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We report the discovery of three binary millisecond pulsars during the Parkes Multibeam Pulsar Survey of the Galactic Plane. The objects are highly recycled and are in orbits of many tens of days about low-mass white-dwarf companions. The eccentricity of one object, PSR J1853+1303, is more than an order of magnitude lower than predicted by the theory of convective fluctuations during tidal circularization. We demonstrate that, under the assumption that the systems are randomly oriented, current theoretical models of the core-mass--orbital-period relation for the progenitors of these systems likely overestimate the white-dwarf masses, strengthening previous concerns about the match of these models to the data. The new objects allow us to update the limits on violation of relativistic equivalence principles to 95% confidence upper limits of 5.6 x 10^-3 for the Strong Equivalence Principle parameter Delta and 4.0 x 10^-20 for the Lorentz-invariance/momentum-conservation parameter alpha_3., 8 pages; accepted to ApJ. Small changes to parameters from including correct Parkes clock corrections; changes/corrections to Figures 1, 4 and 5; minor changes to text and references. The limit on Delta has increased from 0.0055 to 0.0056

A new Bayesian tool for nonlinear model fitting has been developed, that employs a parallel tempering Markov Chain Monte Carlo algorithm with a novel statistical control system. The algorithm has been used to re-analyze precision radial velocity data for HD 73526. For a single planet model, three possible orbits were found with periods of 127.88 +0.37

We explore a new method for investigating the density and velocity structure of the Be star equatorial disk in LS I +61°303, based on its variable radio emission. At a particular radius the method yields a circularly shaped solution locus in the radial (vr) and circular (vc) components of the disk gas velocity, which allows us to set limits on vr and vc at that radius. For a given model of the circular velocity, vc(r), we can derive vr(r) and the gas density, ρ(r), within a constant. While ρ(r) depends on the assumed model for vc(r), we find that the fractional change in gas density at any radius does not. This allows us to study the evolution in ρ(r) with time. The analysis indicates that the previously demonstrated ~4.6 yr modulation in radio properties results from an outward-moving density enhancement or shell in the equatorial disk with a velocity of ~1.0 km s-1. We propose that each new shell ejection may be triggered by the interaction of a short-lived relativistic wind (ejector phase) from the neutron star with the rapidly rotating Be star. Our best estimates of the mass accretion rate of the neutron star are in the range ~0.001 to ~0.01 of the Eddington accretion limit. This translates to an expected luminosity range of ~1035 to ~1036 ergs s-1, which is comparable to estimates of the total X-ray and γ-ray luminosity for LS I +61°303.

The binary star LS I +61303 is remarkable for its periodic radio outbursts every 26.5 days. We recently discovered a 4.4 year periodic modulation of the phase and amplitude of these outbursts using the Gregory-Loredo Bayesian algorithm for detecting periodic signals of unknown shape. In this paper we obtain improved estimates of both the orbital period (P1) and modulation period (P2) based on a larger data set and examine the behavior of the spectral index between 2.2 and 8.3 GHz, versus modulation period. The new estimates are P1 =2 6.4960 ± .0028 days and P2 = 1667 ± 8 days and our best estimate of the radio phase of periastron is 0.4. Analysis of the spectral index data indicates that the optical depth in the synchrotron emission region is always << 1a t 8.3 GHz, and can reach values 2.7 at 2.2 GHz. A test of the precessing Be star model of Lipunov and Nazin indicates that it is unlikely to be the correct mechanism to explain the 1667 day periodic modulation.

We extend the Bayesian method of Gregory and Loredo for the detection of a periodic signal of unknown shape and period to the case where the noise sampling distribution is independent Gaussian. The analysis readily handles nonuniformly sampled data and allows for an unknown noise variance. The method is applied to the radio astronomy data for the interesting X-ray binary system LS I +61°303, which exhibits periodic radio outbursts with a period of 26.5 days. Several authors have suggested that the peak flux density of the outbursts exhibit a periodic or quasi-periodic modulation of approximately 1600 days. Our Bayesian analysis of the outburst peak flux densities provides strong support for such a modulation. We derive the posterior probability density function of the modulation period and the estimated mean shape of the modulation based on the available flux density data. The probability density for the modulation period exhibits a broad peak in the range 1599-1660 days (68% credible region) with a mean value of 1632 days. The rms flux density deviation from the mean shape, amounting to 45 mJy, is much larger than the measurement errors of ≈10 mJy, which suggests additional complexity in the source that is yet to be understood. The next maximum in the long-term flux modulation is predicted to occur near 1999 July 22 (JD 2,451,382).

