arXiv subjects

David P. Huenemoerder

Publications and source records attributed to David P. Huenemoerder.

At least 19 recordsLinked to original sources

arXiv2026

X-ray grating spectroscopy as a mission enhancement

We propose to add instruments to any potential future X-ray mission with focussing optics that is considered in NASA's ASTRA framework. Such an instrument is a necessity to study AGN wind outflows and feedback, find the missing baryons, study the intergalactic medium, and analyze abundances and chemical bonds in dust grains throughout the Milky Way. We conclude that those science goals can be achieved with a spectral resolving power > 3000 in the soft X-ray band (about 10-40 Ang) and an effective area a few times larger than current instruments. We describe a possible mission implementation for a soft X-ray grating spectrometer that can be folded in and out or be mounted permanently in the beam. Such an instrument can reach the requirements for a wide variety of host mission properties. A small UV imager and a UV spectrograph can be mounted on the same platform with independent optics. These added instruments vastly enhance the science capabilities of the host mission for a modest cost (100-200 million $) and with weight and power needs that can be easily accommodated in any major mission.

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arXiv2026

A Multiwavelength View of $ρ$ Oph I: Resolving the X-ray Source Between A and B

We present a multiwavelength analysis of the central stellar pair of $ρ$ Oph, components A and B. Using recent high-resolution \textit{Chandra X-ray Observatory} observations, we demonstrate with high confidence that the dominant X-ray source is $ρ$ Oph B, while $ρ$ Oph A is comparatively X-ray faint. This result contrasts with earlier \textit{XMM-Newton} observations, which, due to limited spatial resolutions, attributed the X-ray emission to $ρ$ Oph A. An analysis of $ρ$ Oph B's X-ray light curves and spectra reveals properties more consistent with a cool star than a hot star. We therefore propose that $ρ$ Oph B is an Algol-like binary system, consisting of a B-type primary and an active, X-ray-emitting GK-type companion.

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arXiv2025

Chandra/HETG and NuSTAR Observations of V750 Ara, a $γ\,$Cas-type Star

We present 197 ks HETG and 95 ks NuSTAR spectra of the $γ\,$Cas-type object V750 Ara. The high-resolution X-ray spectra show that the target is similar to other objects of this class. Data are interpreted under the assumption that the X-rays come from an accreting white dwarf, and our analysis implies an accretion rate of about $3\times10^{-11}M_\odot\mathrm{yr}^{-1}$. Emission lines are weak, and predominantly from hydrogen-like ions: Mg XII, Si XIV, and S XVI. H-like and He-like Fe are both present, but Fe K$α$ fluorescence is weak, being significantly detected only in the NuSTAR spectrum, but was not obviously detected in the HETG dispersed or zeroth-order spectra. The flux was variable above a level expected by Poisson statistics. There were no significant changes in the spectral hardness, though we are limited by lack of soft signal below 1 keV. Emission lines of Mg and Si were strong enough to measure velocity offsets and widths which were found to be marginally inconsistent. The H-like Mg line is consistent with instrumental broadening only, but shows a 300 km/s blueshift. He-like Mg and H-like Si lines have no significant shift in velocity but are broadened by about 1000 km/s. This suggests either different physical origins or velocity structure differing with plasma temperature.

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arXiv2025

X-ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae

We present the first first X-ray polarization measurements of a white dwarf, the intermediate polar EX Hya. We measured significant polarization only in the 2 -- 3 keV energy band with a polarization degree of 8 percent at a $3σ$ significance. No significant polarization was detected above 3 keV, which we attribute to the higher energy bands having lower signal-to-noise. We found that the scattering surface detected by the IXPE is nearly perpendicular to the optical scattering plane, showing that the X-ray scattering surface is the WD and close to the base of the accretion column. Finally, we show how the polarization can be used to estimate the height of the accretion shock above the white dwarf's surface.

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arXiv2024

A Time-Dependent Spectral Analysis of $γ$ Cassiopeiae

We investigated the temporal and spectral features of $γ$ Cassiopeiae's X-ray emission within the context of the white dwarf accretion hypothesis. We find that the variabilities present in the X-ray data show two different signals, one primarily due to absorption and the other due to flickering like in non-magnetic cataclysmic variables. We then use this two-component insight to investigate previously un-reported simultaneous XMM and NuSTAR data. The model fitting results find white dwarf properties consistent with optical studies alongside a significant secondary, thermal source. We propose a secondary shock between the Be decretion disk and white dwarf accretion disk as the source. Finally, we analyzed a unique, low-count rate event of the XMM light curve as potential evidence for the white dwarf encountering Be decretion disk structures.

