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Many high-state cataclysmic variables (CVs) exhibit blue-shifted absorption features in their ultraviolet (UV) spectra – a smoking-gun signature of outflows. However, the impact of these outflows on optical spectra remains much more uncertain. During its recent outburst, the eclipsing dwarf nova V455 And displayed strong optical emission lines whose cores were narrower than expected from a Keplerian disc. Here, we explore whether disc + wind models developed for matching UV observations of CVs can also account for these optical spectra. Importantly, V455 And was extremely bright at outburst maximum: the accretion rate implied by fitting the optical continuum with a standard disc model is |$\dot{M}_{\rm acc} \simeq 10^{-7}~{\rm M}_\odot ~{\rm yr^{-1}}$|⁠. Allowing for continuum reprocessing in the outflow helps to relax this constraint. A disc wind can also broadly reproduce the optical emission lines, but only if the wind is (i) highly mass-loaded, with a mass-loss rate reaching |$\dot{M}_{\rm wind} \simeq 0.4 \dot{M}_{\rm acc}$|⁠ , and/or (ii) clumpy, with a volume filling factor |$f_V \simeq 0.1$|⁠. The same models can describe the spectral evolution across the outburst, simply by lowering |$\dot{M}_{\rm acc}$| and |$\dot{M}_{\rm wind}$|⁠. Extending these models to lower inclinations and into the UV produces spectra consistent with those observed in face-on high-state CVs. We also find, for the first time in simulations of this type, P-Cygni-like absorption features in the Balmer series, as have been observed in both CVs and X-ray binaries. Overall, dense disc winds provide a promising framework for explaining multiple observational signatures seen in high-state CVs, but theoretical challenges persist. [ABSTRACT FROM AUTHOR]

Disc winds are a common feature in accreting astrophysical systems on all scales. In active galactic nuclei (AGNs) and accreting white dwarfs (AWDs), specifically, radiation pressure mediated by spectral lines is a promising mechanism for driving these outflows. Previous hydrodynamical simulations have largely supported this idea, but relied on highly approximate treatments of ionization and radiative transfer. Given the sensitivity of line driving to the ionization state and radiation field in the outflow, here, we present a new method for carrying out 2.5D radiation hydrodynamic simulations that takes full account of the frequency-dependent radiative transfer through the wind, the corresponding ionization state, and the resulting radiative accelerations. Applying our method to AWDs, we find that it is much harder to drive a powerful line-driven outflow when the interaction between matter and radiation is treated self-consistently. This conclusion is robust to changes in the adopted system parameters. The fundamental difficulty is that discs luminous enough to drive such a wind are also hot enough to overionize it. As a result, the mass-loss rates in our simulations are much lower than those found in earlier, more approximate calculations. We also show that the ultraviolet spectra produced by our simulations do not match those observed in AWDs. We conclude that, unless the overionization problem can be mitigated (e.g. by subgrid clumping or a softer-than-expected radiation field), line driving may not be a promising mechanism for powering the outflows from AWDs. These conclusions are likely to have significant implications for disc winds in AGN also. [ABSTRACT FROM AUTHOR]

Blueshifts – or, more accurately, blue asymmetries – in broad emission lines such as C  iv λ1550 are common in luminous quasars and correlate with fundamental properties such as Eddington ratio and broad absorption line (BAL) characteristics. However, the formation of these blueshifts is still not understood, and neither is their physical connection to the BAL phenomenon or accretion disc. In this work, we present Monte Carlo radiative transfer and photoionization simulations using parametrized biconical disc-wind models. We take advantage of the azimuthal symmetry of a quasar and show that we can reproduce C  iv blueshifts provided that (i) the disc-mid-plane is optically thick out to radii beyond the line formation region, so that the receding wind bicone is obscured; and (ii) the system is viewed from relatively low (that is, more face-on) inclinations (≲40°). We show that C  iv emission-line blueshifts and BALs can form in the same wind structure. The velocity profile of the wind has a significant impact on the location of the line formation region and the resulting line profile, suggesting that the shape of the emission lines can be used as a probe of wind-driving physics. While we are successful at producing blueshifts/blue asymmetries in outflows, we struggle to match the detailed shape or skew of the observed emission-line profiles. In addition, our models produce redshifted emission-line asymmetries for certain viewing angles. We discuss our work in the context of the C  iv λ1550 emission blueshift versus equivalent-width space and explore the implications for quasar disc wind physics. [ABSTRACT FROM AUTHOR]

