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A remarkable extension of the high-ion-temperature (high-Ti) regime was obtained in deuterium plasma experiments in the Large Helical Device. In order to clarify transport characteristics in the ion internal transport barrier (ITB) formation with an isotope effect, a dataset of pure deuterium (nD/ne  >  0.8) and pure hydrogen (nH/ne  >  0.8) plasmas in the high-Ti regime were analyzed, and two mechanisms of transport improvement were characterized. A significant reduction of ion heat transport in the core of both deuterium and hydrogen plasmas was observed, indicating ion ITB formation. The dependence of the ion heat diffusivity on temperature ratio (Te/Ti) and normalized Ti-gradient (R/LTi  =  −(R/Ti)(dTi/dr)) was investigated in the core region, in which gyrokinetic simulations with the GKV code predict the destabilization of ion temperature gradient (ITG) modes. The Te/Ti dependence shows ITG-like property, while a significant deviation from the ITG-like property is found in the R/ dependence, indicating suppression of the ITG mode in the large R/ regime and resultant ion ITB formation. In the comparison between deuterium plasma and hydrogen plasma, the lower transport in the deuterium plasma is observed in both ion and electron heat diffusivities, indicating a significant isotope effect. It was found with the nonlinear turbulent transport simulation with GKV that the zonal flow enhancement contributes to the ITG suppression in the deuterium plasma. [ABSTRACT FROM AUTHOR]

The X-rays in intermediate polars (IPs) originate in a compact region near the surface of a magnetic white dwarf (WD) and interact with the complex environment surrounding the emission region. Here we report a case study of two IPs, TV Col and V2731 Oph, with selected archival X-ray observations (NuSTAR, Swift, Suzaku, and XMM-Newton). For TV Col, we were successful in simultaneously accounting for the primary X-rays, the secondary X-rays due to Compton scattering and fluorescence, and the effects of local absorbers. In this case, we were able to demonstrate that the shock height is small, based on the high reflection amplitude; hence, the maximum temperature of the post-shock region can be used to derive the WD mass of 0.735 ± 0.015 M⊙. Despite the high specific accretion rate required to explain the small shock height, we do not detect any spin modulation in our NuSTAR data, consistent with the modest amount of complex absorption seen spectroscopically. We argue that our results are robust because they are based on the joint temporal–spectral analysis of broadband X-ray data. The spectrum of V2731 Oph is more highly absorbed. Through our analysis of the Suzaku data, we present a spectral model with nitrogen overabundance without the previously claimed soft blackbody that should be further explored. We have been unable to constrain the reflection amplitude for V2731 Oph; this and the detection of spin modulation above 10 keV suggest that it may have a tall shock. Hence, we only derive a lower limit to the mass of its WD (>0.9 M⊙). [ABSTRACT FROM AUTHOR]

The current superactive state of the recurrent nova T Coronae Borealis (T CrB) has been observed with unprecedented detail. Previously published observations provide strong evidence that this state is due to an enhancement of the flow of material through the accretion disk, which increased the optical depth of its most internal region, the boundary layer. NuSTAR and Swift observed T CrB in 2015 September, roughly halfway through the rise to optical maximum. In our analysis of these data, we have found that: (i) the UV emission, as observed with Swift/UVOT in 2015, was already as bright as it became in 2017, after the optical peak; (ii) the soft X-ray emission (E ≲ 0.6 keV) observed in 2017 after the optical peak, on the other hand, had not yet developed during the rising phase in 2015; (iii) the hard X-ray emitting plasma (E ≳ 2 keV) had the same temperature and about half the flux of that observed during quiescence in 2006. This phenomenology is akin to that observed during dwarf novae in outburst, but with the changes in the spectral energy distribution happening on a far longer timescale. [ABSTRACT FROM AUTHOR]

The isotope effect on impurities and bulk ion particle transport is investigated by using the deuterium, hydrogen, and isotope mixture plasma in the Large Helical Device (LHD). A clear isotope effect is observed in the impurity transport but not the bulk ion transport. The isotope effects on impurity transport and ion heat transport are observed as a primary and a secondary effect, respectively, in the plasma with an internal transport barrier (ITB). In the LHD, an ion ITB is always transient because the impurity hole triggered by the increase of ion temperature gradient causes the enhancement of ion heat transport and gradually terminates the ion ITB. The formation of an impurity hole becomes slower in the deuterium (D) plasma than the hydrogen (H) plasma. This primary isotope effect on impurity transport contributes the longer sustainment of the ion ITB state because the low ion thermal diffusivity can be sustained as long as the normalized carbon impurity gradient R/Ln,c, where , is above the critical value (∼−5). Therefore, the longer sustainment of the ITB state in the deuterium plasma is considered to be a secondary isotope effect due to the mitigation of the impurity hole. The radial profile of H and D ion density is measured using bulk charge exchange spectroscopy inside the isotope mixture plasma. The decay time of H ion density after the H-pellet injection and the decay time of D ion density after D-pellet injection are almost identical, which demonstrates that there is no significant isotope effect on ion particle transport. [ABSTRACT FROM AUTHOR]

