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Radio emission has been detected from tens of white dwarfs, in particular in accreting systems. Additionally, radio emission has been predicted as a possible outcome of a planetary system around a white dwarf. We searched for 3 GHz radio continuum emission in 846 000 candidate white dwarfs previously identified in Gaia using the Very Large Array Sky Survey (VLASS) Epoch 1 Quick Look Catalogue. We identified 13 candidate white dwarfs with a counterpart in VLASS within 2 arcsec. Five of those were found not to be white dwarfs in follow-up or archival spectroscopy, whereas seven others were found to be chance alignments with a background source in higher resolution optical or radio images. The remaining source, WDJ204259.71+152108.06, is found to be a white dwarf and M-dwarf binary with an orbital period of 4.1 d and long-term stochastic optical variability, as well as luminous radio and X-ray emission. For this binary, we find no direct evidence of a background contaminant, and a chance alignment probability of only ≈2 per cent. However, other evidence points to the possibility of an unfortunate chance alignment with a background radio and X-ray emitting quasar, including an unusually poor Gaia DR3 astrometric solution for this source. With at most one possible radio emitting white dwarf found, we conclude that strong (≳1–3 mJy) radio emission from white dwarfs in the 3 GHz band is virtually non-existent outside of interacting binaries. [ABSTRACT FROM AUTHOR]

The shell of the classical nova V5668 Sgr was resolved by ALMA at the frequency of 230 GHz 927 d after eruption, showing that most of the continuum bremsstrahlung emission originates in clumps with diameter smaller than 1015 cm. Using Very Large Array radio observations, obtained between days 2 and 1744 after eruption, at frequencies between 1 and 35 GHz, we modelled the nova spectra, assuming first that the shell is formed by a fixed number of identical clumps, and afterwards with the clumps having a power-law distribution of sizes, and were able to obtain the clump's physical parameters (radius, density, and temperature). We found that the density of the clumps decreases linearly with the increase of the shell's volume, which is compatible with the existence of a second media, hotter and thinner, in pressure equilibrium with the clumps. We show that this thinner media could be responsible for the emission of the hard X-rays observed at the early times of the nova eruption, and that the clump's temperature evolution follows that of the super-soft X-ray luminosity. We propose that the clumps were formed in the radiative shock produced by the collision of the fast wind of the white dwarf after eruption, with the slower velocity of the thermonuclear ejecta. From the total mass of the clumps, the observed expansion velocity and thermonuclear explosion models, we obtained an approximate value of 1.25 M⊙ for the mass of the white dwarf, a central temperature of 107 K and an accretion rate from the secondary star of 10−9–10−8 M⊙ yr−1. [ABSTRACT FROM AUTHOR]

Weakly accreting black hole X-ray binaries launch compact radio jets that persist even in the quiescent spectral state, at X-ray luminosities ≲ 10−5 of the Eddington luminosity. However, radio continuum emission has been detected from only a few of these quiescent systems, and little is known about their radio variability. Jet variability can lead to misclassification of accreting compact objects in quiescence, and affects the detectability of black hole X-ray binaries in next-generation radio surveys. Here we present the results of a radio monitoring campaign of A0620 − 00, one of the best-studied and least-luminous known quiescent black hole X-ray binaries. We observed A0620 − 00 at 9.8 GHz using the Karl G Jansky Very Large Array on 31 epochs from 2017 to 2020, detecting the source |$\sim 75{{\ \rm per\, cent}}$| of the time. We see significant variability over all time-scales sampled, and the observed flux densities follow a lognormal distribution with μ = 12.5 μJy and σ = 0.22 dex. In no epoch was A0620 − 00 as bright as in 2005 (51 ± 7 μJy), implying either that this original detection was obtained during an unusually bright flare, or that the system is fading in the radio over time. We present tentative evidence that the quiescent radio emission from A0620 − 00 is less variable than that of V404 Cyg, the only other black hole binary with comparable data. Given that V404 Cyg has a jet radio luminosity ∼20 times higher than A0620 − 00, this comparison could suggest that less luminous jets are less variable in quiescence. [ABSTRACT FROM AUTHOR]

Peaking at 3.7 mag on 2020 July 11, YZ Ret was the second-brightest nova of the decade. The nova's moderate proximity (2.7 kpc, from Gaia) provided an opportunity to explore its multiwavelength properties in great detail. Here, we report on YZ Ret as part of a long-term project to identify the physical mechanisms responsible for high-energy emission in classical novae. We use simultaneous Fermi /LAT and NuSTAR observations complemented by XMM–Newton X-ray grating spectroscopy to probe the physical parameters of the shocked ejecta and the nova-hosting white dwarf. The XMM–Newton observations revealed a supersoft X-ray emission which is dominated by emission lines of C  v , C  vi , N  vi , N  vii , and O  viii rather than a blackbody-like continuum, suggesting CO-composition of the white dwarf in a high-inclination binary system. Fermi /LAT-detected YZ Ret for 15 d with the γ -ray spectrum best described by a power law with an exponential cut-off at 1.9 ± 0.6 GeV. In stark contrast with theoretical predictions and in keeping with previous NuSTAR observations of Fermi -detected classical novae (V5855 Sgr and V906 Car), the 3.5–78-keV X-ray emission is found to be two orders of magnitude fainter than the GeV emission. The X-ray emission observed by NuSTAR is consistent with a single-temperature thermal plasma model. We do not detect a non-thermal tail of the GeV emission expected to extend down to the NuSTAR band. NuSTAR observations continue to challenge theories of high-energy emission from shocks in novae. [ABSTRACT FROM AUTHOR]