We present a far-ultraviolet (FUV) study of the globular cluster M30 (NGC 7099). The images were obtained using the Advanced Camera for Surveys (ACS/SBC, F150LP, FUV) and the Wide Field Planetary Camera 2 (WFPC2, F300W, UV) which were both onboard the Hubble Space Telescope. The FUV − UV colour–magnitude diagram (CMD) shows a main sequence (MS) turnoff at FUV ≈ 22 mag and |$FUV-UV\, \approx 3$| mag. The MS extends 4 mag below the turnoff, and a prominent horizontal branch (HB) and blue straggler (BS) sequence can be seen. A total of 1218 MS stars, 185 red giant branch stars, 47 BS stars, and 41 HB stars are identified, along with 78 sources blueward of the MS which consist of white dwarfs (WDs) and objects in the gap between the WDs and the MS that include potential cataclysmic variable (CV) candidates. The radial distribution of the BS population is concentrated towards the cluster centre, indicating that mass segregation has occurred. The blue and red sub-populations of the double BS sequence appear mixed in the ultraviolet CMD, and no significant central concentration of CV candidates is seen in this cluster. [ABSTRACT FROM AUTHOR]

A significant number of tidal disruption events (TDEs) radiate primarily at optical and ultraviolet (UV) wavelengths, with only weak soft X-ray components. One model for this optical excess proposes that thermal X-ray emission from a compact accretion disc is reprocessed to longer wavelengths by an optically thick envelope. Here, we explore this reprocessing scenario in the context of an optically thick accretion disc wind. Using state-of-the-art Monte Carlo radiative transfer and ionization software, we produce synthetic UV and optical spectra for wind and disc-hosting TDEs. Our models are inspired by observations, spanning a realistic range of accretion rates and wind kinematics. We find that such outflows can efficiently reprocess the disc emission and produce the broad Balmer and helium recombination features commonly seen in TDEs and exhibit asymmetric red wings. Moreover, the characteristic colour temperature of the reprocessed spectral energy distribution (SED) is much lower than that of the accretion disc. We show explicitly how changes in black hole mass, accretion rate, and wind properties affect the observed broad-band SED and line spectrum. In general, slower, denser winds tend to reprocess more radiation and produce stronger Balmer emission. Most of the outflows we consider are too highly ionized to produce UV absorption features, but this is sensitive to the input SED. For example, truncating the inner disc at just ≃4 R ISCO lowers the wind ionization state sufficiently to produce UV absorption features for sightlines looking into the wind. [ABSTRACT FROM AUTHOR]

The spectra arising from the discs of nova-like variables show many of the features seen in stellar atmospheres. They are typically modelled either from an appropriated weighted set of stellar atmospheres or a disc atmosphere with energy is dissipated near the disc plane, with the effective temperature distribution expected from a steady-state accretion disc. However, these models generally overpredict the depth of the Balmer jump and the slope of the spectrum in the ultraviolet. The problem is likely due to energy dissipation in the disc atmosphere, which produces a flatter vertical temperature profile than is observed in stars. Here, we provide validation for this hypothesis in the form of spectra generated using the stellar atmosphere code tlusty using a parametric prescription for energy dissipation as a function of depth and closely match the spectrum of the nova-like IX Vel over the wavelength range 1150–6000 Å. [ABSTRACT FROM AUTHOR]