The deuterium (D) operation was initiated in the LHD in 2017. In the first campaign of the D experiments, we successfully extended the high temperature regime in the LHD. The new record of the ion temperature (Ti) of 10 keV associated with the ion internal transport barrier (ITB) was achieved due to several operational optimization. The thermal confinement characteristics of ITB plasmas were compared between hydrogen and D discharges. The effective ion thermal diffusivity of the ion-ITB plasmas was found to be smaller in the D discharges than that in the H discharges. The profiles of the Ti, the electron density, and the impurity of the high Ti plasmas strongly depended on the magnetic configuration and these profiles tended to peaked in the inward-shifted configuration. It was also found that the electron thermal confinement of the electron-ITB plasmas was clearly improved in the deuterium case. The GKV simulation showed the linear growth rate of TEM/ITG reduced in the plasmas with D both for the ion ITB and the electron ITB plasmas and qualitatively agreed with the tendency of the change in the thermal diffusivity obtained from the power balance analysis. [ABSTRACT FROM AUTHOR]

Symbiotic stars in which the symbiotic phenomenon is powered solely by accretion, often at an average rate that is higher than in cataclysmic variable stars, provide an important opportunity to diagnose boundary layers around disk-accreting white dwarfs. Here, we investigate SU Lyncis (SU Lyn), a recently discovered example of a purely accretion-powered symbiotic star, using the first reliable X-ray spectroscopy, obtained with NuSTAR, and ultraviolet (UV) photometry obtained with Swift. SU Lyn has hard, thermal, X-ray emission that is strongly affected by a variable local absorber that has little impact on the UV emission. Its X-ray spectrum is described well using a plasma cooling from kT ≈ 21 keV, with a 3–30 keV luminosity of approximately 4.9 × 1032 erg s−1. The spectrum is also consistent with the presence of reflection with an amplitude of 1.0, although in that case, the best-fit plasma temperature is 20%–25% lower. The UV to X-ray luminosity ratio of SU Lyn changed significantly between 2015 and 2016. We interpret this as a consequence of a drop by almost 90% in the accretion rate. Whereas the UV luminosity of the disk responded linearly, the luminosity of the optically thin (hard X-ray) emission from the boundary layer remained roughly constant because the boundary layer changed from partially optically thick to almost completely optically thin. Under this interpretation, we place a lower limit on the white dwarf mass of 0.7 M⊙ (0.8 M⊙ if we neglect reflection). [ABSTRACT FROM AUTHOR]

Modeling of impurity-seeded plasma in Large Helical Device is presented for the first time by using the three-dimensional transport code EMC3-EIRENE. High and low recycling coefficients for impurity ions are assumed to include low and high absorption rates on wall surfaces due to low and high chemical activity of neon and nitrogen, respectively. Radiation power measured by two bolometer systems and particle flux measured by divertor probes installed in multiple toroidal sections are utilized to determine impurity amount in the plasma. The toroidal uniformity and the non-uniformity of a divertor flux reduction observed experimentally for neon and nitrogen seeding, respectively, are reproduced by the model. Validations by measurements and deviations between the model and the experiment are discussed. [ABSTRACT FROM AUTHOR]

The behavior of carbon impurities in deuterium plasmas and its impact on thermal confinement were investigated in comparison with hydrogen plasmas in the Large Helical Device (LHD). Deuterium plasma experiments have been started in the LHD and high-ion-temperature plasmas with central ion temperature (Ti) of 10 keV were successfully obtained. The thermal confinement improvement could be sustained for a longer time compared with hydrogen plasmas. An isotope effect was observed in the time evolution of the carbon density profiles. A transiently peaked profile was observed in the deuterium plasmas due to the smaller carbon convection velocity and diffusivity in the deuterium plasmas compared with the hydrogen plasmas. The peaked carbon density profile was strongly correlated to the ion thermal confinement improvement. The peaking of the carbon density profile will be one of the clues to clarify the unexplained mechanisms for the formations of ion internal transport barrier and impurity hole on LHD. These results could also lead to a better understanding of the isotope effect in the thermal confinement in torus plasma. [ABSTRACT FROM AUTHOR]

