518 research outputs found
Present-day stress orientations and tectonic provinces of the NW Borneo collisional margin
Extent: 15p.Borehole failure observed on image and dipmeter logs from 55 petroleum wells across the NW Borneo collisional margin were used to determine maximum horizontal stress (ÏH) orientations; combined with seismic and outcrop data, they define seven tectonic provinces. The Baram DeltaâDeepwater Fold-Thrust Belt exhibits three tectonic provinces: its inner shelf inverted province (ÏH is NW-SE, margin-normal), its outer shelf extension province (ÏH is NE-SW, margin-parallel), and its slope to basin floor compression province (ÏH is NW-SE, margin-normal). In the inverted province, ÏH reflects inversion of deltaic normal faults. The ÏH orientations in the extension and compression provinces reflect deltaic gravitational tectonics. The shale and minibasin provinces have been recognized in offshore Sabah. In the shale province, ÏH is N010°E, which aligns around the boundary of a massif of mobile shale. Currently, no data are available to determine ÏH in the minibasin province. In the Balingian province, ÏH is ESE-WNW, reflecting ESE absolute Sunda plate motions due to the absence of a thick detachment seen elsewhere in NW Borneo. The Central Luconia province demonstrates poorly constrained and variable ÏH orientations. These seven provinces result from the heterogeneous structural and stratigraphic development of the NW Borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages.Rosalind C. King, Mark R. P. Tingay, Richard R. Hillis, Christopher K. Morley, and James Clar
On the Origin of Radio Emission in the X-ray States of XTE J1650-500 during the 2001-2002 Outburst
We report on simultaneous radio and X-ray observations of the black hole
candidate XTE J1650-500 during the course of its 2001-2002 outburst. The
scheduling of the observations allowed us to sample the properties of XTE
J1650-50 in different X-ray spectral states, namely the hard state, the steep
power-law state and the thermal dominant state, according to the recent
spectral classification of McClintock & Remillard. The hard state is consistent
with a compact jet dominating the spectral energy distribution at radio
frequencies; however, the current data suggest that its contribution as direct
synchrotron emission at higher energies may not be significant. In that case,
XTE J1650-50 may be dominated by Compton processes (either inverse
Comptonization of thermal disk photons and/or SSC from the base of the compact
jet) in the X-ray regime. We, surprisingly, detect a faint level of radio
emission in the thermal dominant state that may be consistent with the emission
of previously ejected material interacting with the interstellar medium,
similar (but on a smaller angular scale) to what was observed in XTE J1550-564
by Corbel and co-workers. Based on the properties of radio emission in the
steep power-law state of XTE J1650-50, and taking into account the behavior of
other black hole candidates (namely GX 339-4, XTE J1550-564, and XTE J1859+226)
while in the intermediate and steep power-law states, we are able to present a
general pattern of behavior for the origin of radio emission in these two
states that could be important for understanding the accretion-ejection
coupling very close to the black hole event horizon.Comment: Accepted for publication in The Astrophysical Journal. 33 pages, 10
figure
Chandra Discovery of a 100 kpc X-ray Jet in PKS 0637--752
The quasar PKS 0637-753, the first celestial X-ray target of the Chandra
X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to
12 arcsec west of the quasar, coincident with the inner portion of the jet
previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a
redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous
(~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents
a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753
was on-axis and ACIS-S was near the optimum focus. For the inner portion of the
radio jet, the X-ray morphology closely matches that of new ATCA radio images
at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space
VLBI mission show structure aligned with the X-ray jet, placing important
constraints on the X-ray source models. HST images show that there are three
small knots coincident with the peak radio and X-ray emission. Two of these are
resolved, which we use to estimate the sizes of the X-ray and radio knots. The
outer portion of the radio jet, and a radio component to the east, show no
X-ray emission to a limit of about 100 times lower flux.