We present a new catalogue of radio sources in the face-on spiral galaxy M83. Radio observations taken in 2011, 2015, and 2017 with the Australia Telescope Compact Array at 5.5 and 9 GHz have detected 270 radio sources. Although a small number of these sources are background extragalactic sources, most are either H  ii regions or supernova remnants (SNRs) within M83 itself. Three of the six historical supernovae are detected, as is the very young remnant that had been identified in a recent study, which is likely the result of a supernova that exploded in the last ∼100 yr but was missed. All of these objects are generally fading with time. Confusion limits our ability to measure the radio emission from a number of the SNRs in M83, but 64 were detected in unconfused regions, and these have the approximate power-law luminosity function that has been observed in other galaxies. The SNRs in M83 are systematically smaller in diameter and brighter than those that have been detected at radio wavelengths in M33. A number of the radio sources are coincident with X-ray sources in M83; most of these coincident sources turn out to be SNRs. Our dual frequency observations are among the most sensitive to date for a spiral galaxy outside the Local Group; despite this we were not able to place realistic constraints on the spectral indices, and as a result, it was not possible to search for SNRs based on their radio properties alone. [ABSTRACT FROM AUTHOR]

Some tidal disruption events (TDEs) exhibit blueshifted broad absorption lines (BALs) in their rest-frame ultraviolet (UV) spectra, while others display broad emission lines (BELs). Similar phenomenology is observed in quasars and accreting white dwarfs, where it can be interpreted as an orientation effect associated with line formation in an accretion disc wind. We propose and explore a similar unification scheme for TDEs. We present synthetic UV spectra for disc and wind-hosting TDEs, produced by a state-of-the-art Monte Carlo ionization and radiative transfer code. Our models cover a wide range of disc wind geometries and kinematics. Such winds naturally reproduce both BALs and BELs. In general, sightlines looking into the wind cone preferentially produce BALs, while other orientations preferentially produce BELs. We also study the effect of wind clumping and CNO-processed abundances on the observed spectra. Clumpy winds tend to produce stronger UV emission and absorption lines, because clumping increases both the emission measure and the abundances of the relevant ionic species, the latter by reducing the ionization state of the outflow. The main effect of adopting CNO-processed abundances is a weakening of C  iv  1550 Å  and an enhancement of N v  1240 Å  in the spectra. We conclude that line formation in an accretion disc wind is a promising mechanism for explaining the diverse UV spectra of TDEs. If this is correct, the relative number of BAL and BEL TDEs can be used to estimate the covering factor of the outflow. The models in this work are publicly available online and upon request. [ABSTRACT FROM AUTHOR]

X-ray signatures of outflowing gas have been detected in several accreting black hole binaries, always in the soft state. A key question raised by these observations is whether these winds might also exist in the hard state. Here, we carry out the first full-frequency radiation hydrodynamic simulations of luminous (⁠|${L = 0.5 \, L_{\mathrm{\mathrm{ Edd}}}}$|⁠) black hole X-ray binary systems in both the hard and the soft state, with realistic spectral energy distributions (SEDs). Our simulations are designed to describe X-ray transients near the peak of their outburst, just before and after the hard-to-soft state transition. At these luminosities, it is essential to include radiation driving, and we include not only electron scattering, but also photoelectric and line interactions. We find powerful outflows with |${\dot{M}_{\mathrm{ wind}} \simeq 2 \, \dot{M}_{\mathrm{ acc}}}$| are driven by thermal and radiation pressure in both hard and soft states. The hard-state wind is significantly faster and carries approximately 20 times as much kinetic energy as the soft-state wind. However, in the hard state the wind is more ionized, and so weaker X-ray absorption lines are seen over a narrower range of viewing angles. Nevertheless, for inclinations ≳80°, blueshifted wind-formed Fe  xxv and Fe  xxvi features should be observable even in the hard state. Given that the data required to detect these lines currently exist for only a single system in a luminous hard state – the peculiar GRS 1915+105 – we urge the acquisition of new observations to test this prediction. The new generation of X-ray spectrometers should be able to resolve the velocity structure. [ABSTRACT FROM AUTHOR]

