142 research outputs found
Simulating CCDs for the Chandra Advanced CCD Imaging Spectrometer
We have implemented a Monte Carlo algorithm to model and predict the response
of various kinds of CCDs to X-ray photons and minimally-ionizing particles and
have applied this model to the CCDs in the Chandra X-ray Observatory's Advanced
CCD Imaging Spectrometer. This algorithm draws on empirical results and
predicts the response of all basic types of X-ray CCD devices. It relies on new
solutions of the diffusion equation, including recombination, to predict the
radial charge cloud distribution in field-free regions of CCDs. By adjusting
the size of the charge clouds, we can reproduce the event grade distribution
seen in calibration data. Using a model of the channel stops developed here and
an insightful treatment of the insulating layer under the gate structure
developed at MIT, we are able to reproduce all notable features in ACIS
calibration spectra.
The simulator is used to reproduce ground and flight calibration data from
ACIS, thus confirming its fidelity. It can then be used for a variety of
calibration tasks, such as generating spectral response matrices for spectral
fitting of astrophysical sources, quantum efficiency estimation, and modeling
of photon pile-up.Comment: 42 pages, 22 figures; accepted for publication in Nuclear Instruments
and Methods in Physics Research, Section A; paper with high-quality figures
can be found at ftp://ftp.astro.psu.edu/pub/townsley/simulator.p
Nonlinear Dynamics of Aeolian Sand Ripples
We study the initial instability of flat sand surface and further nonlinear
dynamics of wind ripples. The proposed continuous model of ripple formation
allowed us to simulate the development of a typical asymmetric ripple shape and
the evolution of sand ripple pattern. We suggest that this evolution occurs via
ripple merger preceded by several soliton-like interaction of ripples.Comment: 6 pages, 3 figures, corrected 2 typo
A QUASI-VARIATIONAL INEQUALITY PROBLEM IN SUPERCONDUCTIVITY
Accepted versio
Quasivariational solutions for first order quasilinear equations with gradient constraint
We prove the existence of solutions for an evolution quasi-variational
inequality with a first order quasilinear operator and a variable convex set,
which is characterized by a constraint on the absolute value of the gradient
that depends on the solution itself. The only required assumption on the
nonlinearity of this constraint is its continuity and positivity. The method
relies on an appropriate parabolic regularization and suitable {\em a priori}
estimates. We obtain also the existence of stationary solutions, by studying
the asymptotic behaviour in time. In the variational case, corresponding to a
constraint independent of the solution, we also give uniqueness results
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 nonhomogeneous boundary value problem in mass transfer theory
We prove a uniqueness result of solutions for a system of PDEs of
Monge-Kantorovich type arising in problems of mass transfer theory. The results
are obtained under very mild regularity assumptions both on the reference set
, and on the (possibly asymmetric) norm defined in
. In the special case when is endowed with the Euclidean
metric, our results provide a complete description of the stationary solutions
to the tray table problem in granular matter theory.Comment: 22 pages, 2 figure
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
X-ray speed reading: enabling fast, low noise readout for next-generation CCDs
Current, state-of-the-art CCDs are close to being able to deliver all key
performance figures for future strategic X-ray missions except for the required
frame rates. Our Stanford group is seeking to close this technology gap through
a multi-pronged approach of microelectronics, signal processing and novel
detector devices, developed in collaboration with the Massachusetts Institute
of Technology (MIT) and MIT Lincoln Laboratory (MIT-LL). Here we report results
from our (integrated) readout electronics development, digital signal
processing and novel SiSeRO (Single electron Sensitive Read Out) device
characterization.Comment: To appear in SPIE Proceeding of Astronomical Telescopes +
Instrumentation, 202
- …