38 research outputs found
Recommended from our members
Checklist of the Marine Fauna and Flora Reported from the Region of Aransas Pass, Texas
This checklist is of preliminary form with the hope
that this will be a beginning for a more complete annotated
list of the fauna and flora of this region.
In many cases, this list is quite obviously incomplete,
such as the algae, for which there are many species
found on the Port Aransas jetties which have not
been reported in a formal publication. Further work on
the taxonomy or the biota of this region will gradually
fill the incomplete gaps, and the resulting list should
then prove quite beneficial to field investigators in this
area.
The arrangement or this list is phylogenetic, with
reference for each species indicated by the number of each
reference.Marine Scienc
Recommended from our members
Checklist of the Invertebrate Animals Reported from the Region of Port Aransas, Texas
This checklist is of preliminary form with the hope that
this will be a beginning for a more complete annotated list
of the fauna and flora of this region. In many cases this
list is quite obviously incomplete, however further work
on the taxonomy of the biota of this region will gradually
fill the incomplete gaps, and the resulting list should then
prove quite useful to field investigators in this area.Marine Scienc
Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor
The Advanced CCD Imaging Spectrometer (ACIS) is one of two focal-plane
instruments on the Chandra X-ray Observatory. During initial radiation-belt
passes, the exposed ACIS suffered significant radiation damage from trapped
soft protons scattering off the x-ray telescope's mirrors. The primary effect
of this damage was to increase the charge-transfer inefficiency (CTI) of the
ACIS 8 front-illuminated CCDs. Subsequently, the Chandra team implemented
procedures to remove the ACIS from the telescope's focus during high-radiation
events: planned protection during radiation-belt transits; autonomous
protection triggered by an on-board radiation monitor; and manual intervention
based upon assessment of space-weather conditions. However, as Chandra's
multilayer insulation ages, elevated temperatures have reduced the
effectiveness of the on-board radiation monitor for autonomous protection. Here
we investigate using the ACIS CCDs themselves as a radiation monitor. We
explore the 10-year database to evaluate the CCDs' response to particle
radiation and to compare this response with other radiation data and
environment models.Comment: 10 pages, 5 figures. To appear in Proc. SPIE vol. 773
A charge transfer inefficiency correction model for the Chandra Advanced CCD Imaging Spectrometer
Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the
focal plane instruments on the Chandra X-ray Observatory, suffered radiation
damage from exposure to soft protons during passages through the Earth's
radiation belts. The primary effect of the damage was to increase the charge
transfer inefficiency (CTI) of the eight front illuminated CCDs by more than
two orders of magnitude. The ACIS instrument team is continuing to study the
properties of the damage with an emphasis on developing techniques to mitigate
CTI and spectral resolution degradation. We will discuss the characteristics of
the damage, the detector and the particle background and how they conspire to
degrade the instrument performance. We have developed a model for ACIS CTI
which can be used to correct each event and regain some of the lost
performance. The correction uses a map of the electron trap distribution, a
parameterization of the energy dependent charge loss and the fraction of the
lost charge re-emitted into the trailing pixel to correct the pixels in the
event island. This model has been implemented in the standard Chandra data
processing pipeline. Some of the correction algorithm was inspired by the
earlier work on ACIS CTI correction by Townsley et al. (2000; 2002). The
details of the CTI model and how each parameter improves performance will be
discussed, as well as the limitations and the possibilities for future
improvement.Comment: 12 pages, 12 figures, will appear in Proc. SPIE 550
Temperature dependence of charge transfer inefficiency in Chandra X-ray CCDs
Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the
focal plane instruments on the Chandra X-ray Observatory, suffered radiation
damage from exposure to soft protons during passages through the Earth's
radiation belts. The primary effect of the damage was to increase the charge
transfer inefficiency (CTI) of the eight front illuminated CCDs by more than
two orders of magnitude. The ACIS instrument team is continuing to study the
properties of the damage with an emphasis on developing techniques to mitigate
CTI and spectral resolution degradation. We present the initial temperature
dependence of ACIS CTI from -120 to -60 degrees Celsius and the current
temperature dependence after more than six years of continuing slow radiation
damage. We use the change of shape of the temperature dependence to speculate
on the nature of the damaging particles.Comment: 9 pages, 8 figures, to appear in Proc. SPIE vol 6276 "High Energy,
Optical, and Infrared Detectors for Astronomy II
Physics of reverse annealing in high-resistivity Chandra ACIS CCDs
After launch, the Advanced CCD Imaging Spectrometer (ACIS), a focal plane
instrument on the Chandra X-ray Observatory, suffered radiation damage from
exposure to soft protons during passages through the Earth's radiation belts.
