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 $\Delta P_b/P_b = 1.6 \times 10^{-7}$, changing quadratically with time and displaying an orbital period second derivative $\ddot P_b = (1.43 \pm 0.08) \times 10^{-18}\,$s$^{-1}$. The previous measurement of a large negative orbital period derivative reflected only the short-term behavior of the system during the early observations; the orbital period derivative is now positive and increasing rapidly. If, as we suspect, the PSR~B1957+20 system is undergoing quasi-cyclic orbital period variations similar to those found in other close binaries such as Algol and RS CVn, then the $0.025\,M{_\odot}$ companion to PSR~B1957+20 is most likely non-degenerate, convective, and magnetically active.Comment: 9 pages, 3 figures, LaTeX, submitted ApJL 13 Dec. 1993, arz-00

Measurement of gravitational spin-orbit coupling in a binary pulsar system

In relativistic gravity, a spinning pulsar will precess as it orbits a compact companion star. We have measured the effect of such precession on the average shape and polarization of the radiation from PSR B1534+12. We have also detected, with limited precision, special-relativistic aberration of the revolving pulsar beam due to orbital motion. Our observations fix the system geometry, including the misalignment between the spin and orbital angular momenta, and yield a measurement of the precession timescale consistent with the predictions of General Relativity.Comment: 4 pages, accepted to PRL. Version with high-resolution figure 2 available at http://www.astro.ubc.ca/people/stairs/papers/sta04b.ps.g

Probing the Masses of the PSR J0621+1002 Binary System Through Relativistic Apsidal Motion

Orbital, spin and astrometric parameters of the millisecond pulsar PSR J0621+1002 have been determined through six years of timing observations at three radio telescopes. The chief result is a measurement of the rate of periastron advance, omega_dot = 0.0116 +/- 0.0008 deg/yr. Interpreted as a general relativistic effect, this implies the sum of the pulsar mass, m_1, and the companion mass, m_2, to be M = m_1 + m_2 = 2.81 +/- 0.30 msun. The Keplerian parameters rule out certain combinations of m_1 and m_2, as does the non-detection of Shapiro delay in the pulse arrival times. These constraints, together with the assumption that the companion is a white dwarf, lead to the 68% confidence maximum likelihood values of m_1 = 1.70(+0.32 -0.29) msun and m_2 =0.97(+0.27 - 0.15) msun and to the 95% confidence maximum likelihood values of m_1 = 1.70(+0.59 -0.63) msun and m_2 = 0.97(+0.43 -0.24) msun. The other major finding is that the pulsar experiences dramatic variability in its dispersion measure (DM), with gradients as steep as 0.013 pc cm^{-3} / yr. A structure function analysis of the DM variations uncovers spatial fluctuations in the interstellar electron density that cannot be fit to a single power law, unlike the Kolmogorov turbulent spectrum that has been seen in the direction of other pulsars. Other results from the timing analysis include the first measurements of the pulsar's proper motion, mu = 3.5 +/- 0.3 mas / yr, and of its spin-down rate, dP/dt = 4.7 x 10^{-20}, which, when corrected for kinematic biases and combined with the pulse period, P = 28.8 ms, gives a characteristic age of 1.1 x 10^{10} yr and a surface magnetic field strength of 1.2 x 10^{9} G.Comment: Accepted by ApJ, 10 pages, 5 figure