The X-ray emission is difficult to explain with models that successfully
account for extra-nuclear X-ray/radio structures in other active galaxies. We
think the most plausible is a synchrotron self-Compton (SSC) model, but this
would imply extreme departures from the conventional minimum-energy and/or
homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung
models for the jet X-rays, unless multicomponent or ad hoc geometries are
invoked.Comment: 5 Pages, 2 Figures. Submitted to Ap. J. Letter
Scientific basis for safely shutting in the Macondo Well after the April 20, 2010 Deepwater Horizon blowout
As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic pro- files, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement
The Chandra X-ray Observatory Resolves the X-ray Morphology and Spectra of a Jet in PKS 0637-752
The core-dominated radio-loud quasar PKS 0637-752 (z = 0.654) was the first
celestial object observed with the Chandra X-ray Observatory, offering the
early surprise of the detection of a remarkable X-ray jet. Several observations
with a variety of detector configurations contribute to a total exposure time
with the Chandra Advanced CCD Imaging Spectrometer (ACIS; Garmire et al. 2000,
in preparation) of about 100ks. A spatial analysis of all the available X-ray
data, making use of Chandra's spatial resolving power of about 0.4 arcsec,
reveals a jet that extends about 10 arcsec to the west of the nucleus. At least
four X-ray knots are resolved along the jet, which contains about 5% of the
overall X-ray luminosity of the source. Previous observations of PKS 0637-752
in the radio band (Tingay et al. 1998) had identified a kpc-scale radio jet
extending to the West of the quasar. The X-ray and radio jets are similar in
shape, intensity distribution, and angular structure out to about 9 arcsec,
after which the X-ray brightness decreases more rapidly and the radio jet turns
abruptly to the north. The X-ray luminosity of the total source is log Lx ~
45.8 erg/s (2 - 10keV), and appears not to have changed since it was observed
with ASCA in November 1996. We present the results of fitting a variety of
emission models to the observed spectral distribution, comment on the
non-existence of emission lines recently reported in the ASCA observations of
PKS 0637-752, and briefly discuss plausible X-ray emission mechanisms.Comment: 24 pages, includes 8 figures, Accepted for publication in Ap
Bright Giant Pulses from the Crab Nebula Pulsar: Statistical Properties, Pulse Broadening and Scattering due to the Nebula
We report observations of Crab giant pulses made with the Australia Telescope
Compact Array and a baseband recorder system, made simultaneously at two
frequencies, 1300 and 1470 MHz. These observations were sensitive to pulses
with amplitudes \ga 3 kJy and widths \ga 0.5 s. Our analysis led to the
detection of more than 700 such bright giant pulses over 3 hours, and using
this large sample we investigate their amplitude, width, arrival time and
energy distributions. The brightest pulse detected in our data has a peak
amplitude of 45 kJy and a width of 0.5 s, and therefore an
inferred brightness temperature of K. The duration of
giant-pulse emission is typically 1 s, however it can also be as
long as 10 s. The pulse shape at a high time resolution (128 ns) shows
rich diversity and complexity in structure and is marked by an unusually low
degree of scattering. We discuss possible implications for scattering due to
the nebula, and for underlying structures and electron densities.Comment: 8 pages, 8 figures, Accepted for publication in Ap
Testing potential new sites for optical telescopes in Australia
In coming years, Australia may find the need to build new optical telescopes to continue local programmes, contribute to global survey projects, and form a local multi-wavelength connection for the new radio telescopes being built. In this study, we refine possible locations for a new optical telescope by studying remotely sensed meteorological infrared data to ascertain expected cloud coverage rates across Australia, and combine these data with a digital elevation model using a geographic information system. We find that the best sites within Australia for building optical telescopes are likely to be on the highest mountains in the Hamersley Range in northwest Western Australia, while the MacDonnell Ranges in the Northern Territory may also be appropriate. We believe that similar seeing values to Siding Spring should be obtainable and with significantly more observing time at the identified sites. We expect to find twice as many clear nights as at current telescope sites. These sites are thus prime locations for future on-site testing
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