J. M. Weisberg

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.

Evidence for alignment of the rotation and velocity vectors in pulsars

We present strong observational evidence for a relationship between the direction of a pulsars motion and its rotation axis. We show carefully calibrated polarization data for 25 pulsars, 20 of which display linearly polarized emission from the pulse longitude at closest approach to the magnetic pole. Such data allow the determination of the position angle of the linear polarization which in turn reflects the position angle of the rotation axis. Of these 20 pulsars, 10 show an offset between the velocity vector and the polarization position angle which is either less than 10° or more than 80°, a fraction which is very unlikely by random chance. We believe that the bimodal nature of the distribution arises from the presence of orthogonal polarization modes in the pulsar radio emission. In some cases, this orthogonal ambiguity is resolved by observations at other wavelengths so that we conclude that the velocity vector and the rotation axis are aligned at birth. Strengthening the case is the fact that 4 of the 5 pulsars with ages less than 3 Myr show this relationship, including the Vela pulsar. We discuss the implications of these findings in the context of the Spruit & Phinney model of pulsar birth-kicks. We point out that, contrary to claims in the literature, observations of double neutron star systems do not rule out aligned kick models and describe a possible observational test involving the double pulsar system.

Probing the interstellar medium with pulsars on AU scales

We present a new technique, multiepoch observations of 21 cm absorption against high-velocity pulsars, to probe the properties of the cold neutral hydrogen gas (H I) in the interstellar medium (ISM) at AU scales. In three epochs, over a 1.7 yr interval, we find evidence for significant opacity variations toward all of the pulsars in our sample. Small-scale structure in the ISM is detected on a range of scales from 5 AU to 100 AU, over a wide range of distances (50-2600 pc), opacities (tau(sub max) = 0.1 - 2.5) and directions (anticenter, interarm, high latitude, and local ISM). It appears that small-scale structure is a general property of the ISM and is not confined to special lines of sight. A significant fraction (10%-15%) of the cold H I gas is in this form. These opacity variations do not show any strong correlations with such parameters as transverse distance or integrated opacity, and there is no obvious relation between these structures and those seen in the ionized phase of the ISM.

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.

ON PULSAR DISTANCE MEASUREMENTS AND THEIR UNCERTAINTIES

Accurate distances to pulsars can be used for a variety of studies of the Galaxy and its electron content. However, most distance measures to pulsars have been derived from the absorption (or lack thereof) of pulsar emission by Galactic H I gas, which typically implies that only upper or lower limits on the pulsar distance are available. We present a critical analysis of all measured H I distance limits to pulsars and other neutron stars, and translate these limits into actual distance estimates through a likelihood analysis that simultaneously corrects for statistical biases. We also apply this analysis to parallax measurements of pulsars in order to obtain accurate distance estimates and find that the parallax and H I distance measurements are biased in different ways, because of differences in the sampled populations. Parallax measurements typically underestimate a pulsars distance because of the limited distance to which this technique works and the consequential strong effect of the Galactic pulsar distribution (i.e., the original Lutz-Kelker bias), in H I distance limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading to overestimated distances because the bright pulsars on which this technique is applicable are more likely to be nearby given their brightness.

The eclipsing millisecond pulsar PSR 1957 + 20

Information obtained over the past year on the eclipsing millisecond pulsar PSR 1957 + 20 and its orbiting companion is discussed. The pulsar is found to be similar in many ways to other millisecond pulsars: its spin parameters are extremely stable, its period derivative is very small, its profile has a strong interpulse, and its radio spectrum has a steep power-law index. The orbit is nearly circular, and the mass function implies a companion mass not much greater than 0.022 solar. Eclipses last for approximately 56 and 50 min at 318 and 430 MHz, respectively, corresponding to a nu exp - 0.41 + or - 0.09 dependence of eclipse duration on frequency. The available evidence points strongly toward a system in which the radiation from the pulsar heats the companion to the point of ablation, thereby driving a stellar wind that trails outward and behind the companion. 28 refs.

Profile morphology and polarization of young pulsars

We present polarization profiles at 1.4 and 3.1 GHz for 14 young pulsars with characteristic ages less than 75 kyr. Careful calibration ensures that the absolute position angle of the linearly polarized radiation at the pulsar is obtained. In combination with previously published data, we draw three main conclusions about the pulse profiles of young pulsars. (i) Pulse profiles are simple and consist of either one or two prominent components. (ii) The linearly polarized fraction is nearly always in excess of 70 per cent. (iii) In profiles with two components, the trailing component nearly always dominates, only the trailing component shows circular polarization and the position angle swing is generally flat across the leading component and steep across the trailing component. Based on these results, we can make the following generalizations about the emission beams of young pulsars. (i) There is a single, relatively wide cone of emission from near the last open field lines. (ii) Core emission is absent or rather weak. (iii) The height of the emission is between 1 and 10 per cent of the light cylinder radius.

Arecibo 430 MHz Pulsar Polarimetry: Faraday Rotation Measures and Morphological Classifications

We have measured Faraday rotation measures (RMs) at Arecibo Observatory for 36 pulsars, 17 of them new. We combine these and earlier measurements to study the Galactic magnetic field and its possible temporal variations. Many RM values have changed significantly on several-year timescales, but these variations probably do not reflect interstellar magnetic field changes. By studying the distribution of pulsar RMs near the plane in conjunction with the new NE2001 electron density model, we note the following structures in the first Galactic longitude quadrant: (1) the local field reversal can be traced as a null in RM in a 0.5 kpc wide strip interior to the Solar circle, extending ~7 kpc around the Galaxy. (2) Steadily increasing RMs in a 1 kpc wide strip interior to the local field reversal, and also in the wedge bounded by 42° < l < 52°, indicate that the large-scale field is approximately steady from the local reversal in to the Sagittarius arm. (3) The RMs in the 1 kpc wide strip interior to the Sagittarius arm indicate another field reversal in this strip. (4) The RMs in a final 1 kpc wide interior strip, straddling the Scutum arm, also support a second field reversal interior to the Sun, between the Sagittarius and Scutum arms. (5) Exterior to the nearby reversal, RMs from 60° < l < 78° show evidence for two reversals, on the near and far side of the Perseus arm. (6) In general, the maxima in the large-scale fields tend to lie along the spiral arms, while the field minima tend to be found between them. We have also determined polarized profiles of 48 pulsars at 430 MHz. We present morphological pulse profile classifications (Rankin 1983) of the pulsars, based on our new measurements and previously published data.

PSR J1829+2456: a relativistic binary pulsar

We report the discovery of a new binary pulsar, PSR J1829+2456, found during a midlatitude drift-scan survey with the Arecibo telescope. Our initial timing observations show the 41-ms pulsar to be in a 28-h, slightly eccentric, binary orbit. The advance of periastron ˙ ω = 0. ◦ 28 ± 0. ◦ 01 yr −1 is derived from our timing observations spanning 200 d. Assuming that the advance of periastron is purely relativistic and a reasonable range of neutron star masses for PSR J1829+2456, we constrain the companion mass to be between 1.22 and 1.38 M� , making it likely to be another neutron star. We also place a firm upper limit on the pulsar mass of 1.38 M� . The expected coalescence time due to gravitational wave emission is long (∼60 Gyr), and this system will not significantly impact upon calculations of merger rates that are relevant to upcoming instruments such as LIGO.