703 research outputs found
X-Ray Spectral Variability of PKS 2005-489 During the Spectacular November 1998 Flare
We report on monitoring of the BL Lac object PKS 2005-489 by the Rossi X-ray
Timing Explorer (RXTE) in October-December 1998. During these months, the
source underwent a spectacular flare; at its peak on November 10, its 2-10 keV
flux was , over 30 times
brighter than in quiescence. During the rising phase, the X-ray spectrum of PKS
2005-489 hardened considerably, reaching near maximum. During the declining phase, the X-ray spectrum
steepened rapidly, reaching , then became somewhat harder
towards the end of December (). While such behavior has been
seen before, the simplicity, magnitude and duration of this flare allowed us to
study it in great detail. We argue that this flare was caused by either the
injection of particles into the jet or {\it in situ} particle acceleration, and
that the spectral steepening which followed the flare maximum was the result of
synchrotron cooling. Contrary to other recently observed blazar flares (e.g.,
Mkn 501, 3C 279, PKS 2155-304), our results do not imply a major shift in the
location of the synchrotron peak during this flare.Comment: ApJ Letters in press, 6 pages, 2 figures Corrected reference
"Dark Matter" in Accretion Disks
Using Spitzer Space Telescope photometric observations of the eclipsing,
interacting binary WZ Sge, we have discovered that the accretion disk is far
more complex than previously believed. Our 4.5 and 8 micron time series
observations reveal that the well known gaseous accretion disk is surrounded by
an asymmetric disk of dusty material with a radius approximately 15 times
larger than the gaseous disk. This dust ring contains only a small amount of
mass and is completely invisible at optical and near-IR wavelengths, hence
consisting of "dark matter". We have produced a model dust ring using 1 micron
spherical particles with a density of 3 g/cm and with a temperature profile
ranging from 700-1500K. Our discovery about the accretion disk structure and
the presence of a larger, outer dust ring have great relevance for accretion
disks in general, including those in other interacting binary systems, pre-main
sequence stars, and active galaxies.Comment: 34 pages, 8 figures (3 in color). Accepted to Ap
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3-D seismic exploration for the Victorio Peak treasure
In January of 1994, we conducted an extensive seismic experiment to find caverns under Victorio Peak, a bioherm reef structure located approximately 80 miles northeast of Las Cruces, New Mexico. Within Victorio Peak there is reputed to be an extensive network of caverns, tunnels and fissures that contain a large and varied treasure consisting of gold bars, Spanish armor, jewels, coins, and human skeletons. We used an array of seismic sources that included a sledgehammer on the surface of the mountain, a sledgehammer impacting on the walls of two deep fissures within the mountain, as well as blasting caps located in boreholes drilled into the mountain. Approximately 2,000 source positions were recorded by 120 receiver channels consisting of geophones cemented into fissure walls and hydrophones deployed in a deep horizontal borehole drilled at the base of the mountain. The data analysis consisted of measuring reduced traveltime and amplitude of the direct arrival and isolating those source/receiver pairs that exhibited anomalously large direct arrival traveltimes and/or low amplitudes. We have currently identified and located a major amplitude anomaly under the peak that will be drilled and explored during the summer of 1994
Faint Radio Sources in the NOAO Bootes Field. VLBA Imaging and Optical Identifications
As a step toward investigating the parsec-scale properties of faint
extragalactic radio sources, the Very Long Baseline Array (VLBA) was used at
5.0 GHz to obtain phase-referenced images of 76 sources in the NOAO Bootes
field. These 76 sources were selected from the FIRST catalog to have peak flux
densities above 10 mJy at 5 arcsec resolution and deconvolved major diameters
of less than 3 arcsec at 1.4 GHz. Fifty-seven of these faint radio sources were
identified with accretion-powered radio galaxies and quasars brighter than 25.5
mag in the optical I band. On VLA scales at 1.4 GHz, a measure of the
compactness of the faint sources (the ratio of the peak flux density from FIRST
to the integrated flux density from the NVSS catalog) spans the full range of
possibilites arising from source-resolution effects. Thirty of the faint radio
sources, or 39 +9/-7%, were detected with the VLBA at 5.0 GHz with peak flux
densities above 6 sigma ~2 mJy at 2 mas resolution. The VLBA detections occur
through the full range of compactness ratios. The stronger VLBA detections can
themselves serve as phase-reference calibrators, boding well for opening up
much of the radio sky to VLBA imaging. For the adopted cosmology, the VLBA
resolution correponds to 17 pc or finer. Most VLBA detections are unresolved or
slightly resolved but one is diffuse and five show either double or core-jet
structures; the properties of these latter six are discussed in detail. Three
VLBA detections are unidentified and fainter than 25.5 mag in the optical I
band; their properties are highlighted because they likely mark
optically-obscured active nuclei at high redshift.Comment: 30 pages, 6 figures, ID tweaks, to appear in the September 2005 issue
of A
Entry in the ADHD drugs market: Welfare impact of generics and me-toos
Recent decades have seen a growth in treatments for attention deficit hyperactivity disorder (ADHD) including many branded and generic drugs. In the early 2000's, new drug entry dramatically altered market shares. We estimate a demand system for ADHD drugs and assess the welfare impact of new drugs. We find that entry induced large welfare gains by reducing prices of substitute drugs, and by providing alternative delivery mechanisms for existing molecules. Our results suggest that the success of follow-on patented drugs may come from unanticipated innovations like delivery mechanisms, a factor ignored by proposals to retard new follow-on drug approvals
Variable Star Candidates in an ACS Field of M31
A search for variable stars is performed using two epochs of Hubble Space
Telescope (HST) Advanced Camera for Surveys (ACS) imaging data for a 9.28
square arcminute portion of M31. This data set reveals 254 sources that vary by
at least 4-sigma between epochs. The positions and 2-epoch B-band (equivalent)
photometry of these sources are presented. The photometry suggests that this
catalog includes most of the RR Lyrae population of this portion of M31.Comment: 22 pages, 3 tables, 5 figures, accepted for publication in A
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