From grain growth to astromineralogy: Studying dust with X-ray imaging and spectroscopy (Cosmic Dust 2018)
X-ray imaging and spectroscopy can provide a powerful tool for measuring the large end of the dust grain size distribution — important for interpretting infrared extinction as well as understanding grain growth in the diffuse interstellar medium (ISM). In addition, X-ray photoelectric absorption edges observed in high resolution spectra of Galactic X-ray binaries directly reveal the mineral composition of interstellar dust. I will review open problems in the field of astromineralogy, as probed by X-ray extinction. I will describe how observations from the next two X-ray missions — XARM and ARCUS — will answer some of those questions. Finally, I will discuss synergistic opportunities for X-ray telescopes and JWST to provide a more complete picture of dust grain evolution in the diffuse ISM.
Click here for a link to the pdf version of the poster.
Modern Problems in High Resolution X-ray Absorption from the Cold Interstellar Medium (HEAD 2020 in Rosemont, IL)
The X-ray energy band is sensitive to absorption by all abundant metals in the interstellar medium (ISM) — carbon, oxygen, neon, silicon, magnesium, and iron — whether they are in gas or dust form. Photoelectric edges seen in high resolution X-ray spectra of Galactic X-ray point sources allow us to measure absolute abundances of gas and solid phase metals in the ISM, with fewer underlying assumptions. X-ray absorption fine structure (XAFS) in the photoelectric edge also reveals dust grain mineralogies and grain sizes. We review open problems in the field of astromineralogy, including problems in reconciling X-ray absorption spectroscopy with ISM observations at other wavelengths. In the future, high resolution imaging spectroscopy enabled by micro-calorimeters can probe the constituent elements of large dust grains, which are relatively invisible in absorption. Gratings spectroscopy is also necessary to obtain high resolution spectra at soft energies, required for probing the state of neutral carbon and oxygen -- the two most abundant interstellar metals and the two most important actors in the field of astromineralogy.
Click here for a link to the presentation slides.
With accurate cross-sections and higher signal-to-noise, X-ray spectroscopy can directly measure Milky Way gas and dust-phase metal abundances with few underlying assumptions. The X-ray energy band is sensitive to absorption by all abundant interstellar metals — carbon, oxygen, neon, silicon, magnesium, and iron — whether they are in gas or dust form. High resolution X-ray spectra from Galactic X-ray point sources can be used to directly measure metal abundances from all phases of the interstellar medium (ISM) along singular sight lines. We show our progress for measuring the depth of photoelectric absorption edges from neutral ISM metals, using all the observations of bright Galactic X-ray binaries available in the Chandra HETG archive. The cross-sections we use take into account both the absorption and scattering effects by interstellar dust grains on the iron and silicate spectral features. However, there are many open problems for reconciling X-ray absorption spectroscopy with ISM observations in other wavelengths. We will review the state of the field, lab measurements needed, and ways in which the next generation of X-ray telescopes will contribute.
Click here for a link to the talk
Interstellar dust grain composition from high-resolution X-ray absorption edge structure
(click here for PDF)
X-ray light is sufficient to excite electrons from n=1 (K-shell) and n=2 (L-shell) energy levels of neutral interstellar metals, causing a sharp increase in the absorption cross-section. Near the ionization energy, the shape of the photoelectric absorption edge depends strongly on whether the atom is isolated or bound in molecules or minerals (dust). With high resolution X-ray spectroscopy, we can directly measure the state of metals and the mineral composition of dust in the interstellar medium. In addition, the scattering contribution to the X-ray extinction cross-section can be used to gauge grain size, shape, and filling factor. In order to fully take advantage of major advances in high resolution X-ray spectroscopy, lab measurements of X-ray absorption fine structure (XAFS) from suspected interstellar minerals are required. Optical constants derived from the absorption measurements can be used with Mie scattering or anomalous diffraction theory in order to model the full extinction cross-sections from the interstellar medium. Much like quasar spectra are used to probe intergalactic gas, absorption spectroscopy of Galactic X-ray binaries and bright stars will yield key insights to the mineralogy and evolution of dust grains in the Milky Way.
Navigating Atomic Databases with ISIS
(click here for PDF)
As part of the High Resolution X-ray Spectroscopic Software and Tools workshop (an extension of the XMM-Newton Workshop), I gave a presentation about using the Interactive Spectral Interpretation System in order to navigate atomic databases. See this presentation for more detailed descriptions of the philosophy behind XSTARDB.