The origin, geometry, and kinematics of the broad-line region (BLR) gas in quasars and active galactic nuclei (AGN) are uncertain. We demonstrate that clumpy biconical disc winds illuminated by an AGN continuum can produce BLR-like spectra. We first use a simple toy model to illustrate that disc winds make quite good BLR candidates, because they are self-shielded flows and can cover a large portion of the ionizing flux-density (ϕ H - nH) plane. We then conduct Monte Carlo radiative transfer and photoionization calculations, which fully account for self-shielding and multiple scattering in a non-spherical geometry. The emergent model spectra show broad emission lines with equivalent widths and line ratios comparable to those observed in AGN, provided that the wind has a volume filling factor of fV ≲ 0.1. Similar emission line spectra are produced for a variety of wind geometries (polar or equatorial) and for launch radii that differ by an order of magnitude. The line emission arises almost exclusively from plasma travelling below the escape velocity, implying that 'failed winds' are important BLR candidates. The behaviour of a line-emitting wind (and possibly any 'smooth flow' BLR model) is similar to that of the locally optimally emitting cloud model originally proposed by Baldwin et al. (1995), except that the gradients in ionization state and temperature are large-scale and continuous, rather than within or between distinct clouds. Our models also produce UV absorption lines and X-ray absorption features, and the stratified ionization structure can partially explain the different classes of broad absorption line quasars. [ABSTRACT FROM AUTHOR]

We have carried out radiation-hydrodynamic simulations of thermally driven accretion disc winds in low-mass X-ray binaries. Our main goal is to study the luminosity dependence of these outflows and compare with observations. The simulations span the range 0.04 ≤ L acc/ L Edd ≤ 1.0 and therefore cover most of the parameter space in which disc winds have been observed. Using a detailed Monte Carlo treatment of ionization and radiative transfer, we confirm two key results found in earlier simulations that were carried out in the optically thin limit: (i) the wind velocity – and hence the maximum blueshift seen in wind-formed absorption lines – increases with luminosity; (ii) the large-scale wind geometry is quasi-spherical, but observable absorption features are preferentially produced along high-column equatorial sightlines. In addition, we find that (iii) the wind efficiency always remains approximately constant at |${\skew4\dot{M}_{\rm wind}/\skew4\dot{M}_{\rm acc} \simeq 2}$|⁠, a behaviour that is consistent with observations. We also present synthetic Fe xxv and Fe xxvi absorption line profiles for our simulated disc winds in order to illustrate the observational implications of our results. [ABSTRACT FROM AUTHOR]

We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992–1996 revealed a 1.15 d periodicity with a 25 per cent amplitude, hypothesized to be due to metals in a surface accretion spot. We obtained phase resolved HST /Space Telescope Imaging Spectrograph high resolution far-ultraviolet spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate, and radial velocity of GD 394 constant over the observed time-scales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high ionisation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf. [ABSTRACT FROM AUTHOR]

We present disc wind models aimed at reproducing the main features of the strong Na  i resonance line P Cygni profiles in the rapidly accreting pre-main-sequence FU Ori objects. We conducted Monte Carlo radiative transfer simulations for a standard magnetocentrifugally driven wind (MHD) model and our own 'Genwind' models, which allows for a more flexible wind parametrization. We find that the fiducial MHD wind and similar Genwind models, which have flows emerging outward from the inner disc edge, and thus have polar cavities with no absorbing gas, cannot reproduce the deep, wide Na  i absorption lines in FU Ori objects viewed at low inclination. We find that it is necessary to include an 'inner wind' to fill this polar cavity to reproduce observations. In addition, our models assuming pure scattering source functions in the Sobolev approximation at intermediate viewing angles (30° ≲ i ≲ 60°) do not yield sufficiently deep line profiles. Assuming complete absorption yields better agreement with observations, but simple estimates strongly suggest that pure scattering should be a much better approximation. The discrepancy may indicate that the Sobolev approximation is not applicable, possibly due to turbulence or non-monotonic velocity fields; there is some observational evidence for the latter. Our results provide guidance for future attempts to constrain FU Ori wind properties using full MHD wind simulations, by pointing to the importance of the boundary conditions necessary to give rise to an inner wind, and by suggesting that the winds must be turbulent to produce sufficiently deep line profiles. [ABSTRACT FROM AUTHOR]