A Bayesian analysis of the phase variations of the 26.5 day periodic radio outbursts from the high-mass X-ray binary LS I +61°303 demonstrates a clear periodic modulation on a timescale similar to that previously found for the long-term modulation of the outburst peak flux density. Combining the outburst phase and flux information we obtain a phase and flux modulation period of 1584+14-11 days as well as a more accurate outburst period of 26.4917 ± 0.0025 days. From the shape of the phase and outburst flux modulation we find that larger outbursts occur at an earlier orbital phase, closer to periastron, probably as a result of variations in the wind from the rapidly rotating Be star primary. The phase modulation also suggests a rather sudden onset to each new cycle of mass loss by the Be star. The next maximum in long-term flux modulation is predicted to occur between 1999 February and 2000 March (JD 2,451,233-2,451,633).

We briefly summarize the main features of our Bayesian method for detecting and measuring periodic signals in event arrival time data, and compare it to the more common fast Fourier transform (FFT) and epoch folding (EF) methods in a case study using ROSAT observations of the X-ray pulsar PSR 0540-693. Although originally detected using an FFT analysis of Einstein Observatory data (Seward, Harnden, & Helfand), the same approach failed to detect the pulsar signal in ROSAT data. We show that both EF and our Bayesian method can reliably detect the pulsar in the ROSAT data, and we demonstrate several important advantages of the Bayesian method. These include the ease of interpretation of the results and more accurate period determinations. Furthermore, the Bayesian approach accounts for our prior uncertainty in the signal period and phase in a manner that does not require specification of the number and location of "independent" frequencies or phases, an aspect of more traditional approaches that lends a troubling subjectivity to conclusions based on them. Finally, we present the shape of the pulsar light curve derived from the ROSAT data by using our Bayesian method.

An optical, intensified CCD observation of the field of the 3.49 s X-ray pulsar lE 2259+586 is discussed. Two stars with B = 22.0 and B = 21.3 are found to lie 4.''9 and 6.''3 away from the nominal X-ray position determined by the High Resolution Imager (HRI) camera of the Einstein Observatory. The fainter star is identified as the optical counterpart to the pulsar on the basis of recently detected pulsations. No other star with B< or approx. =23 is found within an 8'' radius circle around the HRI position. The absence of a bright optical counterpart rules out a massive binary companion for the pulsar. If the pulsar has a binary companion, it must be a low-mass main-sequence dwarf for a degenerate star. The implication of the binary hypothesis is that the neutron star was formed in a Type I supernova explosion which occurred in a preexisting close binary configuration. The possibility that the pulsar is powered by accreting matter from a molecular cloud observed at the western edge of the supernova remnant surrounding the pulsar is briefly discussed.

The formulation of generalized models tracing the geometry and intensity of the synchrotron emission from precessing, twin, relativistic jets as projected on the plane of the sky is presented. It is shown that neither the shape of the image nor its relative intensities are altered by including the effects of a cosmological redshift and a relative velocity between the source and observer. The models are fitted to the available data for several quasars and radio galaxies and demonstrate the plausibility of the phenomenon. Probable selection effects are considered and diagnostics given for recognizing objects showing this behavior. In the radio galaxies considered, velocities up to about 0.2c and precession periods of 1,000,000 yr are deduced. In the QSOs investigated, velocities of 0.7c and greater are found and periods of order 10,000 yr. In some cases precession cone angles increase with time. Consequences in terms of lifetimes of QSO behavior and binary supermassive objects are discussed.

An attempt to detect thermal radio emission from a compact circumstellar cloud about the infrared star NML Cyg has been carried out at three frequencies, 2.7, 8.1, and 10.5 GHz. Although positive results were obtained with single-dish observations at 10.5 GHz, the radio emission is not from a circumstellar cloud about NML Cyg. Instead it is postulated that the emission is from an H ii region with an angular extent of approx.2'. The red print of the Sky Survey shows a faint nebulosity of comparable angular size overlapping the star's position, lending support to this interpretation. The interferometer observations at 2.7 and 8.1 GHz provide an upper limit on the radio emission from any compact circumstellar cloud about NML Cyg of 2.8 mJy, which is well below the flux density expected for the absorbing cloud postulated by Davies et al. (1972). (AIP)

We preent the results of simultaneous observations of the periodic source LSI+61/sup 0/303 at 5.0 and 10.5 GHz during the radio outburst of 1981 August--September. The source is optically thin during both the latter portion of the rise and the subsequent decay of the outburst. From analysis of the data we derive a source dimension of approx.10/sup 13/ cm and magnetic field strength of approx.1 gauss. We conclude that adiabatic expansion is the dominant energy loss mechanism for the radiating electrons. Fits to the flux density decay indicate an electron energy distribution N(E)proportionalE/sup -1.6plus-or-minus0.1/. The flat spectrum implies continued particledagger injection during the rise of wind and supercritical accretion. The main features of the radio emission are best accounted for by a model of multiple shock fronts produced as a result of supercritical Roche lobe overflow at periastron of a highly eccentric orbit. Some predictions of the model are pointed out.