An effect of the damage was to increase the charge transfer inefficiency (CTI)
of the front illuminated CCDs. As part of the initial damage assessment, the
focal plane was warmed from the operating temperature of -100C to +30C which
unexpectedly further increased the CTI. We report results of ACIS CCD
irradiation experiments in the lab aimed at better understanding this reverse
annealing process. Six CCDs were irradiated cold by protons ranging in energy
from 100 keV to 400 keV, and then subjected to simulated bakeouts in one of
three annealing cycles. We present results of these lab experiments, compare
them to our previous experiences on the ground and in flight, and derive limits
on the annealing time constants.Comment: 9 pages, to appear in Proc. SPIE 7021, "High Energy, Optical and
Infrared Detectors for Astronomy
A Persistent Disk Wind in GRS 1915+105 with NICER
The bright, erratic black hole X-ray binary GRS 1915+105 has long been a
target for studies of disk instabilities, radio/infrared jets, and accretion
disk winds, with implications that often apply to sources that do not exhibit
its exotic X-ray variability. With the launch of NICER, we have a new
opportunity to study the disk wind in GRS 1915+105 and its variability on short
and long timescales. Here we present our analysis of 39 NICER observations of
GRS 1915+105 collected during five months of the mission data validation and
verification phase, focusing on Fe XXV and Fe XXVI absorption. We report the
detection of strong Fe XXVI in 32 (>80%) of these observations, with another
four marginal detections; Fe XXV is less common, but both likely arise in the
well-known disk wind. We explore how the properties of this wind depends on
broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio,
and fractional RMS variability. The trends with count rate and RMS are
consistent with an average wind column density that is fairly steady between
observations but varies rapidly with the source on timescales of seconds. The
line dependence on spectral hardness echoes known behavior of disk winds in
outbursts of Galactic black holes; these results clearly indicate that NICER is
a powerful tool for studying black hole winds.Comment: Accepted for publication in ApJL. Comments welcom
Long-term trends in radiation damage of Chandra X-ray CCDs
Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the
focal plane instruments on the Chandra X-ray Observatory, suffered radiation
damage from exposure to soft protons during passages through the Earth's
radiation belts. Current operations require ACIS to be protected during
radiation belt passages to prevent this type of damage, but there remains a
much slower and more gradual increase. We present the history of ACIS charge
transfer inefficiency (CTI), and other measures of radiation damage, from
January 2000 through June 2005. The rate of CTI increase is low, of order 1e-6
per year, with no indication of step-function increases due to specific solar
events. Based on the time history and CCD location of the CTI increase, we
speculate on the nature of the damaging particles.Comment: 10 pages, 14 figures to appear in Proc. SPIE vol. 5898 "UV, X-ray,
and Gamma-Ray Space Instrumentation for Astronomy XIV
Performance of the Charge Injection Capability of Suzaku XIS
A charge injection technique is applied to the X-ray CCD camera, XIS (X-ray
Imaging Spectrometer) onboard Suzaku. The charge transfer inefficiency (CTI) in
each CCD column (vertical transfer channel) is measured by the injection of
charge packets into a transfer channel and subsequent readout. This paper
reports the performances of the charge injection capability based on the ground
experiments using a radiation damaged device, and in-orbit measurements of the
XIS. The ground experiments show that charges are stably injected with the
dispersion of 91eV in FWHM in a specific column for the charges equivalent to
the X-ray energy of 5.1keV. This dispersion width is significantly smaller than
that of the X-ray events of 113eV (FWHM) at approximately the same energy. The
amount of charge loss during transfer in a specific column, which is measured
with the charge injection capability, is consistent with that measured with the
calibration source. These results indicate that the charge injection technique
can accurately measure column-dependent charge losses rather than the
calibration sources. The column-to-column CTI correction to the calibration
source spectra significantly reduces the line widths compared to those with a
column-averaged CTI correction (from 193eV to 173eV in FWHM on an average at
the time of one year after the launch). In addition, this method significantly
reduces the low energy tail in the line profile of the calibration source
spectrum.Comment: Paper contains 18 figures and 15 tables. Accepted for publication in
PAS