High-temperature and high-density plasmas are achieved by means of real-time control, fast diagnostic, and high-power heating systems. Those systems are precisely controlled via highly integrated electronic components, but can be seriously affected by radiation damage. Therefore, the effects of irradiation on currently used electronic components should be investigated for the control and measurement of Large Helical Device (LHD) deuterium plasmas. For the precise estimation of the radiation field in the LHD torus hall, the MCNP6 code is used with the cross-section library ENDF B-VI. The geometry is modeled on the computer-aided design. The dose on silicon, which is a major ingredient of electronic components, over nine years of LHD deuterium operation shows that the gamma-ray contribution is dominant. Neutron irradiation tests were performed in the OKTAVIAN at Osaka University and the Fast Neutron Laboratory at Tohoku University. Gamma-ray irradiation tests were performed at the Nagoya University Cobalt-60 irradiation facility. We found that there are ethernet connection failures of programmable logic controller (PLC) modules due to neutron irradiation with a neutron flux of 3  ×  106 cm−2 s−1. This neutron flux is equivalent to that expected at basement level in the LHD torus hall without a neutron shield. Most modules of the PLC are broken around a gamma-ray dose of 100 Gy. This is comparable with the dose in the LHD torus hall over nine years. If we consider the dose only, these components may survive more than nine years. For the safety of the LHD operation, the electronic components in the torus hall have been rearranged. [ABSTRACT FROM AUTHOR]

We present multiwavelength observations of the unusual nova V1535 Sco throughout its outburst in 2015. Early radio observations were consistent with synchrotron emission, and early X-ray observations revealed the presence of high-energy (>1 keV) photons. These indicated that strong shocks were present during the first ∼2 weeks of the nova's evolution. The radio spectral energy distribution was consistent with thermal emission from week 2 to week 6. Starting in week 7, the radio emission again showed evidence of synchrotron emission, and there was an increase in X-ray emission, indicating a second shock event. The optical spectra show evidence of at least two separate outflows, with the faster outflow possibly having a bipolar morphology. The optical and near-infrared light curves and the X-ray measurements indicated that the companion star is likely a K giant. [ABSTRACT FROM AUTHOR]

Interacting binaries in which a white dwarf accretes material from a companion—cataclysmic variables (CVs) in which the mass donor is a Roche-lobe filling star on or near the main sequence, and symbiotic stars in which the mass donor is a late type giant—are relatively commonplace. They display a wide range of behaviors in the optical, X-rays, and other wavelengths, which still often baffle observers and theorists alike. Here I review the existing body of research on X-ray emissions from these objects for the benefits of both experts and newcomers to the field. I provide introductions to the past and current X-ray observatories, the types of known X-ray emissions from these objects, and the data analysis techniques relevant to this field. I then summarize of our knowledge regarding the X-ray emissions from magnetic CVs, non-magnetic CVs and symbiotic stars, and novae in eruption. I also discuss space density and the X-ray luminosity functions of these binaries and their contribution to the integrated X-ray emission from the Galaxy. I then discuss open questions and future prospects. [ABSTRACT FROM AUTHOR]

We describe the X-ray emission as observed by Suzaku from five symbiotic stars that we selected for deep Suzaku observations after their initial detection with ROSAT, ASCA, and Swift. We find that the X-ray spectra of all five sources can be adequately fit with absorbed optically thin thermal plasma models, with either single- or multi-temperature plasmas. These models are compatible with the X-ray emission originating in the boundary layer between an accretion disk and a white dwarf. The high plasma temperatures of kT > 3 keV for all five targets were greater than expected for colliding winds. Based on these high temperatures as well as previous measurements of UV variability and UV luminosity and the large amplitude of X-ray flickering in 4 Dra, we conclude that all five sources are accretion-powered through predominantly optically thick boundary layers. Our X-ray data allow us to observe a small optically thin portion of the emission from these boundary layers. Given the time between previous observations and these observations, we find that the intrinsic X-ray flux and the intervening absorbing column can vary by factors of three or more on a timescale of years. However, the location of the absorber and the relationship between changes in accretion rate and absorption are still elusive. [ABSTRACT FROM AUTHOR]