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arXiv2024

Unfolding X-ray Spectral Data: Conditions and Applications

We present conditions for which X-ray spectra can be ``unfolded'' to present accurate representation of the true source spectra. The method we use to unfold the data is implemented in the \textit{Interactive Spectral Interpretation Software} \citep{Houck2000} and distinguishes itself as being model-independent. We find that this method of unfolding makes accurate representations of the true source spectra (1) The detector is high-resolution and (2) The spectrum is not steeply sloped. These criteria are not simple conditions that give concrete determinations; each detector and spectrum must be judged individually. We find that both grating and imaging detectors can be unfolded with minimal distortions as compared to both continuum and local spectral features; the latter CCD detectors being much more energy dependent. We also provide example use cases for unfolding in the context of current generation X-ray observatories and important caveats.

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arXiv2024

Diffraction Gratings for X-ray Spectroscopy

X-ray diffraction gratings play an essential role in high-resolution spectroscopy of astrophysical phenomena. We present some scientific highlights from the X-ray grating spectrometers (XGS) on board of the Chandra and XMM/Newton missions, XGS optical design, and the basic physics of grating diffraction geometry and efficiency. We review design, fabrication, and performance of the currently orbiting transmission and reflection grating elements, followed by descriptions of the state-of-the art of more advanced grating technologies that promise orders-of-magnitude improvements in XGS performance, especially in combination with advanced X-ray telescope mirrors. A few key science questions that require new grating technology are posed, and powerful future mission concepts and recent and approved missions are presented.

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arXiv2024

The Nature of X-Rays from Young Stellar Objects in the Orion Nebula Cluster -- A Chandra HETGS Legacy Project

The Orion Nebula Cluster (ONC) is the closest site of very young ($\sim$ 1 Myrs) massive star formation. The ONC hosts more than 1600 young and X-ray bright stars with masses ranging from $\sim$ 0.1 to 35 $M_\odot$. The Chandra HETGS Orion Legacy Project observed the ONC with the Chandra high energy transmission grating spectrometer (HETGS) for $2.1\,$Ms. We describe the spectral extraction and cleaning processes necessary to separate overlapping spectra. We obtained 36 high resolution spectra which includes a high brilliance X-ray spectrum of $θ^1$ Ori C with over 100 highly significant X-ray lines. The lines show Doppler broadening between 300 and $400\;\mathrm{km}\;\mathrm{s}^{-1}$. Higher spectral diffraction orders allow us to resolve line components of high Z He-like triplets in $θ^1$ Ori C with unprecedented spectral resolution. Long term light curves spanning $\sim$20 years show all stars to be highly variable, including the massive stars. Spectral fitting with thermal coronal emission line models reveals that most sources show column densities of up to a few times $10^{22}\,$cm$^{-2}$ and high coronal temperatures of 10 to 90 MK. We observe a bifurcation of the high temperature component where some stars show a high component of 40 MK, while others show above 60 MK indicating heavy flaring activity. Some lines are resolved with Doppler broadening above our threshold of $\sim200\;\mathrm{km}\;\mathrm{s}^{-1}$, up to $500\;\mathrm{km}\;\mathrm{s}^{-1}$. This data set represents the largest collection of HETGS high resolution X-ray spectra from young pre-MS stars in a single star-forming region to date.

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arXiv2024

Chandra HETG X-ray Spectra and Variability of $π$ Aqr, a $γ$ Cas-type Be star

High-resolution X-ray spectra of $π\,$Aqr, a $γ\,$Cas-type star, obtained with the Chandra/HETG grating spectrometer, revealed emission lines of H-like ions of Mg, Si, S, and Fe, a strong, hard continuum, and a lack of He-like ions, indicating the presence of very hot thermal plasma. The X-ray light curve showed significant fluctuations, with coherent variability at period of about 3400 seconds in one observation. The hardness ratio was relatively constant except for one observation in which the spectrum was much harder and more absorbed. We interpret the X-ray emission as arising from accretion onto the secondary, which is likely a magnetic white dwarf, an intermediate polar system.

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arXiv2024

Observed epochal variations in X-ray lines from the O Supergiant $ζ$ Puppis do not require substantial changes in the wind mass flux

We fit the high resolution \textit{Chandra} X-ray spectra of the O supergiant $ζ$ Puppis using the variable boundary condition (VBC) line model to test the stability of its mass-loss rate between two epochs of observation: 2000 March and 2018 July -- 2019 August. At issue is whether the observed variations are induced by global changes in the cool (unshocked) wind itself or are isolated to the local pockets of hot gas (i.e., changes in the frequency and location of the shocks). Evidence in the literature favored the possibility of a 40 per cent increase in the mass flux of the entire stellar wind, based on X-ray reabsorption from a line-deshadowing-instability-inspired parameterization, whereas our fit parameters are consistent with a constant mass flux with a change in the velocity variations that determine the locations where shocks form. Our results suggest the shocks in the more recent data are formed at somewhat larger radii, mimicking the enhanced blueshifts and increased line fluxes interpreted in the previous analysis as being due to increases in both the X-ray generation and reabsorption from an overall stronger wind.

