Astronomy & Celestial Mechanics
the sky, computed
Concept

Pulse profile — where it appears

The shape of a pulsar's signal, folded over its rotation period to average out variation between pulses. It is a slice through the emission beam along the observer's line of sight, and it is nearly constant for most pulsars. Geodetic precession in a binary moves that slice, so the shape slowly changes.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

One hot spot, seen through more and more gravity. The X-ray flux from a small hot spot at colatitude 60° on a rotating neutron star seen from inclination 60°, over one rotation, each curve divided by its own mean, for no bending, 2GM/Rc² = 0.20, 2GM/Rc² = 0.34, 2GM/Rc² = 0.48 — the last two are a 1.4 M☉ star of 12 km and a 2.0 M☉ star of 12.4 km. Light bending is computed exactly; Doppler effects are left out. With no bending the spot is behind the star for 39 per cent of the turn and the pulsed fraction is 1.00. As the compactness rises the star's gravity bends light from the far side into view: the dark interval shrinks to 27 per cent, then vanishes, and the pulsed fraction falls to 1.00, 0.99, 0.64. The more compact the star, the more of its own back it shows and the smoother its pulse.

The pulse that shows the back of the star

A hot spot on a spinning neutron star flashes once a turn, and the flash is weaker than it would be on an ordinary star because the star's gravity bends light from its far side into view. How much weaker measures the ratio of mass to radius. But on its own the flash cannot tell a compact star from a tilted one — the two trade exactly — and what separates them is the lean the spin puts into the pulse, which measures the radius directly.

stars · Degeneracy A beam the line of sight passes through once a precession. The angle between the line of sight and a pulsar's magnetic pole against the year, for a spin 22 degrees from the orbital axis in an orbit inclined at 47 degrees, a pole 27 degrees from the spin, and a beam 9 degrees in half-width, at the Hulse–Taylor rate of 1.21 degrees a year, a cycle of 296 years. The beam is in view from about 1943 to 2025, 28 per cent of the cycle. The closing date is put in, from the published forecast of about 2025.

A pulsar that relativity is turning out of sight

A spinning body carried round a mass does not keep its axis fixed, because space itself is curved along the path. For the Earth's axis the effect is nineteen thousandths of an arcsecond a year, and for a pulsar in a tight binary it is several degrees. A pulsar is seen only when its beam sweeps across the observer, so that slow turning has a calendar: one of the two pulsars in the double pulsar vanished in 2008, on schedule, because of a term in an expansion that had taken almost a century to measure.

gravitation · Relativistic orbits

Named alongside it

The objects these essays reach for when they reach for this one.

Neutron starBinary pulsarCompactnessEquation of stateGeneral relativityGeodetic precessionGravitational lensingMass radius relationMillisecond pulsarRelativistic beamingThe Shapiro delaySupernova kick

All concepts