Radio continuum observations offer a new window on compact objects in globular clusters compared to typical X-ray or optical studies. As part of the MAVERIC survey, we have used the Australia Telescope Compact Array to carry out a deep (median central noise level ≈4  |$\mu$| Jy beam-1) radio continuum survey of 26 southern globular clusters at central frequencies of 5.5 and 9.0 GHz. This paper presents a catalogue of 1285 radio continuum sources in the fields of these 26 clusters. Considering the surface density of background sources, we find significant evidence for a population of radio sources in seven of the 26 clusters, and also identify at least 11 previously known compact objects (six pulsars and five X-ray binaries). While the overall density of radio continuum sources with 7.25-GHz flux densities ≳ 20  |$\mu$| Jy in typical globular clusters is relatively low, the survey has already led to the discovery of several exciting compact binaries, including a candidate ultracompact black hole X-ray binary in 47 Tuc. Many of the unclassified radio sources near the centres of the clusters are likely to be true cluster sources, and multiwavelength follow-up will be necessary to classify these objects and better understand the demographics of accreting compact binaries in globular clusters. [ABSTRACT FROM AUTHOR]

X-ray observations of shocked gas in novae can provide a useful probe of the dynamics of the ejecta. Here we report on X-ray observations of the nova V959 Mon, which was also detected in GeV gamma-rays with the Fermi satellite. We find that the X-ray spectra are consistent with a two-temperature plasma model with non-solar abundances. We interpret the X-rays as due to shock interaction between the slow equatorial torus and the fast polar outflow that were inferred from radio observations of V959 Mon. We further propose that the hotter component, responsible for most of the flux, is from the reverse shock driven into the fast outflow. We find a systematic drop in the column density of the absorber between days 60 and 140, consistent with the expectations for such a picture. We present intriguing evidence for a delay of around 40 d in the expulsion of the ejecta from the central binary. Moreover, we infer a relatively small (a few times 10−6 M⊙) ejecta mass ahead of the shock, considerably lower than the mass of 104 K gas inferred from radio observations. Finally, we infer that the dominant X-ray shock was likely not radiative at the time of our observations, and that the shock power was considerably higher than the observed X-ray luminosity. It is unclear why high X-ray luminosity, closer to the inferred shock power, is never seen in novae at early times, when the shock is expected to have high enough density to be radiative. [ABSTRACT FROM AUTHOR]

Supernova remnants (SNRs) in Local Group galaxies offer unique insights into the origin of different types of supernovae (SNe). In order to take full advantage of these insights, one must understand the intrinsic and environmental diversity of SNRs in the context of their host galaxies. We introduce a semi-analytic model that reproduces the statistical properties of a radio continuum-selected SNR population, taking into account the detection limits of radio surveys, the range of SN kinetic energies, the measured interstellar medium (ISM) and stellar mass distribution in the host galaxy from multi-wavelength images and the current understanding of electron acceleration and magnetic field amplification in SNR shocks from first-principle kinetic simulations. Applying our model to the SNR population in M33, we reproduce the SNR radio luminosity function with a median SN rate of ~3.1 × 10-3 per year and an electron acceleration efficiency, ∊e ~ 4.2 × 10-3. We predict that the radio visibility times of ~70 per cent of M33 SNRs will be determined by their Sedov-Taylor lifetimes, and correlated with the measured ISM column density, NH (tvis ∝ N -αH, with a ~ 0.33) while the remainingwill have visibility times determined by the detection limit of the radio survey. These observational constraints on the visibility time of SNRs will allow us to use SNR catalogues as 'SN surveys' to calculate SN rates and delay-time distributions in the Local Group. [ABSTRACT FROM AUTHOR]

INTEGRAL Gamma-Ray (IGR) J14091-6108 is a Galactic X-ray source known to have an iron emission line, a hard X-ray spectrum, and an optical counterpart. Here, we report on X-ray observations of the source with XMM-Newton and NuSTAR as well as optical spectroscopy with European Southern Obseratory/Very Large Telescope and National Optical Astronomy Observatory/Southern Astrophysical Research Telescope. In the X-rays, this provides data with much better statistical quality than the previous observations, and this is the first report of the optical spectrum. Timing analysis of the XMM data shows a very significant detection of 576.3 ± 0.6 s period. The signal has a pulsed fraction of 30 ± 3 per cent in the 0.3-12 keV range and shows a strong drop with energy. The optical spectra show strong emission lines with significant variability in the lines and continuum, indicating that they come from an irradiated accretion disc. Based on these measurements, we identify the source as a magnetic cataclysmic variable of intermediate polar (IP) type where the white dwarf spin period is 576.3 s. The X-ray spectrum is consistent with the continuum emission mechanism being due to thermal bremsstrahlung, but partial covering absorption and reflection are also required. In addition, we use the IP mass model, which suggests that the white dwarf in this system has a high mass, possibly approaching the Chandrasekhar limit. [ABSTRACT FROM AUTHOR]