3,130 research outputs found
A 3D finite-element modelling investigation into optimal survey parameters and direct imaging for marine controlled-source electromagnetic surveys
Relatively little is known about marine controlled-source electromagnetic
surveys (MCSEM) used to detect hydrocarbon reservoirs. Typical MCSEM require the
use of inversion to generate a model of the subsurface. We utilize a 3D finite-element
forward model to simulate a MCSEM survey. With the results we were able to
determine the strengths and weaknesses of each transmitter and receiver configuration
that would best detect a shallow hydrocarbon target. Careful selection of the correct
configuration is important as we have found that incorrect transmitter orientation, offset
and receiver measurement component can yield misleading results. Using the ideal
configuration we were able to directly image the hydrocarbon target without the use of
inversion modeling. The direct image is able to show the hydrocarbon target's shape
and edges without any ambiguity. The direct image of the target can potentially be used
to refine 3D inversion modeling, or be used in conjunction with seismic profiles to refine
seismic picks
Infrared emission from kilonovae: the case of the nearby short hard burst GRB 160821B
We present constraints on Ks-band emission from one of the nearest short hard
gamma-ray bursts, GRB 160821B, at z=0.16, at three epochs. We detect a reddened
relativistic afterglow from the jetted emission in the first epoch but do not
detect any excess kilonova emission in the second two epochs. We compare upper
limits obtained with Keck I/MOSFIRE to multi-dimensional radiative transfer
models of kilonovae, that employ composition-dependent nuclear heating and LTE
opacities of heavy elements. We discuss eight models that combine toroidal
dynamical ejecta and two types of wind and one model with dynamical ejecta
only. We also discuss simple, empirical scaling laws of predicted emission as a
function of ejecta mass and ejecta velocity. Our limits for GRB 160821B
constrain the ejecta mass to be lower than 0.03 Msun for velocities greater
than 0.1c. At the distance sensitivity range of advanced LIGO, similar
ground-based observations would be sufficiently sensitive to the full range of
predicted model emission including models with only dynamical ejecta. The color
evolution of these models shows that I-K color spans 7--16 mag, which suggests
that even relatively shallow infrared searches for kilonovae could be as
constraining as optical searches.Comment: Accepted for Publication in Astrophysical Journal Letter
From SN 2010da to NGC 300 ULX-1: Ten Years of Observations of an Unusual High Mass X-Ray Binary in NGC 300
In May 2010, an intermediate luminosity optical transient was discovered in the nearby galaxy NGC 300 by a South African amateur astronomer. In the decade since its discovery, multi-wavelength observations of the misnamed “SN 2010da” have continually reshaped our understanding of this high mass X-ray binary system. In this review, we present an overview of the multi-wavelength observations and attempt to understand the 2010 transient event, and later, the reclassification of this system as NGC 300 ULX-1: a red supergiant + neutron star ultraluminous X-ray source
First Detection of Mid-Infrared Variability from an Ultraluminous X-Ray Source Holmberg II X-1
We present mid-infrared (IR) light curves of the Ultraluminous X-ray Source
(ULX) Holmberg II X-1 from observations taken between 2014 January 13 and 2017
January 5 with the \textit{Spitzer Space Telescope} at 3.6 and 4.5 m in
the \textit{Spitzer} Infrared Intensive Transients Survey (SPIRITS). The mid-IR
light curves, which reveal the first detection of mid-IR variability from a
ULX, is determined to arise primarily from dust emission rather than from a jet
or an accretion disk outflow. We derived the evolution of the dust temperature
( K), IR luminosity (
), mass (
), and equilibrium temperature radius
( AU). A comparison of X-1 with a sample
spectroscopically identified massive stars in the Large Magellanic Cloud on a
mid-IR color-magnitude diagram suggests that the mass donor in X-1 is a
supergiant (sg) B[e]-star. The sgB[e]-interpretation is consistent with the
derived dust properties and the presence of the [Fe II] (
m) emission line revealed from previous near-IR studies of X-1. We
attribute the mid-IR variability of X-1 to increased heating of dust located in
a circumbinary torus. It is unclear what physical processes are responsible for
the increased dust heating; however, it does not appear to be associated with
the X-ray flux from the ULX given the constant X-ray luminosities provided by
serendipitous, near-contemporaneous X-ray observations around the first mid-IR
variability event in 2014. Our results highlight the importance of mid-IR
observations of luminous X-ray sources traditionally studied at X-ray and radio
wavelengths.Comment: 9 page, 4 figures, 1 table, Accepted to ApJ Letter
Nature Versus Nurture: Luminous Blue Variable Nebulae in and near Massive Stellar Clusters at the Galactic Center
Three Luminous Blue Variables (LBVs) are located in and near the Quintuplet
Cluster at the Galactic Center: the Pistol star, G0.120-0.048, and qF362. We
present imaging at 19, 25, 31, and 37 {\mu}m of the region containing these
three LBVs, obtained with SOFIA using FORCAST. We argue that the Pistol and
G0.120-0.048 are identical ``twins" that exhibit contrasting nebulae due to the
external influence of their different environments. Our images reveal the
asymmetric, compressed shell of hot dust surrounding the Pistol Star and
provide the first detection of the thermal emission from the symmetric, hot
dust envelope surrounding G0.120-0.048. Dust and gas composing the Pistol
nebula are primarily heated and ionized by the nearby Quintuplet Cluster stars.
