arXiv subjects

R. M. Williams

Publications and source records attributed to R. M. Williams.

12 recordsLinked to original sources

arXiv2016

Development of a low-level Ar-37 calibration standard

Argon-37 is an environmental signature of an underground nuclear explosion. Producing and quantifying low-level Ar-37 standards is an important step in the development of sensitive field measurement instruments. This paper describes progress at Pacific Northwest National Laboratory in developing a process to generate and quantify low-level Ar-37 standards, which can be used to calibrate sensitive field systems at activities consistent with soil background levels. This paper presents a discussion of the measurement analysis, along with assumptions and uncertainty estimates.

physics.ins-det↗

arXiv2014

Physical Nature of the [S II]-Bright Shell Nebulae N70 and N185

N70 and N185 are two large ($\ge$100 pc in diameter) shell nebulae in the Large Magellanic Cloud (LMC). Their high [\ion{S}{2}]/H$α$ ratios rival those of supernova remnants (SNRs), but they are not confirmed as SNRs. To study their physical nature, we have obtained \emph{XMM-Newton} X-ray observations and high-dispersion long-slit echelle spectroscopic observations of these two nebulae. The X-ray spectra of both nebulae can be well interpreted with an optically thin thermal ($\sim$0.2 keV) plasma with the average LMC abundance in a collisional ionization equilibrium. N70 encompasses the OB association LH114. Although N70 has a modest expansion velocity and essentially thermal radio emission, its diffuse X-ray luminosity ($\sim6.1\times10^{35}$ erg s$^{-1}$) is higher than that from a quiescent superbubble with N70's density, size, and expansion velocity; thus, N70 is most likely a superbubble that is recently energized by an interior SNR. N185 does not contain any known OB association, and its X-ray luminosity is an order of magnitude lower than expected if it is a quiescent superbubble. N185 has nonthermal radio emission and has high-velocity material expanding at nearly 200 km s$^{-1}$, similar to many known SNRs in the LMC. Its X-ray luminosity ($\sim1.9\times10^{35}$ erg s$^{-1}$) is also consistent with that of an evolved SNR. We therefore suggest that N185 is energized by a recent supernova.

astro-ph.HE↗

arXiv2012

DEM L241, a Supernova Remnant containing a High-Mass X-ray Binary

A Chandra observation of the Large Magellanic Cloud supernova remnant DEM L241 reveals an interior unresolved source which is probably an accretion-powered binary. The optical counterpart is an O5III(f) star making this a High-Mass X-ray Binary (HMXB) with orbital period likely to be of order tens of days. Emission from the remnant interior is thermal and spectral information is used to derive density and mass of the hot material. Elongation of the remnant is unusual and possible causes of this are discussed. The precursor star probably had mass > 25 solar masses

astro-ph.HE↗

arXiv2009

Nonthermal X-ray Emission from the N11 Superbubble in the Large Magellanic Cloud

We present the results of X-ray spectral analysis of the superbubble around the OB association LH9 in the H II complex N11 in the Large Magellanic Cloud. Using observations from Suzaku, we have modeled the X-ray emission over the energy range 0.2-7.5 keV. We constrained the thermal spectrum below 2 keV using a thermal plasma model found in a previous XMM-Newton study of the N11 region. We find that the hard X-ray emission (> 2 keV) requires a nonthermal power-law component. The photon index of this component was found to be $Γ=1.72\pm 0.15$. We performed an energy budget analysis for N11 using the known stellar content of LH9. We found that the observed thermal and kinetic energy in the superbubble is only half of the expected mechanical energy injected by stars.

astro-ph.HE↗

arXiv2006

Supernova Remnants in the Magellanic Clouds. VII. Infrared Emission from SNRs

We have used the instruments on the Spitzer Space Telescope to study the Large Magellanic Cloud supernova remnants (SNRs) N11L, N44, N49, N206, N63A, and N157B. The two large SNRs N44 and N206 were not detected in any IRAC or MIPS wavebands; the remainder were detected at one or more wavelengths. In particular, the SNRs N49 and N63A each had features that were evident in all available IRAC and MIPS bands. Each of these two also displayed faint limb emission in the MIPS 24 micron band only. IRS spectra obtained for the N49 SNR showed a number of prominent lines, with little continuum contribution. We therefore suggest that N49, and possibly N63A, are dominated by line emission, with thermal emission from hot dust being at most a secondary component.

astro-ph↗

arXiv2005

Chandra Observation of the Magellanic Cloud Supernova Remnant 0454-672 in N9

A Chandra observation has defined the extent of the SNR B 0454-692 in the LMC H II region N9. The remnant has dimension 2.3" x 3.6" and is elongated in the NS direction. The brightest emission comes from a NS central ridge which includes three bright patches. There is good agreement between X-ray and [O III] and [S II] morphology. The remnant is old enough so that optical data give more information about dynamics than do the X-ray data. The SN energy release was >= 4 x 10^50 ergs and the age is ~3 x 10^4 years. There are several unresolved sources nearby but none are clearly associated with the remnant. The X-ray spectrum is soft and indicates enhanced Fe abundance in the central region, consistent with a Type Ia SN origin, but a Type II origin cannot be ruled out.

astro-ph↗

arXiv2005

Protostars, Dust Globules, and a Herbig-Haro Object in the LMC Superbubble N51D

Using Spitzer Space Telescope and Hubble Space Telescope observations of the superbubble N51D, we have identified three young stellar objects (YSOs) in dust globules, and made the first detection of a Herbig-Haro object outside the Galaxy. The spectral energy distributions of these YSOs suggest young massive stars with disk, envelope, and outflow cavities. The interstellar conditions are used to assess whether the star formation was spontaneous or induced by external pressure.