If some or all Type Ia SNe arise from accretion onto a massive WD from a companion, then the companion will remain in some form after the SN explosion. Tycho-B is an unusual, relatively hot star along the line of sight to Tycho’s SNR – conclusively shown to be a Type Ia – and has been suggested as such a companion. If the interior of Tycho’s SNR contains unshocked Fe, and if Tycho-B is either within the SNR shell or in the background, then one might hope to see evidence of this in the UV spectrum. Such is the case for SN 1006, where spectra of the background Schweizer–Middleditch star, as well as two AGNs, show broad absorption lines of Fe II. To test this idea, we have used STIS on HST to obtain a UV spectrum of Tycho-B. The observed spectrum, however, shows no evidence of Fe II absorption. Furthermore, a luminosity distance estimate using UV and optical spectra of Tycho-B suggests that the star is consistent with a foreground interloper. We conclude either that Tycho-B is nearer than Tycho’s SNR, or that all of the Fe in the interior of Tycho’s SNR is more highly ionized. [ABSTRACT FROM AUTHOR]

Essentially all low-mass X-ray binaries (LMXBs) in the soft state appear to drive powerful equatorial disc winds. A simple mechanism for driving such outflows involves X-ray heating of the top of the disc atmosphere to the Compton temperature. Beyond the Compton radius, the thermal speed exceeds the escape velocity, and mass-loss is inevitable. Here, we present the first coupled radiation-hydrodynamic simulation of such thermally driven disc winds. The main advance over previous modelling efforts is that the frequency-dependent attenuation of the irradiating SED is taken into account. We can therefore relax the approximation that the wind is optically thin throughout which is unlikely to hold in the crucial acceleration zone of the flow. The main remaining limitations of our simulations are connected to our treatment of optically thick regions. Adopting parameters representative of the wind-driving LMXB GRO J1655−40, our radiation-hydrodynamic model yields a mass-loss rate that is ≃5 × lower than that suggested by pure hydrodynamic, optically thin models. This outflow rate still represents more than twice the accretion rate and agrees well with the mass-loss rate inferred from Chandra /HETG observations of GRO J1655−40 at a time when the system had a similar luminosity to that adopted in our simulations. The Fe xxv and Fe xxvi Lyman $$\rm {\alpha }$$ absorption line profiles observed in this state are slightly stronger than those predicted by our simulations but the qualitative agreement between observed and simulated outflow properties means that thermal driving is a viable mechanism for powering the disc winds seen in soft-state LMXBs. [ABSTRACT FROM AUTHOR]

We have carried out a study of the X-ray properties of the supernova remnant (SNR) population in M33 with XMM-Newton, comprising deep observations of eight fields in M33 covering all of the area within the D25 contours, and with a typical luminosity of 7.1 × 1034 erg s-1 (0.2-2.0 keV). Here, we report our work to characterize the X-ray properties of the previously identified SNRs in M33, as well as our search for new X-ray detected SNRs. With our deep observations and large field of view we have detected 105 SNRs at the 3σ level, of which 54 SNRs are newly detected in X-rays, and three are newly discovered SNRs. Combining XMM-Newton data with deep Chandra survey data allows detailed spectral fitting of 15 SNRs, for which we have measured temperatures, ionization time-scales and individual abundances. This large sample of SNRs allows us to construct an X-ray luminosity function, and compare its shape to luminosity functions from host galaxies of differing metallicities and star formation rates to look for environmental effects on SNR properties. We conclude that while metallicity may play a role in SNR population characteristics, differing star formation histories on short time-scales, and small-scale environmental effects appear to cause more significant differences between X-ray luminosity distributions. In addition, we analyse the X-ray detectability of SNRs, and find that in M33 SNRs with higher [S II]/H α ratios, as well as those with smaller galactocentric distances, are more detectable in X-rays. [ABSTRACT FROM AUTHOR]