The effects of dust scattering on high resolution X-ray absorption edge structure
(click here for PDF)
High energy studies of astrophysical dust complement observations of dusty interstellar gas at other wavelengths. With high resolution X-ray spectroscopy, dust scattering significantly enhances the total extinction optical depth and alters the shape of photoelectric absorption edges. This effect is modulated by the dust grain size distribution, spatial location along the line of sight, and the imaging resolution of the X-ray telescope. At soft energies, the spectrum of scattered light is likely to have significant features at the 0.3 keV (C-K), 0.5 keV (O-K), and 0.7 keV (Fe-L) photoelectric absorption edges. We focus in particular on the Fe-L edge, fitting a template for the total extinction to the high resolution spectrum of three X-ray binaries from the Chandra archive: GX 9+9, XTE J1817-330, and Cyg X-1. We discuss ways in which spectroscopy with Chandra and XMM can yield insight into dust obscured objects such as stars, binaries, AGN, and foreground quasar absorption line systems.
Studying interstellar dust grain composition with high resolution spectroscopic imaging
(click here for PDF)
Interstellar dust in the foreground of bright point sources will scatter X-rays over arcminute scales, producing a diffuse `halo' image. The scattering halo intensity is strongly sensitive to the dust grain size distribution, spatial distribution, and composition of dust on the sight line. Currently, Chandra's spatial resolution makes it the best detector available for studying X-ray scattering halos from the diffuse ISM. A Chandra successor with similar resolution but larger effective area will lend a greater opportunity to study scattering echoes from low column densities of the diffuse interstellar or intergalactic medium. More importantly, the combination of high resolution spectroscopy with imaging is uniquely suited to studying the composition of dust grains. At soft energies in particular, the spectrum of scattered light is likely to have significant features at the 0.3 keV (C-K) and 0.5 (O-K) photoelectric absorption edges. This direct probe of ISM dust grain elements will be important for (i) understanding the relative abundances of graphitic grains or PAHs versus silicates, and (ii) measuring the depletion of gas phase elements into solid form.
X-ray Extinction from Dust Scattering
(click here for PDF)
X-ray scattering by dust is a necessary component of ISM extinction that is not included in typical absorption models for the interstellar medium available in XSPEC e.g. phabs, tbabs, and TBnew. Small angle scattering by ISM dust grains affects any X-ray instrument with sub-arcminute resolution e.g. Chandra, Swift, and XMM. Column densities measured from X-ray obscuration will thereby be overestimated if only an ISM absorption model is used. We simulate a number of Chandra spectra to explore the bias in NH and photon index measurements obtained without inclusion of extinction from dust scattering. We then extrapolate to other observatories, Swift and XMM, whose angular resolution ensures capture of some fraction of the scattered light from dust at intermediate distances between the source and the observer.
Atomic Data Unleashed:
Interactive and scriptable interfaces to atomic databases
We present new software developed in ISIS, the Interactive Spectral Interpretation System, to provide high level access to underlying atomic data used in astrophysical plasma models. In particular, modules developed to support spectra computed from the XSTAR database allow for simple line or edge identification and more complex analysis requiring strengths or dependencies of specific features, such as theoretical line ratios. We will show examples for computing diagnostics of astrophysical interest, such as curves of growth and emissivity curves, and for modeling multi-component photoionized plamas (using warmabs and photemis) with newly developed ISIS modules. These software interfaces to atomic data are key to the legacy of Chandra, which has a rich and ever increasing high resolution spectroscopic dataset, requiring increasingly complex analysis.
X-ray Scattering through the Intergalactic Medium:
Time Variability and Ghost Halos
Dust grains polluting the intergalactic medium (IGM) have a chance of being detected through the phenomenon of X-ray scattering, which produces a diffuse arcminute-scale halo around bright X-ray point sources. I present follow up work to Corrales & Paerels (2012) by calculating the expected intensity of intergalactic dust scattering halos using the more exact Mie scattering treatment. This adjustment is necessary to check for large 0.1-1 micron sized dust grains that would interfere with the photometry needed for high precision measurements of cosmological constants. Even with the supreme focusing power of Chandra, I find that the dust scattering halo intensity is much dimmer than the Chandra PSF wings. However, scattered light takes a longer path to reach the observer, causing intergalactic scattering halos to be delayed ~10,000 years. I investigate the possibility of detecting a scattering halo around a quasar that has recently become dim, or ghost halos from a past quasar that is no longer visible.
An archive of select poster and presentation slides.
Light echo from star V838
(NASA/ESA/H E Bond)