654 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
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The first resolved imaging of milliarcsecond-scale jets in Circinus X-1
We present the first resolved imaging of the milliarcsecond-scale jets in the
neutron star X-ray binary Circinus X-1, made using the Australian Long Baseline
Array. The angular extent of the resolved jets is ~20 milliarcseconds,
corresponding to a physical scale of ~150 au at the assumed distance of 7.8
kpc. The jet position angle is relatively consistent with previous
arcsecond-scale imaging with the Australia Telescope Compact Array. The radio
emission is symmetric about the peak, and is unresolved along the minor axis,
constraining the opening angle to be less than 20 degrees. We observe evidence
for outward motion of the components between the two halves of the observation.
Constraints on the proper motion of the radio-emitting components suggest that
they are only mildly relativistic, although we cannot definitively rule out the
presence of the unseen, ultra-relativistic (Lorentz factor >15) flow previously
inferred to exist in this system.Comment: Accepted for publication in MNRAS Letters. 6 pages, 4 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
Disentangling the circumnuclear environs of Centaurus A: II. On the nature of the broad absorption line
We report on atomic gas (HI) and molecular gas (as traced by CO(2-1))
redshifted absorption features toward the nuclear regions of the closest
powerful radio galaxy, Centaurus A (NGC 5128). Our HI observations using the
Very Long Baseline Array allow us to discern with unprecedented sub-parsec
resolution HI absorption profiles toward different positions along the 21 cm
continuum jet emission in the inner 0."3 (or 5.4 pc). In addition, our CO(2-1)
data obtained with the Submillimeter Array probe the bulk of the absorbing
molecular gas with little contamination by emission, not possible with previous
CO single-dish observations. We shed light with these data on the physical
properties of the gas in the line of sight, emphasizing the still open debate
about the nature of the gas that produces the broad absorption line (~55 km/s).
First, the broad H I line is more prominent toward the central and brightest 21
cm continuum component than toward a region along the jet at a distance ~ 20
mas (or 0.4 pc) further from it. This suggests that the broad absorption line
arises from gas located close to the nucleus, rather than from diffuse and more
distant gas. Second, the different velocity components detected in the CO(2-1)
absorption spectrum match well other molecular lines, such as those of
HCO+(1-0), except the broad absorption line that is detected in HCO+(1-0) (and
most likely related to that of the H I). Dissociation of molecular hydrogen due
to the AGN seems to be efficient at distances <= 10 pc, which might contribute
to the depth of the broad H I and molecular lines.Comment: 17 pages, 9 figures, accepted for publication in Ap
Internal gamma gamma-opacity in Active Galactic Nuclei and the consequences for the TeV observations of M87 and Cen A
Low Luminosity Active Galactic Nuclei (LLAGNs) possess the characteristic
features of more luminous Active Galactic Nuclei (AGNs) but exhibit a much
lower nuclear Halpha luminosity than their more luminous counterparts. M87 (NGC
4486) and Centaurus A (NGC 5128, CenA) are well-studied nearby LLAGNs. As an
additional feature they show gamma-radiation up to TeV (10^{12}eV) energies,
but the origin of this radiation is not resolved. The coincident observation of
a radio and TeV flare in M87 suggests that the TeV radiation is produced within
around 50-100 gravitational radii of the central supermassive black hole,
depending on the assumed value of the mass of the black hole. Strong radiation
fields can be produced in the central region of an (LL)AGN, e.g., by the
accretion flow around the black hole, the jet plasma, or stars closely orbiting
the black hole. These radiation fields can lead to the absorption of emitted
TeV photons, and in fact high optical depths of such fields can make TeV
detection from inner regions impossible. In this paper we consider the
accretion flow around the black hole as the most prominent source for such a
radiation field and we accordingly calculate the probability for absorption of
TeV photons produced near the black holes in M87 and CenA assuming a low
luminosity Shakura-Sunyaev Disk (SSD). We find that the results are very
different for between the two LLAGNs. While the inner region of M87 is
transparent for TeV radiation up to 15TeV, the optical depth in CenA is >> 1,
leading to an absorption of TeV photons that might be produced near the central
black hole. These results imply either that the TeV gamma production sites and
processes are different for both sources, or that LLAGN black holes do not
accrete (at least only) in form of a low luminosity SSD.Comment: 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
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