Dustin K. Swarm

We present the first first X-ray polarization measurements of a white dwarf (WD), the intermediate polar EX Hya. We measured significant polarization only in the 2–3 keV energy band with a polarization degree of 8% at a 3 σ 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 Imaging X-ray Polarimetry Explorer appears to be 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 WD’s surface.

We present a 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 × 10 ⁻¹¹ M _⊙ 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 fluorescence is weak, being significantly detected only in the NuSTAR spectrum, but it 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 that were found to be marginally inconsistent. The H-like Mg line is consistent with instrumental broadening only but shows a 300 km s ⁻¹ blueshift. He-like Mg and H-like Si lines have no significant shift in velocity but are broadened by about 1000 km s ⁻¹ . This suggests either different physical origins or velocity structure differing with plasma temperature.

We investigated the temporal and spectral features of γ Cassiopeiae’s X-ray emission within the context of the white dwarf (WD) 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 nonmagnetic cataclysmic variables. We then use this two-component insight to investigate previously unreported simultaneous XMM and NuSTAR data. The results of model fitting find WD properties consistent with optical studies alongside a significant secondary, thermal source. We propose a secondary shock between the Be decretion disk and WD accretion disk as the source. Finally, we analyzed a unique, low-count-rate event of the XMM light curve as potential evidence for the WD encountering Be decretion disk structures.

High-resolution X-ray spectra of π Aqr, a γ Cas-type star, obtained with the Chandra/HETG 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 a period of about 3400 s 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.

Astronomers are increasingly faced with a deluge of information, and finding worthwhile targets of study in the sea of data can be difficult. Outlier identification studies are a method that can be used to focus investigations by presenting a smaller set of sources that could prove interesting because they do not follow the trends of the underlying population. We apply a principal component analysis (PCA) and an unsupervised random forest algorithm (uRF) to sources from the Chandra Source Catalog v.2 (CSC2). We present 119 high-significance sources that appear in all repeated applications of our outlier identification algorithm (OIA). We analyze the characteristics of our outlier sources and crossmatch them with the SIMBAD database. Our outliers contain several sources that were previously identified as having unusual or interesting features by studies. This OIA leads to the identification of interesting targets that could motivate more detailed study.