R. J. Dewey

Monte Carlo simulations of radio pulsars and their progenitors

Standard models of binary evolution were applied to a model of the main-sequence population to trace the paths by which a massive star may evolve into a neutron star. Using three different models of binary evolution, the relative number of neutron stars formed by each path was calculated. It was found that none of the models were able to reproduce both the observed velocity distribution of radio pulsars and the observed incidence of binary pulsars. 59 references.

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.

Results of two surveys for fast pulsars

Results are reported from two surveys designed to detect fast pulsars - one carried out at 390 MHz using the 92 m telescope at Green Bank, and the other done at 430 MHz with the 305 m Arecibo antenna. Twenty-five new pulsars were discovered. Six pulsars were detected with periods under 300 msec, but none was seen with under 100 msec. One of the new pulsars, PSR 1855 + 09, has a period of 5.362 msec and is a member of a binary system with orbital period 12.3 d; it is the third millisecond pulsar and the sixth binary pulsar known. The 1.5 msec pulsar PSR 1937 + 21, was detected at Arecibo. The two surveys place strong upper limits on the population of pulsars with periods of 10-100 msec in the Galaxy. 33 references.

The formation of the double pulsar PSR J0737−3039A/B

Recent timing observations of the double pulsar J0737−3039A/B have shown that its transverse velocity is extremely low, only 10 km s −1 , and nearly in the plane of the Galaxy. With this new information, we rigorously re-examine the history and formation of this system, determining estimates of the pre-supernova companion mass, supernova kick and misalignment angle between the pre- and post-supernova orbital planes. We find that the progenitor to the recently formed ‘B’ pulsar was probably less than 2 M � , lending credence to suggestions that this object may not have formed in a normal supernova involving the collapse of an iron core. At the same time, the supernova kick was likely non-zero. A comparison to the history of the double neutron star binary B1534+12 suggests a range of possible parameters for the progenitors of these systems, which should be taken into account in future binary population syntheses and in predictions of the rate and spatial distribution of short gamma-ray burst events.

Studies of the Relativistic Binary Pulsar PSR B1534+12. II. Origin and Evolution

We have recently measured the angle between the spin and orbital angular momenta of PSR B1534+12 to be either 25° ± 4° or 155° ± 4°. This misalignment was almost certainly caused by an asymmetry in the supernova explosion that formed its companion neutron star. Here we combine the misalignment measurement with measurements of the pulsar and companion masses, the orbital elements, proper motion, and interstellar scintillation. We show that the orbit of the binary in the Galaxy is inconsistent with a velocity kick large enough to produce a nearly antialigned spin axis, so the true misalignment must be ~25°. Similar arguments lead to bounds on the mass of the companion star immediately before its supernova: 3 ± 1 M☉. The result is a coherent scenario for the formation of the observed binary. After the first supernova explosion, the neutron star that would eventually become the observed pulsar was in a Be/X-ray-type binary system with a companion of at least 10-12 M☉. During hydrogen (or possibly helium) shell burning, mass transfer occurred in a common envelope phase, leaving the neutron star in a roughly half-day orbit with a helium star with mass above ~3.3 M☉. A second phase of mass transfer was then initiated by Roche lobe overflow during shell helium burning, further reducing both the helium star mass and orbital period before the second supernova. Scenarios that avoid Roche lobe overflow by the helium star require larger helium star masses and predict space velocities inconsistent with our measurements. The companion neutron star experienced a velocity kick of 230 ± 60 km s-1 at birth, leading to a systemic kick to the binary of 180 ± 60 km s-1. The direction of the kick was roughly opposed to the instantaneous orbital velocity of the companion.

A new binary pulsar in a highly eccentric orbit

We report the discovery of PSR 2303+46, the fifth radio pulsar known to be in a gravitationally bound orbit around another star. The pulsar period (1.066 s) and the orbital eccentricity (0.658) are the largest amount the five binary systems, while the orbital period (12./sup d/34) lies near the middle of the range. Evolutionary considerations suggest strongly that the companion is another neutron star. The general relativistic precession of periastron should be observable within 1 or 2 yr and, when measured, will specify the total mass of the two stars.

Improved parameters for four binary pulsars

Timing observations of four of the seven known binary pulsars, PSRs 0655 + 64, 0820 + 02, 1831 - 00, and 2303 + 46, have yielded much improved measurements of their celestial coordinates, periods, period derivatives, and orbital parameters. New upper limits for the orbital eccentricity and rate of change of orbital period of 2 x 10 to the -5th and 1.4 x 10 to the -11th s/s, respectively, are obtained for PSR 0655 + 64. For PSR 0820 + 02, a tight limit of 0.016 deg/yr is placed on the rate of advance of the longitude of periastron. A reliable period derivative of (1.43 + or - 0.18) x 10 to the -17th s/s is obtained for PSR 1831 - 00, and a measurement of the general relativistic advance of periastron is measured at 0.0092 + or - 0.0018/yr for PSR 2303 + 46. The latter value implies a total system mass of 2.3 + or - 0.6 solar masses, probably divided about equally between the pulsar and a neutron star companion. 25 references.

A new way to make Thorne-Zytkow objects

We have found a new way to make Thorne-Zytkow objects, which are massive stars with degenerate neutron cores. The asymmetric kick given to the neutron star formed when the primary of a massive tight binary system explodes as a supernova sometimes has the appropriate direction and amplitude to place the newly formed neutron star into a bound orbit with a pericenter distance smaller than the radius of the secondary. Consequently, the neutron star becomes embedded in the secondary. Thorne-Zytkow objects are expected to look like extreme M-type supergiants, assuming that they can avoid a runaway neutrino instability. Accretion onto the embedded neutron star will produce either an isolated, spun-up neutron star (possibly a short-period pulsar) or a black hole. Whether neutron star or black hole remnants predominate depends on the lifetime of Thorne-Zytkow objects, the accretion rates involved, and the maximum neutron star mass, none of which are definitively understood.

Age constraints in the double pulsar system J0737–3039

We investigate the age constraints that can be placed on the double pulsar system using models for the spin-down of the first-born 22.7-ms Pulsar A and the 2.77-s Pulsar B with characteristic ages of 210 and 50 Myr, respectively. Standard models assuming dipolar spin-down of both pulsars suggest that the time since the formation of Pulsar B is ∼50 Myr, that is, close to Pulsar Bs characteristic age. However, adopting models which account for the impact of Pulsar As relativistic wind on Pulsar Bs spin-down, we find that the formation of Pulsar B took place either 80 or 180 Myr ago, depending on the interaction mechanism. Formation 80 Myr ago, closer to Pulsar Bs characteristic age, would result in the contribution from J0737-3039 to the inferred coalescence rates for double neutron star binaries increasing by 40 per cent. The 180 Myr age is closer to Pulsar As characteristic age and would be consistent with the most recent estimates of the coalescence rate. The new age constraints do not significantly impact recent estimates of the kick velocity, tilt angle between pre- and post-supernova orbital planes or pre-supernova mass of Pulsar Bs progenitor.

Limits on Planets Orbiting Massive Stars from Radio Pulsar Timing

When a massive star collapses to a neutron star, rapidly losing over one-half its mass in a symmetric supernova explosion, any planets orbiting the star will be unbound. However, to explain the observed space velocity and binary fraction of radio pulsars, an asymmetric kick must be given to the neutron star at birth. Occasionally, this kick will prevent the resulting pulsar-planet system from unbinding. We estimate the survival probability of a Jupiter-type planet in an asymmetric supernova explosion and show that if a typical massive star has one such planet, then several known nonmillisecond pulsars should possess planetary companions in highly eccentric orbits