221 research outputs found
An Analysis of Potential Tax Incentives to Increase Charitable Giving in Puerto Rico
Compares options for improving tax incentives for charitable giving, including lifting the ceiling on deductions as a percentage of adjusted gross income, and estimated effects on nonprofits in Puerto Rico, where average giving is high relative to AGI
The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources Toward the Anticenter
We report multifrequency Very Long Baseline Array observations of twelve
active galactic nuclei seen toward the Galactic anticenter. All of the sources
are at |b| < 10 degrees and seven have |b| < 0.5 degrees. Our VLBA observations
can detect an enhancement in the angular broadening of these sources due to an
extended H II disk, if the orientation of the H II disk in the outer Galaxy is
similar to that of the H I disk. Such an extended H II disk is suggested by the
C IV absorption in a quasar's spectrum, the appearance of H I disks of nearby
spiral galaxies, and models of Ly-alpha cloud absorbers and the Galactic
fountain. We detect eleven of the twelve sources at one or more frequencies;
nine of the sources are compact and suitable for an angular broadening
analysis. A preliminary analysis of the observed angular diameters suggests
that the H II disk does not display considerable warping or flaring and does
not extend to large Galactocentric distances (R >~ 100 kpc). A companion paper
(Lazio & Cordes 1997) combines these observations with those in the literature
and presents a more comprehensive analysis.Comment: 19 pages, LaTeX2e with AASTeX macro aaspp4, accepted for publication
in ApJS, Vol. 115, 1998 April; Figures 1, 3, and 4 included, for figures of
individual sources see
http://astrosun.tn.cornell.edu/students/lazio/Anticenter/anticenterI.htm
Angular Broadening of Intraday Variable AGN. II. Interstellar and Intergalactic Scattering
We analyze a sample of 58 multi-wavelength, Very Long Baseline Array
observations of active galactic nuclei (AGN) to determine their scattering
properties. Approximately 75% of the sample consists of AGN that exhibit
centimeter-wavelength intraday variability (interstellar scintillation) while
the other 25% do not show intraday variability. We find that interstellar
scattering is measurable for most of these AGN, and the typical broadening
diameter is 2 mas at 1 GHz. We find that the scintillating AGN are typically at
lower Galactic latitudes than the non-scintillating AGN, consistent with the
scenario that intraday variability is a propagation effect from the Galactic
interstellar medium. The magnitude of the inferred interstellar broadening
measured toward the scintillating AGN, when scaled to higher frequencies, is
comparable to the diameters inferred from analyses of the light curves for the
more well-known intraday variable sources. However, we find no difference in
the amount of scattering measured toward the scintillating versus
non-scintillating AGN. A consistent picture is one in which the scintillation
results from localized regions ("clumps") distributed throughout the Galactic
disk, but which individually make little contribution to the angular
broadening. Of the 58 AGN observed, 37 (64%) have measured redshifts. At best,
a marginal trend is found for scintillating (non-scintillating) AGN to have
smaller (larger) angular diameters at higher redshifts. We also use our
observations to try to constrain the possibility of intergalactic scattering.
While broadly consistent with the scenario of a highly turbulent intergalactic
medium, our observations do not place significant constraints on its
properties.Comment: 13 pages, 4 figures; AASTeX format; ApJ in pres
The Microarcsecond Sky and Cosmic Turbulence
Radio waves are imprinted with propagation effects from ionized media through
which they pass. Owing to electron density fluctuations, compact sources
(pulsars, masers, and compact extragalactic sources) can display a wide variety
of scattering effects. These scattering effects, particularly interstellar
scintillation, can be exploited to provide *superresolution*, with achievable
angular resolutions (<~ 1 microarcsecond) far in excess of what can be obtained
by very long baseline interferometry on terrestrial baselines. Scattering
effects also provide a powerful sub-AU probe of the microphysics of the
interstellar medium, potentially to spatial scales smaller than 100 km, as well
as a tracer of the Galactic distribution of energy input into the interstellar
medium through a variety of integrated measures. Coupled with future gamma-ray
observations, SKA observations also may provide a means of detecting fainter
compact gamma-ray sources. Though it is not yet clear that propagation effects
due to the intergalactic medium are significant, the SKA will either detect or
place stringent constraints on intergalactic scattering.Comment: 20 pages, 8 figures in 8 PostScript files, to appear in "Science with
the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy
Reviews (Elsevier: Amsterdam
Genesis Lunar Outpost: Program/Requirements Document for an Early Stage Lunar Outpost
Second document in a series from the Space Architecture Design Group. Developed under the first year of a three-year grant from NASA, the program covers humans factors, environment-behavior, structural, construction processes, life-system and related needs for an 8-12 person lunar base for assignments lasting up to 20 months. The program includes sections on base master planning, base operations, command center, technical support habitat and all related design requirements.https://dc.uwm.edu/caupr_mono/1053/thumbnail.jp
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array
This paper presents a search for radio transients at a frequency of 73.8 MHz
(4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength
Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope,
located on the site of the Very Large Array in New Mexico. The field of view of
the individual dipoles was essentially the entire sky, and the number of
dipoles was sufficiently small that a simple software correlator could be used
to make all-sky images. From 2006 October to 2007 February, we conducted an
all-sky transient search program, acquiring a total of 106 hr of data; the time
sampling varied, being 5 minutes at the start of the program and improving to 2
minutes by the end of the program. We were able to detect solar flares, and in
a special-purpose mode, radio reflections from ionized meteor trails during the
2006 Leonid meteor shower. We detected no transients originating outside of the
solar system above a flux density limit of 500 Jy, equivalent to a limit of no
more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x
10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate
is comparable to that determined from previous all-sky transient searches, but
at a lower frequency than most previous all-sky searches. We believe that the
LWDA illustrates how an all-sky imaging mode could be a useful operational
model for low-frequency instruments such as the Low Frequency Array, the Long
Wavelength Array station, the low-frequency component of the Square Kilometre
Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A
Scintillation-Induced Intermittency in SETI
We consider interstellar scintillations as a cause of intermittency in radio
signals from extraterrestrial intelligence (ETI). We demonstrate that
scintillations are very likely to allow initial detections of narrowband
signals from distant sources (> 100 pc), while making redetections improbable.
We consider three models in order to assess the non-repeating, narrowband
events found in recent SETI and to analyze large surveys in general: (I)
Radiometer noise; (II) A population of constant Galactic sources undergoing
interstellar scintillation,; and (III) Real, transient signals (or hardware
errors) of either terrestrial or ET origin.
We apply likelihood and Bayesian tests of the models to The Planetary
Society/Harvard META data. We find that Models II and III are both highly
preferred to Model I, but that Models II and III are about equally likely.
Ruling out Model II in favor of Model III requires many more reobservations
than were conducted in META *or* the reobservation threshold must be much lower
than was used in META. *We cannot rule out the possibility that META events are
real, intrinsically steady ETI signals.*
We recommend that future surveys use thresholds far below the typical
false-alarm threshold to lessen the effects of intermittency. The threshold
level is best defined in terms of the recording and computational technology
that is available at a cost commensurate with other survey costs.Comment: 59 pages, LaTeX using aaspp4 style file, 12 figures in 14 PostScript
figures, ApJ, in press, 1997 Oct.
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