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The dominant radioactive energy source powering Type Ia supernova light curves is expected to switch from the decay of $^{56}$Co to $^{57}$Co at very late epochs. We use archival HST images of SN1992A obtained more than 900 days after explosion to constrain its cobalt isotopic abundance ratio and compare it to the well-studied event SN2011fe. We confirm the $^{57}$Co / $^{56}$Co ratio for SN2011fe of $0.026\pm 0.004$ found by arXiv:1608.01155, consistent with a "double degenerate" progenitor scenario. For SN1992A, we find a ratio of $0.034\pm 0.010$, but the large uncertainty does not allow us to differentiate between progenitor models, 5 pages, 1 figure. Accepted by RNAAS on 17/11/2020

AE Aqr objects are a class of cataclysmic variable stars in which the rapidly rotating magnetosphere of the white dwarf (WD) primary centrifugally expels most infalling gas before it can accrete onto the WD. The expulsion of the accretion flow via this "magnetic propeller" extracts angular momentum from the WD and produces large-amplitude, aperiodic flares in optical photometry. The eponymous AE Aqr is the only confirmed member of this class of object, but recently, Thorstensen (2020) discovered a candidate AE Aqr system: LAMOST J024048.51+195226.9. Using survey photometry, we measure a refined orbital period for this system and identify a shallow, previously unrecognized eclipse during which the system's frequent AE Aqr-like flaring episodes cease. A dedicated follow-up study is still necessary to test the proposed AE Aqr classification for LAMOST J024048.51+195226.9, but should it be confirmed, the eclipse of its flare-production region will offer a new means of studying the magnetic propeller phenomenon., accepted for publication, RNAAS

The intermediate polar FO Aquarii experienced its first-reported low-accretion states in 2016, 2017, and 2018. We establish that these low states occurred shortly after the system's white dwarf (WD) began spinning down, after having spent a quarter-century spinning up. FO Aquarii is the only intermediate polar whose period derivative has undergone a sign change, and it has now done so twice. By combining our spin-pulse timings with previous data, we determine that the WD's spin period has varied quasi-sinusoidally since the system's discovery, and an extrapolation predicts that the white dwarf was spinning down during newly discovered low states in photographic plates from 1964, 1965, and 1974. Thus, FO Aquarii's low states appear to occur exclusively during epochs of spindown. Additionally, our time-series photometry of the 2016-18 low states reveals that the mode of accretion is extremely sensitive to the accretion rate; when the system is fainter than V~14.0, the accretion onto the WD is largely stream-fed, but when it is brighter, it is almost exclusively disk-fed. The system's grazing eclipse remained detectable throughout all observations, confirming the uninterrupted presence of a disk-like structure, regardless of the accretion state. Our observations are consistent with theoretical predictions that during the low states, the accretion disk dissipates into a ring of diamagnetic blobs. Finally, a new XMM-Newton observation from 2017 indicates that the system's anomalously soft X-ray spectrum and diminished X-ray luminosity in the wake of the 2016 low state appear to be long-lasting changes compared to pre-2016 observations., peer-reviewed

We present the selection algorithm and anticipated results for the Time Domain Spectroscopic Survey (TDSS). TDSS is an SDSS-IV eBOSS subproject that will provide initial identification spectra of approximately 220,000 luminosity-variable objects (variable stars and AGN) across 7,500 square degrees selected from a combination of SDSS and multi-epoch Pan-STARRS1 photometry. TDSS will be the largest spectroscopic survey to explicitly target variable objects, avoiding pre-selection on the basis of colors or detailed modeling of specific variability characteristics. Kernel Density Estimate (KDE) analysis of our target population performed on SDSS Stripe 82 data suggests our target sample will be 95% pure (meaning 95% of objects we select have genuine luminosity variability of a few magnitudes or more). Our final spectroscopic sample will contain roughly 135,000 quasars and 85,000 stellar variables, approximately 4,000 of which will be RR Lyrae stars which may be used as outer Milky Way probes. The variability-selected quasar population has a smoother redshift distribution than a color-selected sample, and variability measurements similar to those we develop here may be used to make more uniform quasar samples in large surveys. The stellar variable targets are distributed fairly uniformly across color space, indicating that TDSS will obtain spectra for a wide variety of stellar variables including pulsating variables, stars with significant chromospheric activity, cataclysmic variables and eclipsing binaries. TDSS will serve as a pathfinder mission to identify and characterize the multitude of variable objects that will be detected photometrically in even larger variability surveys such as LSST., 22 pages, 18 figures, accepted by ApJ

In this thesis, we present results from optical and X-ray studies of compact binary systems. These observations are used to constrain orbital parameters - mainly periods and inclinations for several cataclysmic variables - and examine the accretion structures within the systems. In Chapters 3 and 4, optical and X-ray studies of two interesting cataclysmic variables systems, Lanning 386 and MASTER OTJ192328.22+612413.5, are presented. The X-ray and optical data of these objects suggest that both systems have a high inclination and harbour weakly magnetic white dwarfs with spin periods 17-22 min. There is also a strong focus on studying the long term behaviour in intermediate polars in this thesis. Chapter 5 presents results from Kepler observations of FO Aquarii taken over 69 consecutive days. The shape of the light curve is consistent with the outer edge of the accretion disc being eclipsed and the detected period is longer than the last reported period of 1254.284(16) s, suggesting that FO Aqr was now spinning down, and had a positive P˙ at the time of the observations. Chapter 6 continues the work on FO Aquarii, and presents X-ray data taken during a low accretion state. The X-ray spectrum shows an increase in the ratio of the soft X-ray flux to the hard X-ray flux due to a change in the partial covering fraction of the white dwarf and a change in the hydrogen column density within the disc. XMM-Newton observations of FO Aqr during the subsequent recovery suggest that the system had not yet returned to its typical high state by November 2016. The spin period of the white dwarf in 2014 and 2015 is refined to 1254.3342(8) s. Chapter 7 focuses on CSS120422:J111127+571239, an “evolved main-sequence” channel cataclysmic variable. We present spectroscopy of CSS120422:J111127+571239 using the COS FUV instrument on the Hubble Space Telescope and using the MODS spectrograph on the Large Binocular Telescope. The UV spectrum shows Si IV, N V and He II, but no detectable C IV. The anomalous nitrogen/carbon ratio is seen in a small number of other CVs and confirms a unique binary evolution. We also present and compare the optical spectrum of V418 Ser and advocate that it is also an evolved main-sequence system. Finally, Chapter 8 presents results on UW Coronae Borealis, a low mass X-ray x Abstract binary that shows both Type 1 X-ray and optical bursts. The orbital period is refined to 110.97680(1) min. 17 optical bursts are presented, with 10 of these bursts being resolved temporally. The average e-folding time of 19 ± 3s for the bursts is consistent with the previously found value. Optical bursts are observed during a previously identified gap in orbital phase centred on φ = 0.967, meaning the reprocessing site is not eclipsed as previously thought. Finally, the apparent P-Cygni profiles present in some of the atomic lines in the optical spectra are due to transient absorption, which suggests the origin of the absorption is not a wind from the primary.