Double detonations in double white dwarf (WD) binaries undergoing unstable mass transfer have emerged in recent years as one of the most promising Type Ia supernova (SN Ia) progenitor scenarios. One potential outcome of this "dynamically driven double-degenerate double-detonation" (D 6 ) scenario is that the companion WD survives the explosion and is flung away with a velocity equal to its > 1000 km s −1 pre-SN orbital velocity. We perform a search for these hypervelocity runaway WDs using Gaia's second data release. In this paper, we discuss seven candidates followed up with groundbased instruments. Three sources are likely to be some of the fastest known stars in the Milky Way, with total Galactocentric velocities between 1000 and 3000 km s −1 , and are consistent with having previously been companion WDs in pre-SN Ia systems. However, although the radial velocity of one of the stars is > 1000 km s −1 , the radial velocities of the other two stars are puzzlingly consistent with 2 Shen et al. 0.The combined five-parameter astrometric solutions from Gaia and radial velocities from follow-up spectra yield tentative 6D confirmation of the D 6 scenario. The past position of one of these stars places it within a faint, old SN remnant, further strengthening the interpretation of these candidates as hypervelocity runaways from binary systems that underwent SNe Ia.

The planetary nebula stage is the ultimate fate of stars with masses one to eight times that of the Sun (M(⊙)). The origin of their complex morphologies is poorly understood, although several mechanisms involving binary interaction have been proposed. In close binary systems, the orbital separation is short enough for the primary star to overfill its Roche lobe as the star expands during the asymptotic giant branch phase. The excess gas eventually forms a common envelope surrounding both stars. Drag forces then result in the envelope being ejected into a bipolar planetary nebula whose equator is coincident with the orbital plane of the system. Systems in which both stars have ejected their envelopes and are evolving towards the white dwarf stage are said to be double degenerate. Here we report that Henize 2-428 has a double-degenerate core with a combined mass of ∼1.76M(⊙), which is above the Chandrasekhar limit (the maximum mass of a stable white dwarf) of 1.4M(⊙). This, together with its short orbital period (4.2 hours), suggests that the system should merge in 700 million years, triggering a type Ia supernova event. This supports the hypothesis of the double-degenerate, super-Chandrasekhar evolutionary pathway for the formation of type Ia supernovae.

Long-term ("superorbital") periods or modulations have been detected in a wide variety of both low and high-mass X-ray binaries at X-ray and optical wavelengths. A variety of mechanisms have been proposed to account for the variability properties, such as precessing and/or warped accretion discs, amongst others. The All Sky Monitor on board the Rossi X-ray Timing Explorer provides the most extensive (~15 years) and sensitive X-ray archive for studying such behaviour. It is also clear that such variations can be intermittent and/or a function of X-ray spectral state. Consequently, we use a time-dependent Dynamic Power Spectrum method to examine how these modulations vary with time in 25 X-ray binaries for which superorbital periodicities have been previously reported. Our aim is to characterize these periodicities in a completely systematic way. Some (such as Her X-1 and LMC X-4) are remarkably stable, but others show a range of properties, from even longer variability time-scales to quite chaotic behaviour.Comment: 17 pages, 26 figures, 4 tables, Accepted for publication in MNRAS on 4 November 201

ABSTRACT. Two identical new instruments, the Sutherland High-speed Optical Cameras (SHOC), have been developed for use on the South African Astronomical Observatory's (SAAO) 1.9, 1.0 and 0.75 m telescopes at Sutherland. The SHOC systems are fast-frame-rate, accurately-timed, high-quality, visible-wavelength imagers. Each system consists of a camera, global positioning system (GPS), control computer and peripherals. The primary component is an Andor iXon X3 888 UVB camera, utilizing a 1024 × 1024 pixel, frame-transfer, thermoelectrically-cooled, back-illuminated CCD. One of SHOC's most important features is that it can achieve frame rates of between 1 and 20 frames/s during normal operation (dependent on binning and subframing) with microsecond timing accuracy on each frame (achieved using frame-by-frame GPS triggering). Frame rates can be increased further, and fainter targets observed, by making use of SHOC's electron-multiplying (EM) modes. SHOC is therefore ideally suited to time domain astronomy where high frame rates and extremely accurate timing are critical. Here, we present details of the instrument components, characteristics measured during commissioning, science demonstrations, and development plans. Attention is specifically given to exploration of the signal-to-noise (S/N) parameter space as a function of EM and conventional modes. These results enable observers to optimize instrumental settings for their observations and clearly demonstrate the advantages and potential pitfalls of the EM modes.