588 research outputs found
Removal of spacecraft-surface particulate contaminants by simulated micrometeoroid impacts
A series of hypervelocity impacts has been conducted in an exploding lithium-wire accelerator to examine with a far-field holographic system the removal of particulate contaminants from external spacecraft surfaces subjected to micrometeoroid bombardment. The impacting projectiles used to simulate the micrometeoroids were glass spheres nominally 37 microns in diameter, having velocities between 4 and 17 km/sec. The particulates were glass spheres nominally 25, 50, and 75 microns in diameter which were placed on aluminum targets. For these test, particulates detached had velocities that were log-normally distributed. The significance of the log-normal behavior of the ejected-particulate velocity distribution is that the geometric mean velocity and the geometric standard deviation are the only two parameters needed to model completely the process of particles removed or ejected from a spacecraft surface by a micrometeoroid impact
Development of a high-altitude airborne dial system: The Lidar Atmospheric Sensing Experiment (LASE)
The ability of a Differential Absorption Lidar (DIAL) system to measure vertical profiles of H2O in the lower atmosphere was demonstrated both in ground-based and airborne experiments. In these experiments, tunable lasers were used that required real-time experimenter control to locate and lock onto the atmospheric H2O absorption line for the DIAL measurements. The Lidar Atmospheric Sensing Experiment (LASE) is the first step in a long-range effort to develop and demonstrate an autonomous DIAL system for airborne and spaceborne flight experiments. The LASE instrument is being developed to measure H2O, aerosol, and cloud profiles from a high-altitude ER-2 (extended range U-2) aircraft. The science of the LASE program, the LASE system design, and the expected measurement capability of the system are discussed
Hubble Space Telescope observations of the NUV transit of WASP-12b
We present new observations of four closely-spaced NUV transits of the hot
Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the
phase resolution of the observed NUV light curve relative to previous
observations, while minimising the temporal variation of the system. We observe
significant excess NUV absorption during the transit, with mean normalised
in-transit fluxes of , i.e. 2-5
deeper than the optical transit level of for a uniform stellar
disk (the exact confidence level depending on the normalisation method used).
We further observe an asymmetric transit shape, such that the post-conjunction
fluxes are overall 2-3 higher than pre-conjunction values, and
characterised by rapid variations in count rate between the pre-conjunction and
out of transit levels. We do not find evidence for an early ingress to the NUV
transit as suggested by earlier HST observations. However, we show that the NUV
count rate observed prior to the optical transit is highly variable, but
overall 2.2-3.0 below the post-transit values and comparable
in depth to the optical transit, possibly forming a variable region of NUV
absorption from at least phase 0.83, limited by the data coverage.Comment: Accepted into the Astrophysical Journa
Effects of trichlorobenzene on natural phytoplankton populations
Natural phytoplankton assemblages from an offshore station in Lake Michigan were exposed to individual isomers of trichlorobenzene (TCB) and incubated in situ for a 24 h period. One set of exposures was initiated with a lake assemblage collected at 0330 h from 30 m and the TCB isomers added at 0400 h. The second exposure experiment was initiated with an assemblage from 30 m collected at 1530 h and the TCB isomers added at 1600 h.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44445/1/10646_2004_Article_BF00368534.pd
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization
calculations of broad emission line clouds in quasars, spanning seven orders of
magnitude in hydrogen ionizing flux and particle density. The equivalent widths
of 42 quasar emission lines are presented as contours in the particle density -
ionizing flux plane for a typical incident continuum shape, solar chemical
abundances, and cloud column density of . Results are
similarly given for a small subset of emission lines for two other column
densities ( and ), five other incident
continuum shapes, and a gas metallicity of 5 \Zsun. These graphs should prove
useful in the analysis of quasar emission line data and in the detailed
modeling of quasar broad emission line regions. The digital results of these
emission line grids and many more are available over the Internet.Comment: 16 pages, LaTeX (AASTeX aaspp4.sty); to appear in the 1997 ApJS: full
contents of the 9 photoionization grids presented in this paper may be found
at http://www.pa.uky.edu/~korista/grids/grids.htm
Early multi-wavelength emission from Gamma-ray Bursts: from Gamma-ray to X-ray
The study of the early high-energy emission from both long and short
Gamma-ray bursts has been revolutionized by the Swift mission. The rapid
response of Swift shows that the non-thermal X-ray emission transitions
smoothly from the prompt phase into a decaying phase whatever the details of
the light curve. The decay is often categorized by a steep-to-shallow
transition suggesting that the prompt emission and the afterglow are two
distinct emission components. In those GRBs with an initially steeply-decaying
X-ray light curve we are probably seeing off-axis emission due to termination
of intense central engine activity. This phase is usually followed, within the
first hour, by a shallow decay, giving the appearance of a late emission hump.