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arXiv2023

Survey of X-rays from Massive Stars Observed at High Spectral Resolution with Chandra

Identifying trends between observational data and the range of physical parameters of massive stars is a critical step to the still-elusive full understanding of the source, structure, and evolution of X-ray emission from the stellar winds, requiring a substantial sample size and systematic analysis methods. The \emph{Chandra} data archive as of 2022 contains 37 high resolution spectra of O, B, and WR stars, observed with the \emph{Chandra}/HETGS and of sufficient quality to fit the continua and emission line profiles. Using a systematic approach to the data analysis, we explore morphological trends in the line profiles (i.e., O, Ne, Mg, Si) and find that the centroid offsets of resolved lines versus wavelength can be separated in three empirically-defined groups based on the amount of line broadening and centroid offset. Using \ion{Fe}{17} (15.01 Å, 17.05 Å) and \ion{Ne}{10} $α$ (12.13 Å) lines which are prevalent among the sample stars, we find a well-correlated linear trend of increasing Full Width Half Maximum (FWHM) with faster wind terminal velocity. The H-like/He-like total line flux ratio for strong lines displays different trends with spectral class depending on ion species. Some of the sources in our sample have peculiar properties (e.g., magnetic and $γ$ Cas-analogue stars) and we find that these sources stand out as outliers from more regular trends. Finally, our spectral analysis is presented summarily in terms of X-ray spectral energy distributions in specific luminosity for each source, plus tables of line identifications and fluxes.

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arXiv2023

Clumpy wind studies and the non-detection of cyclotron line in OAO 1657-415

Winds of massive stars are suspected to be inhomogeneous (or clumpy), which biases the measures of their mass loss rates. In High Mass X-ray Binaries (HMXBs), the compact object can be used as an orbiting X-ray point source to probe the wind and constrain its clumpiness. We perform spectro-timing analysis of the HMXB OAO 1657-415 with non-simultaneous NuSTAR and NICER observations. We compute the hardness ratio from the energy-resolved light curves, and using an adaptive rebinning technique, we thus select appropriate time segments to search for rapid spectral variations on timescales of a few hundreds to thousands of seconds. Column density and intensity of Iron K$α$ line were strongly correlated, and the recorded spectral variations were consistent with accretion from a clumpy wind. We also illustrate a novel framework to measure clump sizes, masses in HMXBs more accurately based on absorption measurements and orbital parameters of the source. We then discuss the limitations posed by current X-ray spacecrafts in such measurements and present prospects with future X-ray missions. We find that the source pulse profiles show a moderate dependence on energy. We identify a previously undetected dip in the pulse profile visible throughout the NuSTAR observation near spin phase 0.15 possibly caused by intrinsic changes in accretion geometry close to the neutron star. We do not find any evidence for the debated cyclotron line at $\sim$ 36\,keV in the time-averaged or the phase-resolved spectra with NuSTAR.

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arXiv2022

Testing the Reliability of X-rays as a Tool for Constraining Mass-loss Rates of Hot Stars

We fit a new line shape model to \textit{Chandra} X-ray spectra of the O supergiant $ζ$ Puppis to test the robustness of mass-loss rates derived from X-ray wind line profiles against different assumed heating models. Our goal is to track the hot gas by replacing the common assumption that it is proportional to the cool gas emission measure. Instead of assuming a turn-on radius for the hot gas (as appropriate for the line-deshadowing instability internal to the wind), we parametrize the hot gas in terms of a mean-free path for accelerated low-density gas to encounter slower high-density material. This alternative model is equally successful as previous approaches at fitting X-ray spectral lines in the 5 -- 17 Å wavelength range. We find that the characteristic radii where the hottest gas appears is inversely proportional to line formation temperature, suggesting that stronger shocks appear generally closer to the surface. This picture is more consistent with pockets of low-density, rapid acceleration at the lower boundary than with an internally generated wind instability. We also infer an overall wind mass-loss rate from the profile shapes with a technique used previously in the literature. In doing so, we find evidence that the mass-loss rate derived from X-ray wind line profiles is not robust with respect to changes in the specific heating picture used.

