Paul E. Boynton

Decrease in the orbital period of Hercules X-1

From a pulse-timing analysis of Ginga observations of the binary X-ray pulsar Her X-1 obtained during the interval 1989 April-June we have determined local orbital parameters for a Short High state. We have also determined an orbital epoch in the adjacent Main High state. By comparing these orbital solutions with previously published results, we have detected a decrease in the orbital period for Her X-1 at an average rate of dot-P/P = (- 1.32 +/- 0.16) x 10(exp -8) yr(exp -1) over the interval 1971-1989. This is substantially larger than the value predicted from current estimates of the mass transfer rate, and motivates consideration of other mechanisms of mass transfer and/or mass loss. A second result from these observations is a close agreement between orbital parameters determined separately in Main High and Short High states. This agreement places strong constraints on the obliquity of the stellar companion, HZ Her, if undergoing forced precession with a 35 day period. As a consequence further doubt is placed on the slaved-disk model as the underlying cause of the 35 day cycle in Her X-1.

The 35 Day Evolution of the Hercules X-1 Pulse Profile: Ginga Observations and Their Implications

We observed Her X-1 using the Ginga observatory in the spring of 1989 with the primary intention of studying the evolution of the pulse profile through the course of the 35 day X-ray HIGH-LOW cycle in that source. These observations cover 16 separate days in two MAIN HIGH states and in the intervening SHORT HIGH state. We have augmented these data with four additional Ginga observations of Her X-1 taken for other purposes but useful for our study. We present light curves in the 1-37 keV energy band for the seven high states covered by these data together with a representative sample of pulse profiles. The signal-to-noise ratio for these profiles is generally excellent, and collectively they provide a sound base for studying the evolution of the pulse profile. Of particular utility is the 1989 May observation, which for the first time provides extensive coverage of a short high state at high photon counting rate. By combining pulse phase and frequency information from all three high state observations in 1989, we are able to determine the phase alignment of main high and short high pulse profiles with high confidence. We identify components in the Her X-1 pulse profile by their distinctive spectral signatures, and we establish the existence of a definite, repetitive pattern of pulse shape variations tied to the 35 day high-low cycle. Comparing pre-Ginga observations of pulse profiles superposed according to the 35 day phase indicates that this pattern has persisted over the past two decades. Moreover, the pulse phase alignment of the 1989 data allows the identification of components common to the main high and short high pulse profiles. One of the key elements of this pattern is the rapid change in pulse profile that occurs roughly 7 days into each main high state. In examining possible mechanisms for this interval of accelerated evolution, we are led to consider a class of models that involve dynamical changes in accretion flow geometry arising from neutron star obliquity. These models, however, suggest asymmetries in the X-ray illumination of the companion star that may conflict with extensive optical observations of the Her X-1 system. By contrast, kinematic changes in the geometrical aspect of obscuring matter flows near or within the magnetosphere that are tied to accretion disk precession provide a mechanism that may be consistent with observations.

Discovery of orbital decay in SMC X-1

We report on the results of three observations of the binary X-ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X-ray pulsations yield Doppler delay curves which, in turn, enable the most accurate determination of the SMC X-1 orbital parameters available to date. Epochs of phase zero for the 3.9 day orbit were determined for 1987 May, 1988 August, and 1989 August with accuracies of 13, 0.6, and 3 s, respectively. These epochs are combined with two previous determinations of the orbital epoch to yield the rate of change in the orbital period dot-P(orb)/P(orb) = ( 3.36 +/- 0.02) x 10(exp -6) yr(exp -1). An interpretation of the orbital decay is made in the context of tidal evolution, with consideration of the influence of the increasing moment of inertia of the companion star due to its nuclear evolution. We find that, while the orbital decay is probably driven by tidal interactions, the asynchronism between the orbit and the rotation of the companion star is most likely maintained by the evolutionary expansion of the companion star (Sk 160) rather than via the Darwin instability. In this case Sk 160 is likely to be in the hydrogen shell burning phase of its evolution. Finally, a discussion is presented of the relation among the time scales for stellar evolution (less than 10(exp 7) yr), orbital decay (3 x 10(exp 5) yr), and neutron-star spin-up in the SMC X-1 system (2000 yr). In particular, we present the result of a self-consistent calculation for the histories of the spin of the neutron star and the mass transfer in this system. A plausible case can be made for the spin-up time scale being directly related to the lifetime of the luminous X-ray phase which will end in a common-envelope phase within a time of less than approx. 10(exp 4) yr.

