We present results and applications of high-precision timing measurements of the binary millisecond pulsar J1012+5307. Combining our radio timing measurements with results based on optical observations, we derive complete 3D velocity information for this system. Correcting for Doppler effects, we derive the intrinsic spin parameters of this pulsar and a characteristic age of 8.6±1.9 Gyr. Our upper limit for the orbital eccentricity of only 8×10−7 (68 per cent confidence level) is the smallest ever measured for a binary system. We demonstrate that this makes the pulsar an ideal laboratory in which to test certain aspects of alternative theories of gravitation. Our precision measurements suggest deviations from a simple pulsar spin-down timing model, which are consistent with timing noise and the extrapolation of the known behaviour of slowly rotating pulsars.

/pdf/precision-timing-measurements-of-psr-j1012-5307-xt4ykxauuc.pdf

Precision timing measurements of PSR J1012+5307

Context. A large number of low-mass (≲0.20 M ⊙ ) helium white dwarfs (He WDs) have recently been discovered. The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main-sequence star companion. There appear to be discrepancies between the current theoretical modelling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario yet remains to be fully understood. Aims. Here we investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is to examine the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass. Our calculations are then compared to the most recent observational data.Methods. Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in a large number of close-orbit LMXBs with different initial values of donor star mass, neutron star mass, orbital period, and strength of magnetic braking. Subsequently, we followed the evolution of the detached low-mass proto-He WDs, including stages with residual shell hydrogen burning and vigorous flashes caused by unstable CNO burning.Results. We find that the time between Roche-lobe detachment until the low-mass proto-He WD reaches the WD cooling track is typically Δt proto = 0.5−2 Gyr, depending systematically on the WD mass and therefore on its luminosity. The lowest WD mass for developing shell flashes is ~0.21 M ⊙ for progenitor stars of mass M 2 ≤ 1.5 M ⊙ (and ~0.18 M ⊙ for M 2 = 1.6 M ⊙ ).Conclusions. The long timescale of low-mass proto-He WD evolution can explain a number of recent observations, including some MSP systems hosting He WD companions with very low surface gravities and high effective temperatures. We find no evidence for Δt proto to depend on the occurrence of flashes and thus question the suggested dichotomy in thermal evolution of proto-WDs.

/pdf/the-timescale-of-low-mass-proto-helium-white-dwarf-evolution-2bufmsulzm.pdf

The timescale of low-mass proto-helium white dwarf evolution

A large number of low-mass (< 0.20 M_sun) helium white dwarfs (He WDs) have recently been discovered. The majority of these are orbiting another WD or a millisecond pulsar (MSP) in a close binary system; a few examples are found to show pulsations or to have a main-sequence star companion. There appear to be discrepancies between the current theoretical modelling of such low-mass He WDs and a number of key observed cases, indicating that their formation scenario remains to be fully understood. Here we investigate the formation of detached proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is to examine the thermal evolution and the contraction phase towards the WD cooling track and investigate how this evolution depends on the WD mass. Our calculations are then compared to the most recent observational data. Numerical calculations with a detailed stellar evolution code were used to trace the mass-transfer phase in a large number of close-orbit LMXBs. Subsequently, we followed the evolution of the detached low-mass proto-He WDs, including stages with residual shell hydrogen burning and vigorous flashes caused by unstable CNO burning. We find that the time between Roche-lobe detachment until the low-mass proto-He WD reaches the WD cooling track is typically Delta_t_proto = 0.5 - 2 Gyr, depending systematically on the WD mass and therefore on its luminosity. The lowest WD mass for developing shell flashes is ~0.21 M_sun for progenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun). The long timescale of low-mass proto-He WD evolution can explain a number of recent observations, including some MSP systems hosting He WD companions with very low surface gravities and high effective temperatures. We find no evidence for Delta_t_proto to depend on the occurrence of flashes and thus question the suggested dichotomy in thermal evolution of proto-WDs.

/pdf/the-timescale-of-low-mass-proto-helium-white-dwarf-evolution-1zoqmznx0i.pdf

We present a detailed calculation of the evolution of low-mass (<0.25 M⊙) helium white dwarfs. These white dwarfs (the optical companions to binary millisecond pulsars) are formed via long-term, low-mass binary evolution. After detachment from the Roche lobe, the hot helium cores have a rather thick hydrogen layer with mass between 0.01 and 0.06 M⊙. As a result of mixing between the core and outer envelope, the surface hydrogen content (Xsurf) is 0.5–0.35, depending on the initial value of the heavy element Z and the initial secondary mass. We found that the majority of our computed models experience one or two hydrogen shell flashes. We found that the mass of the helium dwarf in which the hydrogen shell flash occurs depends on the chemical composition. The minimum helium white dwarf mass in which a hydrogen flash takes place is 0.213 M⊙ (Z=0.003), 0.198 M⊙ (Z=0.01), 0.192 M⊙ (Z=0.02) or 0.183 M⊙ (Z=0.03). The duration of the flashes (independent of chemical composition) is between a few ×106 and a few ×107 yr. In several flashes the white dwarf radius will increase so much that it forces the model to fill its Roche lobe again. Our calculations show that the cooling history of the helium white dwarf depends dramatically on the thickness of the hydrogen layer. We show that the transition from a cooling white dwarf with a temporarily stable hydrogen-burning shell to a cooling white dwarf in which almost all residual hydrogen is lost in a few thermal flashes (via Roche lobe overflow) occurs between 0.183 and 0.213 M⊙ (depending on the heavy element value).

Cooling curves and initial models for low-mass white dwarfs (<0.25 M⊙) with helium cores

We present the discovery of a further ve recycled pulsar systems in the midGalactic latitude portion of the High Time Resolution Universe (HTRU) Survey. The pulsars have rotational periods ranging from 2 ms to 66 ms, and four are in binary systems with orbital periods between 10.8 hours and 9.0 days. Three of these binary systems are particularly interesting; PSR J1227 6208 has a pulse period of 34.5 ms and the highest mass function of all pulsars with near-circular orbits. The circular orbit suggests that the companion is not another neutron star, so future timing experiments may reveal one of the heaviest white dwarfs ever found (> 1.3 M ). Timing observations of PSR J1431 4715 indicate that it is eclipsed by its companion which has a mass indicating it belongs to the redback class of eclipsing millisecond pulsars. PSR J1653 2054 has a companion with a minimum mass of only 0:08 M , placing it among the class of pulsars with low-mass companions. Unlike the majority of such systems, however, no evidence of eclipses is seen at 1.4 GHz.

The High Time Resolution Universe survey - XI. Discovery of five recycled pulsars and the optical detectability of survey white dwarf companions

Age of a millisecond binary pulsar

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Age of a millisecond binary pulsar