Using Binary Pulsars to Test Lorentz Symmetry in the Gravitational Sector

I review some of the major developments in the theoretical background and experimental uses of binary pulsars to explore local Lorentz invariance in the gravitational sector and its possible violation.

A Temporal Map in Geostationary Orbit: The Cover Etching on the EchoStar XVI Artifact

Geostationary satellites are unique among orbital spacecraft in that they experience no appreciable atmospheric drag. After concluding their respective missions, geostationary spacecraft remain in orbit virtually in perpetuity. As such, they represent some of human civilizations longest lasting artifacts. With this in mind, the EchoStar XVI satellite, to be launched in fall 2012, will play host to a time capsule intended as a message for the deep future. Inspired in part by the Pioneer Plaque and Voyager Golden Records, the EchoStar XVI Artifact is a pair of gold-plated aluminum jackets housing a small silicon disc containing one hundred photographs. The Cover Etching, the subject of this paper, is etched onto one of the two jackets. It is a temporal map consisting of a star chart, pulsar timings, and other information describing the epoch from which EchoStar XVI came. The pulsar sample consists of 13 rapidly rotating objects, 5 of which are especially stable, having spin periods < 10 ms and extremely small spindown rates. In this paper, we discuss our approach to the time map etched onto the cover and the scientific data shown on it; and we speculate on the uses that future scientists may have for its data. The other portions of the EchoStar XVI Artifact will be discussed elsewhere.

Timing Measurements of the Relativistic Binary Pulsar PSR B1913+16

We present results of more than three decades of timing measurements of the first known binary pulsar, PSR B1913+16. Like most other pulsars, its rotational behavior over such long time scales is significantly affected by small-scale irregularities not explicitly accounted for in a deterministic model. Nevertheless, the physically important astrometric, spin, and orbital parameters are well determined and well decoupled from the timing noise. We have determined a significant result for proper motion, $mu_{alpha} = -1.43pm0.13$, $mu_{delta}=-0.70pm0.13$ mas yr$^{-1}$. The pulsar exhibited a small timing glitch in May 2003, with ${Delta f}/f=3.7times10^{-11}$, and a smaller timing peculiarity in mid-1992. A relativistic solution for orbital parameters yields improved mass estimates for the pulsar and its companion, $m_1=1.4398pm0.0002 M_{sun}$ and $m_2=1.3886pm0.0002 M_{sun}$. The systems orbital period has been decreasing at a rate $0.997pm0.002$ times that predicted as a result of gravitational radiation damping in general relativity. As we have shown before, this result provides conclusive evidence for the existence of gravitational radiation as predicted by Einsteins theory.

A Search for Neutron Star Precession and Interstellar Magnetic Field Variations via Multiepoch Pulsar Polarimetry

In order to study precession and interstellar magnetic field variations, we measured the polarized position angle of 81 pulsars at several-month intervals for four years. We show that the uncertainties in a single-epoch measurement of position angle is usually dominated by random pulse-to-pulse jitter of the polarized subpulses. Even with these uncertainties, we find that the position angle variations in 19 pulsars are significantly better fitted (at the 3 {sigma} level) by a sinusoid than by a constant. Such variations could be caused by precession, which would then indicate periods of ~ (200 - 1300) d and amplitudes of ~(1 - 12) degrees. We narrow this collection to four pulsars that show the most convincing evidence of sinusoidal variation in position angle. Also, in a handful of pulsars, single discrepant position angle measurements are observed which may result from the line of sight passing across a discrete ionized, magnetized structure. We calculate the standard deviation of position angle measurements from the mean for each pulsar, and relate these to limits on precession and interstellar magnetic field variations.

Arecibo HI Absorption Measurements of Pulsars and the Electron Density at Intermediate Longitudes in the First Galactic Quadrant

We have used the Arecibo telescope to measure the HI absorption spectra of eight pulsars. We show how kinematic distance measurements depend upon the values of the galactic constants R_o and Theta_o, and we select our preferred current values from the literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48 degrees < l < 70 degrees is [0.017 (-0.007,+0.012) (68% c.l.)] cm^(-3). This is 0.75 (-0.22,+0.49) (68% c.l.) of the value calculated by the Cordes & Lazio (2002) galactic electron density model. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the galactic center, at 30 degrees<l<48 degrees. Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16: d=(9.0 +/- 3) kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating HI structure on a ~500 AU scale.