Pulsars in binary systems

2.5 Pulsars in binary systems

As can be inferred from Figure 1

, only a few percent 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, often 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 6.2-ms pulsar accompanied by at least three terrestrial-mass bodies [403

, 298 , 402

] while B1620–26, an 11-ms pulsar in the globular cluster M4, is part of a triple system with a $1 –2 M Jupiter$ planet [377 , 17 , 376 , 332 ] orbiting a neutron star–white dwarf stellar binary system. The current limits from pulsar timing favour a roughly 45-yr mildly-eccentric ( $e ∼ 0.16$ ) orbit with semi-major axis $∼$ 25 AU. 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 ( $∼$ 80% of the observed sample) than around the normal pulsars ( $≲$ 1%). In general, binary systems with low-mass companions ( $≲ 0.7 M ⊙$ – predominantly white dwarfs) have essentially circular orbits: $10 −5 ≲ e ≲ 0.01$ . Binary pulsars with high-mass companions ( $≳ 1M ⊙$ – massive white dwarfs, other neutron stars or main sequence stars) tend to have more eccentric orbits, $0.15 ≲ e ≲ 0.9$ .