astro-ph↗

arXiv2005

Supernova Remnants in the Magellanic Clouds. VI. The DEML316 Supernova Remnants

The DEML316 system contains two shells, both with the characteristic signatures of supernova remnants (SNRs). We analyze Chandra and XMM-Newton data for DEML316, investigating its spatial and spectral X-ray features. Our Chandra observations resolve the structure of the northeastern SNR (Shell A) as a bright inner ring and a set of "arcs" surrounded by fainter diffuse emission. The spectrum is well fit by a thermal plasma model with temperature ~1.4 keV; we do not find significant spectral differences for different regions of this SNR. The southwestern SNR (Shell B) exhibits an irregular X-ray outline, with a brighter interior ring of emission including a bright knot of emission. Overall the emission of the SNR is well described by a thermal plasma of temperature ~0.6 keV. The Bright Knot, however, is spectrally distinct from the rest of the SNR, requiring the addition of a high-energy spectral component consistent with a power-law spectrum of photon index 1.6--1.8. We confirm the findings of Nishiuchi et al. (2001) that the spectra of these shells are notably different, with Shell A requiring a high iron abundance for a good spectral fit, implying a Type Ia origin. We further explicitly compare abundance ratios to model predictions for Type Ia and Type II supernovae. The low ratios for Shell A (O/Fe of 1.5 and Ne/Fe of 0.2) and the high ratios for Shell B (O/Fe of 30--130 and Ne/Fe of 8--16) are consistent with Type Ia and Type II origins, respectively. The difference between the SNR progenitor types casts some doubt on the suggestion that these SNRs are interacting with one another.

astro-ph↗

arXiv2005

Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 SNR

The N206 supernova remnant (SNR) in the Large Magellanic Cloud (LMC) has long been considered a prototypical "mixed morphology" SNR. Recent observations, however, have added a new twist to this familiar plot: an elongated, radially-oriented radio feature seen in projection against the SNR face. Utilizing the high resolution and sensitivity available with the Hubble Space Telescope, Chandra, and XMM-Newton, we have obtained optical emission-line images and spatially resolved X-ray spectral maps for this intriguing SNR. Our findings present the SNR itself as a remnant in the mid to late stages of its evolution. X-ray emission associated with the radio "linear feature" strongly suggests it to be a pulsar-wind nebula (PWN). A small X-ray knot is discovered at the outer tip of this feature. The feature's elongated morphology and the surrounding wedge-shaped X-ray enhancement strongly suggest a bow-shock PWN structure.

astro-ph↗

arXiv2004

Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC

We present the first X-ray detection of SNR 0450-70.9 the largest known supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical images and high-dispersion spectra, and radio continuum maps. Optical images of SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments along the eastern and western rims and within the shell interior. The interior filaments have higher [S II]/Halpha ratios and form an apparent inner shell morphology. The X-ray emission region is smaller than the full extent of the optical shell, with the brightest X-ray emission found within the small interior shell and on the western rim of the large shell. The expansion velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s. The radio image shows central brightening and a fairly flat radio spectral index over the SNR. However, no point X-ray or radio source corresponding to a pulsar is detected and the X-ray emission is predominantly thermal. Therefore, these phenomena can be most reasonably explained in terms of the advanced age of the large SNR. Using hydrodynamic models combined with a nonequilibrium ionization model for thermal X-ray emission, we derived a lower limit on the SNR age of about 45,000 yr, well into the later stages of SNR evolution. Despite this, the temperature and density derived from spectral fits to the X-ray emission indicate that the remnant is still overpressured, and thus that the development is largely driven by hot gas in the SNR interior.

astro-ph↗

arXiv2001

Chandra Reveals the X-ray Glint in the Cat's Eye

We have obtained Chandra ACIS-S observations of NGC 6543, the Cat's Eye Nebula. The X-ray emission from NGC 6543 is clearly resolved into a point source at the central star and diffuse emission confined within the central elliptical shell and two extensions along the major axis. Spectral analysis of the diffuse component shows that the abundances of the X-ray-emitting gas are similar to those of the fast (1,750 km/s) stellar wind but not those of the nebula. Furthermore, the temperature of this gas is ~1.7 million K, which is 100 times lower than the expected post-shock temperature of the fast stellar wind. The combination of low temperature and wind abundances is puzzling. The thermal pressure of this hot gas is about twice the pressure in the cool nebular shell; thus, the hot gas plays an essential role in the ongoing evolution of the nebula.

astro-ph↗

arXiv1999

Simplicial minisuperspace models in the presence of a scalar field

We generalize simplicial minisuperspace models associated with restricting the topology of the universe to be that of a cone over a closed connected combinatorial $3-$manifold by considering the presence of a massive scalar field. By restricting all the interior edge lengths and all the boundary edge lengths to be equivalent and the scalar field to be homogenous on the $3-$space, we obtain a family of two dimensional models that include some of the most relevant triangulations of the spatial universe. After studying the analytic properties of the action in the space of complex edge lengths we determine its classical extrema. We then obtain steepest descents contours of constant imaginary action passing through Lorentzian classical geometries yielding a convergent wavefunction of the universe, dominated by the contributions coming from these extrema. By considering these contours we justify semiclassical approximations based on those classical solutions, clearly predicting classical spacetime in the late universe. These wavefunctions are then evaluated numerically. For all of the models examined we find wavefunctions predicting Lorentzian oscillatory behaviour in the late universe.

gr-qc↗