System Barycenter Research Articles

Definition of Very Long Baseline Interferometry (VLBI) Group Delay and Relativistic Correction Model With Picosecond Precision

In this paper, the incorrect definition of VLBI coordinate time delay in solar system barycenter frame (B-frame) is clarified, thus correcting the error of sign of the individual term in VLBI relativistic time delay model of transformation published earlier. It is proved that the terms, i. e. the tropospheric refraction delay, the axis offset refraction delay, and the gravitational delay caused by the major planets which are respectively multiplied by the velocity and the acceleration vector of the geocenter, should be taken into account in the model mentioned above under the precision of 1 ps. Finally, the VLBI relativistic time delay model with precision of better than 1 ps is obtained.

A millisecond pulsar in an eclipsing binary

We have discovered a remarkable pulsar with period 1.6 ms, moving in a nearly circular 9.17-h orbit around a low-mass companion star. At an observing frequency of 430 MHz, the pulsar, PSR1957 + 20, is eclipsed once each orbit for about 50 minutes. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds—presumably as a result of propagation through plasma surrounding the companion. The pulsar's orbit about the system barycentre has a radius of 0.089 light seconds projected on to the line of sight. The observed orbital period and size, together with the fact that eclipses occur, imply a surprisingly low companion mass, only a few per cent of the mass of the Sun.

Compressed planetary and lunar ephemerides

A package of FORTRAN software has been developed which provides planetary and lunar positions, with respect to the solar system barycenter, for all times in the interval 18012049; positions agree to 1 milliarcsecond with those generated by Jet Propulsion Laboratory Development Ephemeris 200 (DE200). The system consists of approximately 800 kilobytes of ephemeris files and 40 kilobytes of programs, totalling 5% of the storage required by DE200. After removal of reference orbits, segments of DE200 positions were fitted by finite Chebyshev series of degree 40. The Chebyshev coefficients were rounded to integer multiples of a suitable unit and packed to form the ephemeris files.

Prolonged minima and the 179-yr cycle of the solar inertial motion

We employ the JPL long ephemeris DE-102 to study the inertial motion of the Sun for the period A.D. 760–2100. Defining solar orbits with reference to the Sun's successive close approaches to the solar system barycenter, occurring at mean intervals of 19.86 yr, we find simple relationships linking the inertial orientation of the solar orbit and the amplitude of the precessional rotation of the orbit with the occurrence of the principal prolonged solar activity minima of the current millenium (the Wolf, Sporer, and Maunder minima). The progression of the inertial orientation parameter is controlled by the 900-yr ‘great inequality’ of the motion of Jupiter and Saturn, while the precessional rotation parameter is linked with the 179-yr cycle of the solar inertial motion previously identified by Jose (1965). A new prolonged minimum of solar activity may be imminent.

Spacetime coordinates in the geocentric reference frame

A geocentric relativistic reference frame is established which is close to the conventional non-relativistic equatorial frame of reference. Within post-Newtonian approximation the worldline of the geocentre is used to connect points by spacelike geodesics on the equal proper time hypersurface and to establish a properly chosen tetrad reference frame. Points on the earth surface and near the earth-space are coordinated making use of the Frobenius matrix of integrating factors which connects the geocentric orthonormal tetrad with the tangent spacetime of relativistic pseudo-Riemann geometry. The gravity field of the earth and its relative velocity with respect to the solar system barycentre cause coordinate effects of the order of 10 cm for topocentric point positioning.

Weak dynamical effects in the Uranian ring system

ABSTRACTA number of weak gravitational forces were examined for their kinematic effects on the Uranian ring system, in an attempt to account for residual errors of ~2 km in the ring positions and a few times 10-3deg/day in the apse precession rates. The principal conclusions are as follows: (1) perturbations by the five known satellites are too small to have observable effects with the best current estimates of the satellite masses. (2) Limits can be set on the masses of inter-ring “shepherd” satellites from their influence on precession rates and their radial perturbations on the rings. For example, the maximum mass of a shepherd satellite between rings 4 and 5 is ~ 1019gm. (3) Using the best available ring parameters, there is no convincing evidence that the rings are associated with resonances with known satellites. The only close association is of rings 5, α and γ with three-body resonances due to Miranda and Ariel. It is more likely that the putative shepherd satellites may occupy resonances. (4) The known satellites can cause anomalous residuals by changing the position of Uranus relative to the system barycenter during the occultation. The maximum change, so far, during the 1977 March 10 event, was 1.6 km. This effect will be investigated with the MIT Uranus ring model.

Has the Sun a companion star?

PULSARS are accurate timekeepers. They are believed to be rotating neutron stars, with strong magnetic fields, and the energy they radiate is at the expense of their rotational kinetic energy1. As each pulsar ages, its period P (relative to the Solar System barycentre) slowly increases, and its period derivative P( = dP/dt) slowly decreases. Certain interesting pulsars (displayed in Table 1) have anomalously small period derivatives, and rather surprisingly, are found grouped together in the same region of the sky (shown in Fig. 1). I suggest here, as an explanation of the peculiar properties of these pulsars, that the barycentre of the Solar System is accelerated, possibly because the Sun is a member of a binary system and has a hitherto undetected companion star.

Results of Optical Timing Measurements of the Crab Nebula Pulsar

Absolute time of arrival measurements were made of the optical pulses from the Crab pulsar between September 1969 and April 1970; they were corrected to the solar system barycentre. The fit to the timing data indicates that the slowdown is due to magnetic dipole radiation, but there are significant deviations indicating the presence of small fluctuations and major jumps. There is no evidence of the quasi-sinusoidal behaviour reported by Arecibo. These measurements allowed an integrated light curve to be constructed with high precision.