Two different responses to an energetic-particle-driven magnetohydrodynamic (MHD) instability, modulation of the turbulence amplitude associated with the MHD instability and dynamical changes in the radial electric field (Er) synchronized with bursting MHD activities, are found around the edge plasma in neutral beam injection (NBI) heated plasmas of the Heliotron J device using multiple Langmuir probes. The nonlinear phase relationship between the MHD activity and broadband fluctuation is found from bicoherence and envelope analysis applied to the probe signals. The structural changes of the Er profile appear in perfect synchronization with the periodic MHD activities, and radial transport of fast ions are observed around the last closed flux surface as a radial delay of the ion saturation current signals. Moreover, distortion of the MHD mode structure is clarified in each cycle of the MHD activities using beam emission spectroscopy diagnostics, suggesting that the fast ion distribution in real and/or velocity spaces is distorted in the core plasma, which can modify the radial electric field structure through a redistribution process of the fast ions. These observations suggest that such effects as a nonlinear coupling with turbulence and/or the modification of radial electric field profiles are important and should be incorporated into the study of energetic particle driven instabilities in burning plasma physics. [ABSTRACT FROM AUTHOR]

Impurity seeding to reduce the divertor heat load was conducted in the large helical device (LHD) using neon (Ne) and krypton (Kr) puffing. Radiation enhancement and reduction of the divertor heat load were observed. In the LHD, the ratio between the total radiated power and the heating power, f rad = Prad/Pheating, is limited up to around 30% in hydrogen plasmas even for high density plasma just below the radiative collapse (ne, bar  >  1   ×   1020 m−3), where ne, bar is the line averaged density. With Ne seeding, the ratio could be raised to 52% at ne, bar ~ 1.3   ×   1019 m−3, albeit with a slight reduction in confinement. f rad ~ 30% could be sustained for 3.4 s using multi-pulse Ne seeding at ne, bar ~ 4   ×   1019 m−3. The localized supplemental radiation was observed along the helical divertor X-points (HDXs) which is similar to the estimated structure by the EMC3-EIRENE code. Kr seeding was also conducted at ne, bar ~ 3.1   ×   1019 m−3. f rad ~ 25% was obtained without a significant change in stored energy. The radiation enhancement had a slower time constant. The supplemental radiation area of the Kr seeded plasma moved from the HDXs to the core plasma. Highly charged states of Kr ions are considered to be the dominant radiators from the plasma core region. [ABSTRACT FROM AUTHOR]

We present the Suzaku XIS observation of V2491 Cyg (Nova Cyg 2008 No. 2) obtained in quiescence, more than two years after the outburst. The nova was detected as a very luminous source in a wide spectral range from soft to hard X-rays. A very soft blackbody-like component peaking at 0.5 keV indicates that we observe either remaining, localized hydrogen burning on the surface of the white dwarf, or accretion onto a magnetized polar cap. In the second case, V2491 Cyg is a candidate “soft intermediate polar.” We obtained the best fit for the X-ray spectra with several components: two of thermal plasma, a blackbody and a complex absorber. The latter is typical of intermediate polars. The X-ray light curve shows a modulation with a period of ∼38 minutes. The amplitude of this modulation is strongly energy-dependent and reaches a maximum in the range 0.8–2.0 keV. We discuss the origin of the X-ray emission and pulsations, and the likelihood of the intermediate polar scenario. [ABSTRACT FROM AUTHOR]

Determining reliable distances to classical novae is a challenging but crucial step in deriving their ejected masses and explosion energetics. Here we combine radio expansion measurements from the Karl G. Jansky Very Large Array with velocities derived from optical spectra to estimate an expansion parallax for nova V959 Mon, the first nova discovered through its γ-ray emission. We spatially resolve the nova at frequencies of 4.5–36.5 GHz in nine different imaging epochs. The first five epochs cover the expansion of the ejecta from 2012 October to 2013 January, while the final four epochs span 2014 February–May. These observations correspond to days 126 through 199 and days 615 through 703 after the first detection of the nova. The images clearly show a non-spherical ejecta geometry. Utilizing ejecta velocities derived from three-dimensional modeling of optical spectroscopy, the radio expansion implies a distance between 0.9 ± 0.2 and 2.2 ± 0.4 kpc, with a most probable distance of 1.4 ± 0.4 kpc. This distance implies a γ-ray luminosity of erg s−1, which is much less than the prototype γ-ray-detected nova, V407 Cyg, possibly due to the lack of a red giant companion in the V959 Mon system. V959 Mon also has a much lower γ-ray luminosity than other classical novae detected in γ-rays to date, indicating a range of at least a factor of 10 in the γ-ray luminosities for these explosions. [ABSTRACT FROM AUTHOR]