The Triple Pulsar System PSR B1620-26 in M4

The millisecond pulsar PSR B1620-26, in the globular cluster M4, has a white dwarf companion in a half-year orbit. Anomalously large variations in the pulsar's apparent spin-down rate have suggested the presence of a second companion in a much wider orbit. Using timing observations made on more than seven hundred days spanning eleven years, we confirm this anomalous timing behavior. We explicitly demonstrate, for the first time, that a timing model consisting of the sum of two non-interacting Keplerian orbits can account for the observed signal. Both circular and elliptical orbits are allowed, although highly eccentric orbits require improbable orbital geometries. The motion of the pulsar in the inner orbit is very nearly a Keplerian ellipse, but the tidal effects of the outer companion cause variations in the orbital elements. We have measured the change in the projected semi-major axis of the orbit, which is dominated by precession-driven changes in the orbital inclination. This measurement, along with limits on the rate of change of other orbital elements, can be used to significantly restrict the properties of the outer orbit. We find that the second companion most likely has a mass m~0.01 Msun --- it is almost certainly below the hydrogen burning limit (m<0.036 Msun, 95% confidence) --- and has a current distance from the binary of ~35 AU and orbital period of order one hundred years. Circular (and near-circular) orbits are allowed only if the pulsar magnetic field is ~3x10^9 G, an order of magnitude higher than a typical millisecond pulsar field strength. In this case, the companion has mass m~1.2x10^-3 Msun and orbital period ~62 years.Comment: 12 pages, 6 figures, 3 tables. Very minor clarifications and rewording. Accepted for publication in the Astrophys.

Timing measurements and proper motions of 74 pulsars using the Nanshan radio telescope

We have measured the positions of 74 pulsars from regular timing observations using the Nanshan radio telescope at Urumqi Observatory between 2000 January and 2004 August (MJD 51500 -- 53240). Proper motions were determined for these pulsars by comparing their current positions with positions given in pulsar catalogues. We compare our results to earlier measurements in the literature and show that, in general, the values agree. New or improved proper motions are obtained for 16 pulsars. The effect of period fluctuations and other timing noise on the determination of pulsar positions is investigated. For our sample, the mean and rms transverse velocities are 443 and 224 km/s respectively, agreeing with previous work even though we determine distances using the new NE2001 electron density model.Comment: 9 pages, 7 figures and 3 tables. Accepted by MNRA

Discovery of an optical bow-shock around pulsar B0740-28

We report the discovery of a faint H-alpha pulsar wind nebula (PWN) powered by the radio pulsar B0740-28. The characteristic bow-shock morphology of the PWN implies a direction of motion consistent with the previously measured velocity vector for the pulsar. The PWN has a flux density more than an order of magnitude lower than for the PWNe seen around other pulsars, but, for a distance 2 kpc, it is consistent with propagation through a medium of atomic density n_H ~ 0.25 cm^{-3}, and neutral fraction of 1%. The morphology of the PWN in the area close to the pulsar is distinct from that in downstream regions, as is also seen for the PWN powered by PSR B2224+65. In particular, the PWN associated with PSR B0740-28 appears to close at its rear, suggesting that the pulsar has recently passed through a transition from low density to high density ambient gas. The faintness of this source underscores that deep searches are needed to find further examples of optical pulsar nebulae.Comment: 5 pages, 1 figure, to appear in Astronomy & Astrophysics Letter

Measurement of Relativistic Orbital Decay in the PSR B1534+12 Binary System

We have made timing observations of binary pulsar PSR B1534+12 with radio telescopes at Arecibo, Green Bank, and Jodrell Bank. By combining our new observations with data collected up to seven years earlier, we obtain a significantly improved solution for the astrometric, spin, and orbital parameters of the system. For the first time in any binary pulsar system, no fewer than five relativistic or "post-Keplerian" orbital parameters are measurable with useful accuracies in a theory-independent way. We find the orbital period of the system to be decreasing at a rate close to that expected from gravitational radiation damping, according to general relativity, although the precision of this test is limited to about 15% by the otherwise poorly known distance to the pulsar. The remaining post-Keplerian parameters are all consistent with one another and all but one of them have fractional accuracies better than 1%. By assuming that general relativity is the correct theory of gravity, at least to the accuracy demanded by this experiment, we find the masses of the pulsar and companion star each to be 1.339+-0.003 Msun and the system's distance to be d = 1.1+-0.2 kpc, marginally larger than the d ~ 0.7 kpc estimated from the dispersion measure. The increased distance reduces estimates of the projected rate of coalescence of double neutron-star systems in the universe, a quantity of considerable interest for experiments with terrestrial gravitational wave detectors such as LIGO.Comment: 17 pages, 4 figures, submitted to the Ap