SDSS J1507+52 is an eclipsing cataclysmic variable (CV) consisting of a cool, non-radially pulsating white dwarf and an unusually small substellar secondary. The system has a high space velocity and a very short orbital period of about 67 min, well below the usual minimum period for CVs. To explain the existence of this peculiar system, two theories have been proposed. One suggests that SDSS J1507+52 was formed from a detached white dwarf-brown dwarf binary. The other theory proposes that the system is a member of the Galactic halo population. Here, we present the ultraviolet (UV) spectroscopy of SDSS J1507+52 obtained with the Hubble Space Telescope with the aim of distinguishing between these two theories. The UV flux of the system is dominated by the emission from the accreting white dwarf. Fits to model stellar atmospheres yield physical parameter estimates of km s and . These fits suggest a distance towards SDSS J1507+52 of pc. The quoted uncertainties include systematic errors associated with the adopted fitting windows and interstellar reddening. Assuming that there is no contribution to the UV flux from a hot, optically thick boundary layer, we find a much higher than previously estimated from eclipse analyses. The strongly subsolar metallicity we infer for SDSS J1507+52 is consistent with that of halo stars at the same space velocity. We therefore conclude that SDSS J1507+52 is a member of the Galactic halo. [ABSTRACT FROM AUTHOR]

Using archival Rossi X-Ray Timing Explorer data, we show that the ultracompact X-ray binary in NGC 1851 exhibits large amplitude X-ray flux variations of more than a factor of 10 on time-scales of days to weeks and undergoes sustained periods of months where the time-averaged luminosity varies by factors of 2. Variations of this magnitude and time-scale have not been reported previously in other ultracompact X-ray binaries. Mass transfer in ultracompact binaries is thought to be driven by gravitational radiation and the predicted transfer rates are so high that the discs of ultracompact binaries with orbits as short as that of this object should not be susceptible to ionization instabilities. Therefore, the variability characteristics we observe were unexpected and need to be understood. We briefly discuss a few alternatives for producing the observed variations in light of the fact that the viscous time-scale of the disc is of the order of a week, comparable to the shorter time-scale variation that is observed but much less than the longer term variation. We also discuss the implications for interpretation of observations of extragalactic binaries if the type of variability seen in the source in NGC 1851 is typical. [ABSTRACT FROM AUTHOR]

We present seven nights' blue (4300–5000 Å) spectroscopy of the nova-like variable star V3885 Sgr. The line spectrum shows a typical combination of broad absorption and emission in Hγ, Hβ and He i, which is associated with the accretion disc. We also observe antiphased narrow emission, which we attribute to irradiation of the secondary star. The He iiλ4686 and N iii–C iii–C iv emission lines are devoid of structure and are most likely formed in an outflow. We measure radial velocity shifts in the absorption and emission lines, from which we fit an orbital period of . From the velocity semi-amplitudes of the disc and companion star, we are able to constrain the binary mass ratio to . The phase-folded spectra provide dense coverage of the entire orbital cycle. Doppler tomograms of the hydrogen and He i lines reveal spiral structure in the accretion disc and the irradiated donor star. We believe that this is the first unambiguous detection of spiral waves in a nova-like variable. [ABSTRACT FROM AUTHOR]

QU Car is listed in cataclysmic variable star catalogues as a nova-like variable. This littlestudied, yet bright interacting binary is re-appraised here in the light of new high-quality ultraviolet interstellar line data obtained with STIS on-board the Hubble Space Telescope. The detection of a component of interstellar absorption at a mean local standard of rest velocity of -14 km s[sup -1] indicates that the distance to QU Car may be ∼2 kpc or more — a considerable increase on the previous lower-limiting distance of 500 pc. If so, the bolometric luminosity of QU Car could exceed 10[sup 37] erg s[sup -1]. This would place this binary in the luminosity domain occupied by known compact-binary supersoft X-ray sources. Even at 500 pc, QU Car appears to be the most luminous nova-like variable known. New intermediate dispersion optical spectroscopy of QU Car spanning 3800-7000 Å is presented. These data yield the discovery that C IV λλ5801,12 is present as an unusually prominent emission line in an otherwise low-contrast line spectrum. Using measurements of this and other lines in a recombination line analysis, it is shown that the C/He abundance as proxied by the n(C[sup 4+])/n(He[sup 2+]) ratio may be as high as 0.06 (an order of magnitude higher than the solar ratio). Furthermore, the C/O abundance ratio is estimated to be greater than 1. These findings suggest that the companion in QU Car is a carbon star. If so, it would be the first example of a carbon star in such a binary. An early-type R star best matches the required abundance pattern and could escape detection at optical wavelengths provided the distance to QU Car is ∼2 kpc or more. [ABSTRACT FROM AUTHOR]