We present Hubble Space Telescope UV spectra of the 4.6 h period double white dwarf SDSS J125733.63+542850.5. Combined with Sloan Digital Sky Survey optical data, these reveal that the massive white dwarf (secondary) has an effective temperature T2 =
13030 +/- 70 +/- 150 K and a surface gravity log g2 = 8.73 +/- 0.05 +/- 0.05 (statistical and systematic uncertainties respectively), leading to a mass of M2 = 1.06 Msun. The temperature of the extremely low-mass white dwarf (primary) is substantially lower at T1 = 6400 +/- 37 +/- 50 K, while its surface gravity is poorly constrained by the data. The relative flux contribution of the two white dwarfs across the spectrum provides a radius ratio of R1/R2 = 4.2, which, together with evolutionary models, allows us to calculate the cooling ages. The secondary massive white dwarf has a cooling age of about 1 Gyr, while that of the primary low-mass white dwarf is likely to be much longer, possibly larger than 5 Gyrs, depending on its mass and the strength of chemical diffusion. These results unexpectedly suggest that the low-mass white dwarf formed long before the massive white dwarf, a puzzling discovery which poses a paradox for binary evolution.
We present ULTRACAM photometry and X-Shooter spectroscopy of the eclipsing double white dwarf binary CSS 41177, the only such system that is also a double-lined spectroscopic binary. Combined modelling of the light curves and radial velocities yield
masses and radii for both white dwarfs without the need to assume mass-radius relations. We find that the primary white dwarf has a mass of M1 = 0.38(2) Msun and a radius of R1 = 0.0222(4) Rsun. The secondary white dwarfs mass and radius are M2 = 0.32(1) Msun and R2 = 0.0207(4) Rsun, and its temperature and surface gravity (T2 = 11678(313) K, log(g2) = 7.32(2)) put it close to the white dwarf instability strip. However, we find no evidence for pulsations to roughly 0.5% relative amplitude. Both masses and radii are consistent with helium white dwarf models with thin hydrogen envelopes of 0.0001 Mstar. The two stars will merge in 1.14 Gyr due to angular momentum loss via gravitational wave emission.
We present ULTRACAM photometry of ES Cet, an ultracompact binary with a 620s orbital period. The mass transfer in systems such as this one is thought to be driven by gravitational radiation, which causes the binary to evolve to longer periods since t
he semi-degenerate donor star expands in size as it loses mass. We supplement these ULTRACAM+WHT data with observations made with smaller telescopes around the world over a nine year baseline. All of the observations show variation on the orbital period, and by timing this variation we track the period evolution of this system. We do not detect any significant departure from a linear ephemeris, implying a donor star that is of small mass and close to a fully degenerate state. This finding favours the double white dwarf formation channel for this AM CVn star. An alternative explanation is that the system is in the relatively short-lived phase in which the mass transfer rate climbs towards its long-term value.
With orbital periods of the order of tens of minutes or less, the AM Canum Venaticorum stars are ultracompact, hydrogen deficient binaries with the shortest periods of any binary subclass, and are expected to be among the strongest gravitational wave
sources in the sky. To date, the only known eclipsing source of this type is the P = 28 min binary SDSS J0926+3624. We present multiband, high time resolution light curves of this system, collected with WHT/ULTRACAM in 2006 and 2009. We supplement these data with additional observations made with LT/RISE, XMM_Newton and the Catalina Real-Time Transient Survey. From light curve models we determine the mass ratio to be q = M2 / M1 = 0.041 +/- 0.002 and the inclination to be 82.6 +/- 0.3 deg. We calculate the mass of the primary white dwarf to be 0.85 +/- 0.04 solar masses and the donor to be 0.035 +/- 0.003 solar masses, implying a partially degenerate state for this component. We observe superhump variations that are characteristic of an elliptical, precessing accretion disc. Our determination of the superhump period excess is in agreement with the established relationship between this parameter and the mass ratio, and is the most precise calibration of this relationship at low q. We also observe a quasi-periodic oscillation in the 2006 data, and we examine the outbursting behaviour of the system over a 4.5 year period.
We present high time-resolution multicolour optical observations of the quiescent X-ray transients GRS1124-684 (=GU Mus) and Cen X-4 (=V822 Cen) obtained with ULTRACAM. Superimposed on the secondary stars ellipsoidal modulation in both objects are la
rge flares on time-scales of 30-60 min, as well as several distinct rapid flares on time-scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi-periodic oscillations are observed and the power density spectra of GRS1124-684 and Cen X-4 can be described by a power-law. From the colour-colour diagrams of the flare events, for GRS1124-684 we find that the flares can be described by hydrogen gas with a density of N_H~10^24 nucleons cm^-2, a temperature of ~8000 K and arising from a radius of ~0.3 Rsun. Finally we compile the values for the transition radius (the radius of the hot advection-dominated accretion flow) estimated from quasi-periodic oscillations and/or breaks in the power density spectrum for a variety of X-ray transients in different X-ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius.
We present high speed photometric observations of the eclipsing dwarf nova IP Peg taken with the triple-beam camera ULTRACAM mounted on the William Herschel Telescope. The primary eclipse in this system was observed twice in 2004, and then a further
sixteen times over a three week period in 2005. Our observations were simultaneous in the Sloan u, g and r bands. By phase-folding and averaging our data we make the first significant detection of the white dwarf ingress in this system and find the phase width of the white dwarf eclipse to be 0.0935 +/- 0.0003, significantly higher than the previous best value of between 0.0863 and 0.0918. The mass ratio is found to be q = M2 /M1 = 0.48 +/- 0.01, consistent with previous measurements, but we find the inclination to be 83.8 +/- 0.5 deg, significantly higher than previously reported. We find the radius of the white dwarf to be 0.0063 +/- 0.0003 solar radii, implying a white dwarf mass of 1.16 +/- 0.02 solar masses. The donor mass is 0.55 +/- 0.02 solar masses. The white dwarf temperature is more difficult to determine, since the white dwarf is seen to vary significantly in flux, even between consecutive eclipses. This is seen particularly in the u-band, and is probably the result of absorption by disc material. Our best estimate of the temperature is 10,000 - 15,000K, which is much lower than would be expected for a CV with this period, and implies a mean accretion rate of less than 5 times 10^-11 solar masses per year, more than 40 times lower than the expected rate.