Further Ginga Observations of PSR B0540–69

We report the pulse-timing history of PSR B0540-69, based on all applicable data from the Ginga X-ray observatory. Using these data, together with other, overlapping optical and X-ray observations, we have determined an improved value for the braking index, n=2.080, as well as a new estimate for the celestial position of PSR B0540-69. Our results are based on an analysis of local pulse frequencies, using a methodology designed to efficiently utilize the sparsely sampled observations of PSR B0540-69. This strategy rests on iteratively determining a sequence of successively more precise pulse ephemerides from successively more precise estimates of local pulse frequencies. We also present improved X-ray and optical pulse profiles and evidence for a close alignment between profiles in these two energy regimes. We may have detected a small, Crab-like glitch in the pulse frequency of this pulsar in a reanalysis of previously published optical data.

The timing noise of PSR 0823+26, 1706-16, 1749-28, 2021+51 and the anomalous braking indices

We have investigated the stability of the pulse frequency second derivatives (ν) of PSR 0823+26, 1706-16, 1749-28 and 2021+51 that show significant quadratic trends in their pulse-frequency histories in order to determine whether the observed second derivatives are secular or if they arise as part of noise processes. We have used time-of-arrival (TOA) data extending to more than three decades, which are the longest time-spans ever taken into account in pulse-timing analyses. We investigated the stability of the pulse-frequency second derivative in the framework of low-resolution noise power spectra estimated from the residuals of pulse frequency and TOA data. We have found that the ν terms of these sources arise from the red torque noise in the fluctuations of pulse-frequency derivatives, which may originate from the external torques from the magnetosphere of the pulsar.

Testing Newtonian gravity in the laboratory

The search for ever smaller violations of Einsteins weak equivalence principle continues to be an important experimental goal at the close of the 20th century, with string theory providing new motivations. Currently, the most sensitive laboratory and terrestrial-scale tests are carried out with instrumentation based on the 18th century concept of the torsion pendulum, but not with 18th century technology. The torsion pendulum has experienced a renaissance since the pioneering work of Robert Dicke in the early 1960s. This presentation describes a new observable associated with large-amplitude pendulum oscillations that provides measurement of extremely small torques with significant freedom from effects that may limit the traditional, non-cryogenic applications of the torsion pendulum.

Testing the inverse-square law of gravity: a new class of torsion pendulum null experiments

We discuss two null experiments designed to test the inverse-square law of gravity in the critical range between and with higher precision than earlier work. To do so, we have devised a torsion pendulum detector whose mass distribution is specifically configured to provide high-sensitivity detection of a uniquely non-Newtonian derivative of the potential (the horizontal derivative of the Laplacian), rather than looking for a small deviation from the expected power-law dependence on distance of a Newtonian field derivative. This method provides a stronger null test of the gravitational inverse-square law force because it is less sensitive to imperfections in the source mass. We discuss the design of these experiments and estimate their performance relative to currently established experimental limits on inverse-square law violation.

Nutating subreflector for a millimeter wave telescope

Nutating a Cassegrain telescope’s secondary mirror is a convenient method of steering the telescope beam through a small angle. Using this principle we have constructed a high performance beam switch for a millimeter wave telescope. A low mass, graphite‐epoxy laminate secondary mirror is driven by linear electric motors operated in a frequency compensated control loop. By design, the nutator exerts little net oscillating torque on the telescope structure, resulting in virtually vibration free operation. The inherent versatility of beam switching by subreflector nutation allowed us to test a variety of switching waveforms without making any hardware changes. To reduce the peak stresses on the mechanical linkage between the mirror and the motors and to attain a minimum transition time between mirror positions, we use a digitally generated driving waveform tailored to the system’s transfer function. The nutator can shift the telescope beam by 10 arcminutes, a 1.25° rotation of the 75‐cm‐diam secondary mirror...

Gravitation physics at BGPL

Abstract We report progress on a program of gravitational physics experiments using cryogenic torsion pendula undergoing large-amplitude torsion oscillation. This program includes tests of the gravitational inverse square law and of the weak equivalence principle. Here, we describe our ongoing search for inverse-square-law violation at a strength down to 10−5 of standard gravity. The low-vibration environment provided by the Battelle Gravitation Physics Laboratory (BGPL) is uniquely suited to this study.

Optical Observations of Binary X-Ray Sources

Compact galactic X-ray sources are undeniably an exciting discovery by the New Astronomy of X-ray observations. Some of these objects were studied briefly, and without much notice, with optical telescopes long before they were known to be sources of high-energy photons. These earlier observations gave no indication of the special properties which compel us at this time to consider the possiblity that systems of this class contain such bizarre objects as neutron stars or black holes.