2.5 Pulsars in binary systems
As can be inferred from Figure 1, only 4% of all known pulsars in the Galactic disk are members of
binary systems. Timing measurements (see Section 4) place useful constraints on the masses of the
companions which, supplemented by observations at other wavelengths, tell us a great deal about their
nature. The present sample of orbiting companions are either white dwarfs, main sequence stars
or other neutron stars. Two notable hybrid systems are the “planet pulsars” B1257+12 and
B1620
26. PSR B1257+12 is a
pulsar accompanied by at least three terrestrial-mass
bodies [346
, 244, 345
] while B1620
26, an
pulsar in the globular cluster M4, is
part of a triple system with a
planet [317, 17, 315, 284
] orbiting a neutron
star-white dwarf stellar binary system. The current limits from pulsar timing favour a roughly
low-eccentricity (
) orbit with semi-major axis
. Despite several
tentative claims over the years, no other convincing cases for planetary companions to pulsars
exist. Orbiting companions are much more common around millisecond pulsars (
of the
observed sample) than around the normal pulsars (
). Binary systems below the line
have low-mass companions (
- predominantly white dwarfs) and essentially circular
orbits:
. Binary pulsars with high-mass companions (
- massive
white dwarfs, other neutron stars or main sequence stars) tend to have more eccentric orbits,
.