Joseph H. Taylor

Pulsar distances and the galactic distribution of free electrons

We describe a quantitative model for the distribution of free electrons in the Galaxy, with particular emphasis on its utility for estimating pulsar distances from dispersion measures. Contrary to past practice, we abandon the assumption of an axisymmetric Galaxy. Instead, we explicitly incorporate spiral arms, the shapes and locations of which are derived from existing radio and optical observations of H II regions. Additional parameters of the model include the electron densities of outer and inner axisymmetric components, as well as of the spiral arms; scale lengths for the r- and z-dependences of the axisymmetric features and the width and scale height of the arms; and fluctuation parameters used to relate the dispersion and scattering contributions of the outer, inner, and spiral arm components of the model

On the orbital period change of the binary pulsar PSR-1913+16

The theoretical significance of the raw observational parameter called the rate of orbital period change, P b obs , of the binary pulsar PSR 1913+16 is reexamined. It is shown that the current precision (∼ 0.8 %) on the determination of P b obs makes it necessary to take explicitly into account the effects of the galactic accelerations of the pulsar and the Sun, and that of the proper motion of the pulsar. Several other possible contributions to P b obs are (re)examined and found negligible. As the value of the galactic contribution to P b /P b depends explicitly on the distance to the pulsar, say d, the determination of d from dispersion measurements is reexamined.

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 timescales 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, μα = –1.43 ± 0.13, μδ = –0.70 ± 0.13 mas yr–1. The pulsar exhibited a small timing glitch in 2003 May, with Δf/f = 3.7 × 10–11, 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 = 1.4398 ± 0.0002 M ☉ and m 2 = 1.3886 ± 0.0002 M ☉. The systems orbital period has been decreasing at a rate 0.997 ± 0.002 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.

Timing behavior of 96 radio pulsars

We present results from observations of 104 pulsars made between 1989 August and 1993 April, including timing solutions for 96 of them. Pulse profiles were recorded at four frequencies in the range 0.4-1.64 GHz, yielding topocentric pulse arrival times with uncertainties of order 10(exp -3) periods. Models fitted to the timing data yield accurate positions, periods, period derivatives, and dispersion measures for each pulsar. Nine of the measured period derivatives are new, and most of the parameters represent improvements upon previous measurements. In a few cases we correct some erroneous parameter values from the published literature. A glitch was observed in the PSR B1800-21 pulse arrival times, and we fit a simple exponential model to the post-glitch recovery. We present graphs of the observed pulse shapes and their evolution with frequency, a table of measured pulase widths, and quantitative estimates of the long-term timing stability of each pulsar.

Gamma Radiation from PSR B1055–52

The telescopes on the Compton Gamma Ray Observatory (CGRO) have observed PSR B1055-52 a number of times between 1991 and 1998. From these data a more detailed picture of the gamma radiation from this source has been developed, showing several characteristics that distinguish this pulsar: the light curve is complex; there is no detectable unpulsed emission; the energy spectrum is flat, with no evidence of a sharp high-energy cutoff up to greater than 4 GeV. Comparisons of the gamma-ray data with observations at longer wavelengths show that no two of the known gamma-ray pulsars have quite the same characteristics; this diversity makes interpretation in terms of theoretical models difficult.

The Coalescence Rate of Double Neutron Star Systems

We estimate the coalescence rate of close binaries with two neutron stars (NS) and discuss the prospects for the detection of NS-NS inspiral events by ground-based gravitational-wave observatories, such as LIGO. We derive the Galactic coalescence rate using the observed sample of close NS-NS binaries (PSR B1913+16 and PSR B1534+12) and examine in detail each of the sources of uncertainty associated with the estimate. Specifically, we investigate (1) the dynamical evolution of NS-NS binaries in the Galactic potential and the vertical scale height of the population, (2) the pulsar lifetimes, (3) the effects of the faint end of the radio pulsar luminosity function and their dependence on the small number of observed objects, (4) the beaming fraction, and (5) the extrapolation of the Galactic rate to extragalactic distances expected to be reachable by LIGO. We find that the dominant source of uncertainty is the correction factor (up to 200) for faint (undetectable) pulsars. All other sources are much less important, each with uncertainty factors smaller than 2. Despite the relatively large uncertainty, the derived coalescence rate is consistent with previously derived upper limits, and is more accurate than rates obtained from population studies. We obtain a most conservative lower limit that the detection rate by LIGO II of about 2 events per year. Our upper limit on the rate is between 300 and 1000 events per year.

Observations of the Crab pulsar and nebula by the EGRET telescope on the Compton Gamma-Ray Observatory

The Crab pulsar and nebula were observed three times in 1991 April to June by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO): April 23 to May 7, May 16 to 30, and June 8 to 15. The results of analysis of the gamma-ray emission in the energy range from 50 MeV to more than 10 GeV are reported. The observed gamma-ray light curve exhibits two peaks separated in phase by 0.40 +/- 0.02, consistent with previous observations. The total pulsed emission from the Crab pulsar is found to be well represented by a power-law spectrum, softer than the spectrum measured by COS B (Clear et al., 1987). The interpulse emission has a harder spectrum than either of the pulses. The evidence for pulsed emission above 5 GeV in the EGRET data is not conclusive. Unpulsed emission in the energy range 50 MeV to 5 GeV was detected, with an indication of a hardening of the unpulsed spectrum above about 1 GeV. There was a significant change in the light curve over the 2 months of these observations, although the shape of the spectrum remained constant.

General relativistic geodetic spin precession in binary pulsar b1913+16: mapping the emission beam in two dimensions

We have carefully measured the pulse profile of the binary pulsar PSR B1913+16 at 21 cm wavelength for 20 years, in order to search for variations that result from general relativistic geodetic precession of the spin axis. The profile width is found to decrease with time in its inner regions, while staying essentially constant on its outer skirts. We fitted these data to a model of the beam shape and precession geometry. Four equivalent solutions are found, but evolutionary considerations and polarization data select a single preferred model. While the current data sample only a limited range of latitudes owing to the long precessional cycle, the preferred model shows a beam elongated in the latitudinal direction and hourglass-shaped.

A search for low-luminosity pulsars

The results of Phase I of the Princeton-NRAO pulsar survey, carried out at 390 MHz using the 92 m telescope at Green Bank, West Virginia, are presented. This search discovered 34 new pulsars and detected 49 previously known ones. As has been the case with all previous surveys, no pulsars with intrinsic luminosities less than 0.3 mJy/sq kpc have been found. Because of the high sensitivity of the survey and its coverage of nearly 2 sr of sky, the present results imply that such low-luminosity pulsars do not constitute a large portion of the total active pulsar population in the Galaxy. Also in common with previous surveys, the sensitivity of this one deteriorates rather quickly for pulse periods less than a few tenths of a second. The extent to which this loss of sensitivity has biased the period distribution of known pulsars is discussed. 14 references.

ORBITAL VARIABILITY IN THE ECLIPSING PULSAR BINARY PSR B1957+20

We have conducted timing observations of the eclipsing millisecond binary pulsar PSR~B1957+20, extending the span of data on this pulsar to more than five years. During this time the orbital period of the system has varied by roughly