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arXiv2021

The Colliding Winds of WR 25 in High Resolution X-rays

WR 25 is a colliding-wind binary star system comprised of a very massive O2.5If*/WN6 primary and an O-star secondary in a 208-day period eccentric orbit. These hot stars have strong, highly-supersonic winds which interact to form a bright X-ray source from wind-collision-shocks whose conditions change with stellar separation. Different views through the WR and O star winds are afforded with orbital phase as the stars move about their orbits, allowing for exploration of wind structure in ways not easy or even possible for single stars. We have analyzed an on-axis Chandra/HETGS spectrum of WR 25 obtained shortly before periastron when the X-rays emanating from the system are the brightest. From the on-axis observations, we constrain the line fluxes, centroids, and widths of various emission lines, including He-triplets of Si XIII and Mg XI. We have also been able to include several serendipitous off-axis HETG spectra from the archive and study their flux variation with phase. This is the first report on high-resolution spectral studies of WR 25 in X-rays.

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arXiv2020

Correlated X-ray and optical variability in the O-type supergiant zeta Puppis

Analysis of the recent long exposure Chandra X-ray observation of the early-type O star zeta Pup shows clear variability with a period previously reported in optical photometric studies. These 813 ks of HETG observations taken over a roughly one year time span have two signals of periodic variability: a high significance period of 1.7820 +/- 0.0008 day, and a marginal detection of periodic behavior close to either 5 day or 6 day period. A BRITE-Constellation nanosatellite optical photometric monitoring, using near-contemporaneous observations to the Chandra data, confirms a 1.78060 +/- 0.00088 day period for this star. The optical period coincides with the new Chandra period within their error ranges, demonstrating a link between these two wavebands and providing a powerful lever for probing the photosphere/wind connection in this star. The phase lag of the X-ray maximum relative to the optical maximum is approximately phi=0.45, but consideration of secondary maxima in both datasets indicates possibly two hot spots on the star with an X-ray phase lag of phi=0.1 each. The details of this periodic variation of the X-rays are probed by displaying a phased and trailed X-ray spectrum and by constructing phased light curves for wavelength bands within the HETG spectral coverage, ranging down to bands encompassing groups of emission lines. We propose that the 1.78 day period is the stellar rotation period and explore how stellar bright spots and associated co-rotating interacting regions or CIRs could explain the modulation of the optical and X-ray output for this star and their phase difference.

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arXiv2020

A Deep Exposure in High Resolution X-Rays Reveals the Hottest Plasma in the $ζ\,$Puppis Wind

We have obtained a very deep exposure (813 ks) of $ζ\,$Puppis (O4 supergiant) with the Chandra/HETG Spectrometer. Here we report on analysis of the 1-9 Å region, especially well suited for Chandra, which has a significant contribution from continuum emission between well separated emission lines from high-ionization species. These data allow us to study the hottest plasma present through the continuum shape and emission line strengths. Assuming a powerlaw emission measure distribution which has a high-temperature cut-off, we find that the emission is consistent with a thermal spectrum having a maximum temperature of 12 MK. This implies an effective wind shock velocity of $900\,\mathrm{km\,s^{-1}}$, well below the wind terminal speed of $2250\,\mathrm{km\,s^{-1}}$. For X-ray emission which forms close to the star, the speed and X-ray flux are larger than can be easily reconciled with strictly self-excited line-deshadowing-instability models, suggesting a need for a fraction of the wind to be accelerated extremely rapidly right from the base. This is not so much a dynamical instability as a nonlinear response to changing boundary conditions.

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arXiv2018

Winds of Massive Stars: High Resolution X-ray Spectra of Stars in NGC 3603

The cluster NGC 3603 hosts some of the most massive stars in the Galaxy. With a modest 50 ks exposure with the Chandra High Energy Grating Spectrometer, we have resolved emission lines in spectra of several of the brightest cluster members which are of WNh and O spectral types. This observation provides our first definitive high-resolution spectra of such stars in this nearby starburst region. The stars studied have broadened X-ray emission lines, some with blue-shifted centroids, and are characteristic of massive stellar winds with terminal velocities around 2000--3000 km/s. X-ray luminosities and plasma temperatures are very high for both the WNh and O stars studied. We conclude that their X-rays are likely the result of colliding winds.

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arXiv2015

Probing Wolf-Rayet Winds: Chandra/HETG X-Ray Spectra of WR 6

With a deep Chandra/HETGS exposure of WR 6, we have resolved emission lines whose profiles show that the X-rays originate from a uniformly expanding spherical wind of high X-ray-continuum optical depth. The presence of strong helium-like forbidden lines places the source of X-ray emission at tens to hundreds of stellar radii from the photosphere. Variability was present in X-rays and simultaneous optical photometry, but neither were correlated with the known period of the system or with each other. An enhanced abundance of sodium revealed nuclear processed material, a quantity related to the evolutionary state of the star. The characterization of the extent and nature of the hot plasma in WR 6 will help to pave the way to a more fundamental theoretical understanding of the winds and evolution of massive stars.

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