The northern region of the Pistol nebula is decelerated due to the interaction
with the high-velocity (2000 km/s) winds from adjacent Wolf-Rayet Carbon (WC)
stars. With the DustEM code we determine that the Pistol nebula is composed of
a distribution of very small, transiently-heated grains (10-~35 {\AA}) and that
it exhibits a gradient of decreasing grain size from the south to the north due
to differential sputtering by the winds from the WC stars. Dust in the
G0.120-0.048 nebula is primarily heated by the central star; however, the
nebular gas is ionized externally by the Arches Cluster. Unlike the Pistol
nebula, the G0.120-0.048 nebula is freely expanding into the surrounding
medium. Given independent dust and gas mass estimates we find that the Pistol
and G0.120-0.048 nebulae exhibit similar gas-to-dust mass ratios of ~310 and
~290, respectively. Both nebulae share identical size scales (~ 0.7 pc) which
suggests that they have similar dynamical timescales of ~10^5 yrs, assuming a
shell expansion velocity of v_exp 60 km/s.Comment: 18 pages, 7 figures, accepted to Ap
An Infrared Study of the Circumstellar Material Associated with the Carbon Star R Sculptoris
The asymptotic giant branch (AGB) star R Sculptoris (R Scl) is one of the
most extensively studied stars on the AGB. R Scl is a carbon star with a
massive circumstellar shell () which
is thought to have been produced during a thermal pulse event years
ago. To study the thermal dust emission associated with its circumstellar
material, observations were taken with the Faint Object InfraRed CAMera for the
SOFIA Telescope (FORCAST) at 19.7, 25.2, 31.5, 34.8, and 37.1 m. Maps of
the infrared emission at these wavelengths were used to study the morphology
and temperature structure of the spatially extended dust emission. Using the
radiative transfer code DUSTY and fitting the spatial profile of the emission,
we find that a geometrically thin dust shell cannot reproduce the observed
spatially resolved emission. Instead, a second dust component in addition to
the shell is needed to reproduce the observed emission. This component, which
lies interior to the dust shell, traces the circumstellar envelope of R Scl. It
is best fit by a density profile with where
and dust mass of
. The strong departure from an
law indicates that the mass-loss rate of R Scl has not been constant.
This result is consistent with a slow decline in the post-pulse mass-loss which
has been inferred from observations of the molecular gas.Comment: 10 pages, 10 figures, accepted to Ap
Old supernova dust factory revealed at the Galactic center
Dust formation in supernova ejecta is currently the leading candidate to
explain the large quantities of dust observed in the distant, early Universe.
However, it is unclear whether the ejecta-formed dust can survive the hot
interior of the supernova remnant (SNR). We present infrared observations of
~0.02 of warm (~100 K) dust seen near the center of the ~10,000
yr-old Sgr A East SNR at the Galactic center. Our findings signify the
detection of dust within an older SNR that is expanding into a relatively dense
surrounding medium ( ~ 100 ) and has survived the
passage of the reverse shock. The results suggest that supernovae may indeed be
the dominant dust production mechanism in the dense environment of early
Universe galaxies.Comment: 25 pages, 5 figures. Includes supplementary materials. Published
Online March 19 2015 on Science Expres
A 3D finite-element modelling investigation into optimal survey parameters and direct imaging for marine controlled-source electromagnetic surveys
Relatively little is known about marine controlled-source electromagnetic
surveys (MCSEM) used to detect hydrocarbon reservoirs. Typical MCSEM require the
use of inversion to generate a model of the subsurface. We utilize a 3D finite-element
forward model to simulate a MCSEM survey. With the results we were able to
determine the strengths and weaknesses of each transmitter and receiver configuration
that would best detect a shallow hydrocarbon target. Careful selection of the correct
configuration is important as we have found that incorrect transmitter orientation, offset
and receiver measurement component can yield misleading results. Using the ideal
configuration we were able to directly image the hydrocarbon target without the use of
inversion modeling. The direct image is able to show the hydrocarbon target's shape
and edges without any ambiguity. The direct image of the target can potentially be used
to refine 3D inversion modeling, or be used in conjunction with seismic profiles to refine
seismic picks
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