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 $\mu$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 ($T_\mathrm{d}\sim600 - 800$ K), IR luminosity ($L_\mathrm{IR}\sim3\times10^4$ $\mathrm{L}_\odot$), mass ($M_\mathrm{d}\sim1-3\times10^{-6}$ $\mathrm{M}_\odot$), and equilibrium temperature radius ($R_\mathrm{eq}\sim10-20$ 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] ($\lambda=1.644$ $\mu$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 ($M_{shell}\sim 7.3\times10^{-3}~M_{\odot}$) which is thought to have been produced during a thermal pulse event $\sim2200$ 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 $\mu$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 $n \propto r^{\alpha}$ where $\alpha=0.75^{+0.45}_{-0.25}$ and dust mass of $M_d=9.0^{+2.3}_{-4.1}\times10^{-6}~M_{\odot}$. The strong departure from an $r^{-2}$ 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 $M_\odot$ 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 ($n_e$ ~ 100 $\mathrm{cm}^{-3}$) 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