The late emission hump can last for up to a day, and hence, although faint, is
energetically very significant. The energy emitted during the late emission
hump is very likely due to the forward shock being constantly refreshed by
either late central engine activity or less relativistic material emitted
during the prompt phase. In other GRBs the early X-ray emission decays
gradually following the prompt emission with no evidence for early temporal
breaks, and in these bursts the emission may be dominated by classical
afterglow emission from the external shock as the relativistic jet is slowed by
interaction with the surrounding circum-burst medium. At least half of the GRBs
observed by Swift also show erratic X-ray flaring behaviour, usually within the
first few hours. The properties of the X-ray flares suggest that they are due
to central engine activity. Overall, the observed wide variety of early
high-energy phenomena pose a major challenge to GRB models.Comment: Accepted for publication in the New Journal of Physics focus issue on
Gamma Ray Burst
The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.: Data and preliminary Modeling
The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst
of the recurrent nova RS Oph was observed twice with Chandra and once with
XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after
outburst. We confirm a 35-sec period on day 54.0 and found that it originates
from the SSS emission and not from the shock. We discus the bound-free
absorption by neutral elements in the line of sight, resonance absorption lines
plus self-absorbed emission line components, collisionally excited emission
lines from the shock, He-like intersystem lines, and spectral changes during an
episode of high-amplitude variability. We find a decrease of the oxygen K-shell
absorption edge that can be explained by photoionization of oxygen. The
absorption component has average velocities of -1286+-267 km/s on day 39.7 and
of -771+-65 km/s on day 66.9. The wavelengths of the emission line components
are consistent with their rest wavelengths as confirmed by measurements of
non-self absorbed He-like intersystem lines. We have evidence that these lines
originate from the shock rather than the outer layers of the outflow and may be
photoexcited in addition to collisional excitations. We found collisionally
excited emission lines that are fading at wavelengths shorter than 15A that
originate from the radiatively cooling shock. On day 39.5 we find a systematic
blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i
ratios which indicates either high densities or significant UV radiation near
the plasma where the emission lines are formed. During the phase of strong
variability the spectral hardness light curve overlies the total light curve
when shifted by 1000sec. This can be explained by photoionization of neutral
oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.Comment: 16 pages, 10 figures, 4 tables. Accepted by ApJ; v2: Co-author
Woodward adde
Bridging the gap between stellar-mass black holes and ultraluminous X-ray sources
The X-ray spectral and timing properties of ultraluminous X-ray sources
(ULXs) have many similarities with the very high state of stellar-mass black
holes (power-law dominated, at accretion rates greater than the Eddington
rate). On the other hand, their cool disk components, large characteristic
inner-disk radii and low characteristic timescales have been interpreted as
evidence of black hole masses ~ 1000 Msun (intermediate-mass black holes). Here
we re-examine the physical interpretation of the cool disk model, in the
context of accretion states of stellar-mass black holes. In particular, XTE
J1550-564 can be considered the missing link between ULXs and stellar-mass
black holes, because it exhibits a high-accretion-rate, low-disk-temperature
state (ultraluminous branch). On the ultraluminous branch, the accretion rate
is positively correlated with the disk truncation radius and the bolometric
disk luminosity, while it is anti-correlated with the peak temperature and the
frequency of quasi-periodic-oscillations. Two prototypical ULXs (NGC1313 X-1
and X-2) also seem to move along that branch. We use a phenomenological model
to show how the different range of spectral and timing parameters found in the
two classes of accreting black holes depends on both their masses and accretion
rates. We suggest that ULXs are consistent with black hole masses ~ 50-100
Msun, moderately inefficiently accreting at ~20 times Eddington.Comment: 11 pages, accepted for publication in Astrophysics and Space Science.
Based on work presented at the Fifth Stromlo Symposium, Australian National
University, Dec 200
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