a challenge and a guide to future modeling and will thus eventually
help us understand pair cascade processes in pulsar magnetospheres. The data
were also used to set new, and less biased, upper limits to the surface
temperature of the neutron star for different models of the neutron star
atmosphere.Comment: 32 pages, 6 figures submitted to the Astrophysical journa
A multi-wavelength search for a counterpart of the unidentified gamma-ray source 3EG J2020+4017 (2CG078+2)
In search of the counterpart to the brightest unidentified gamma-ray source
3EG J2020+4017 we report on new X-ray and radio observations of the gamma-Cygni
field with the Chandra X-ray Observatory and with the Green Bank Telescope. We
also report on reanalysis of archival ROSAT data. With Chandra it became
possible for the first time to measure the position of the putative gamma-ray
counterpart RX J2020.2+4026 with sub-arcsec accuracy and to deduce its X-ray
spectral characteristics. These observations demonstrate that RX J2020.2+4026
is associated with a K field star and therefore is unlikely to be the
counterpart of the bright gamma-ray source 2CG078+2 in the SNR G78.2+2.1 as had
been previously suggested. The Chandra observation detected 37 additional X-ray
sources which were correlated with catalogs of optical and infrared data.
Subsequent GBT radio observations covered the complete 99% EGRET likelihood
contour of 3EG J2020+4017 with a sensitivity limit of L_820 ~ 0.1 mJy kpc^2
which is lower than most of the recent deep radio search limits. If there is a
pulsar operating in 3EG J2020+4017 this sensitivity limit suggests that the
pulsar either does not produce significant amounts of radio emission or that
its geometry is such that the radio beam does not intersect with the line of
sight. Finally, reanalysis of archival ROSAT data leads to a flux upper limit
of f_x < 1.8 x 10^-13 erg s^-1 cm^-2 for a putative point-like X-ray source
located within the 68% confidence contour of 3EG J2020+4017. Adopting the SNR
age of 5400 yrs and assuming a spin-down to X-ray energy conversion factor of
10^-4 this upper limit constrains the parameters of a putative neutron star as
a counterpart for 3EG J2020+4017 to be P >= 160 (d/1.5 kpc)^-1 ms, P_dot >= 5 x
10^-13 (d/1.5 kpc)^-1 s s^-1 and B_perp >= 9 x 10^12 (d/1.5 kpc)^-1 G.Comment: Accepted for publication in ApJ. A PDF version of the accepted paper
with higher resolution images is available at
ftp://www.xray.mpe.mpg.de/people/web/gamma-Cygn
The Identification of the X-ray Counterpart to PSR J2021+4026
We report the probable identification of the X-ray counterpart to the
gamma-ray pulsar PSR J2021+4026 using imaging with the Chandra X-ray
Observatory ACIS and timing analysis with the Fermi satellite. Given the
statistical and systematic errors, the positions determined by both satellites
are coincident. The X-ray source position is R.A. 20h21m30.733s, Decl. +40 deg
26 min 46.04sec (J2000) with an estimated uncertainty of 1.3 arsec combined
statistical and systematic error. Moreover, both the X-ray to gamma-ray and the
X-ray to optical flux ratios are sensible assuming a neutron star origin for
the X-ray flux. The X-ray source has no cataloged infrared-to-visible
counterpart and, through new observations, we set upper limits to its optical
emission of i' >23.0 mag and r' > 25.2mag. The source exhibits an X-ray
spectrum with most likely both a powerlaw and a thermal component. We also
report on the X-ray and visible light properties of the 43 other sources
detected in our Chandra observation.Comment: Accepted for publication in the Astrophysical Journa
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