Physics of Relativistic Objects in Compact Binaries: From Birth to Coalescence

Abstract

In 1971, only four years after the discovery of the first radio pulsar, the first neutron star in a close binary was discovered: the 4.84 s X-ray pulsar Centaurus X-3, which is moving in a 2.087 day orbit around an O-star with a mass > 16M ⊙ [135]. Several more of these High Mass X-ray Binaries (HMXBs) were discovered soon after and it was found that, contrary to what is observed in radio pulsars, the pulse periods of several of these X-ray pulsars are steadily decreasing in the course of time, moving to shorter and shorter values on timescales of order 104 years. It was soon realized that the same accretion process of matter flowing over from the massive companion star that is the cause of the X-ray emission, also causes this “spin-up”. The matter flow in the binary system has angular momentum - derived from the system's orbital motion - and this angular momentum is fed to the neutron star, causing its rotation rate to increase. A few years later, the suggestion was made by [6] that these pulsating X-ray sources in binaries may later in life, when their massive companion stars have exploded as a supernova, become observable as radio pulsars. Such pulsars, which had a history of accretion and spin-up in binaries were later given the name “recycled pulsars” [119]. In 1973 it was calculated [176] that before the second supernova explosion in a HMXB takes place, the orbit of the system will have become very narrow, as a consequence of extensive mass transfer to the neutron star and loss of mass with high angular momentum from the system, leading to final orbital periods of only a few hours. The resulting close system then consists of a helium star (the helium core of the massive companion) plus the neutron star. In 1974 the Hulse-Taylor binary radio pulsar PSRB 1913+16 was discovered, which in addition to its very narrow and eccentric orbit (Porb = 7.75 h, e = 0.615) appeared to have very abnormal characteristics as a radio pulsar: its magnetic field strength is only 2 x 1010 G, some two orders of magnitude lower than that of the other pulsars then known, and its spin period is abnormally short (0.059 s), which at the time made it the second fastest radio pulsar known, after the Crab pulsar (P = 0.033 s). Its orbital period and eccentricity were almost exactly what one would obtain if the helium star in the 4 hour orbit binary (resulting from a HMXB like Centaurus X-3, as calculated in 1973) would explode as a supernova and itself would leave a neutron star. This model for the origin of the Hulse-Taylor binary pulsar was therefore proposed immediately after its discovery [44, 31]. It was thought in these days that the magnetic fields of neutron stars decay on a relatively short timescale, of order 5 million years. The abnormally weak magnetic field of PSRB1913 +16 therefore led [139] to the suggestion that the observed pulsar is the oldest of the two neutron stars in the system, which after a long period of field decay had been spun up by accretion in an X-ray binary system, before the second star exploded. It was subsequently shown [142] that this spin-up idea is the only explanation possible for this peculiar combination of rapid spin and weak magnetic field observed in PSRB 1913+16. This then immediately implies that the companion of this pulsar must also be a neutron star. The reason for this is that during the phases of accretion, orbital shrinking and spin up, the orbit of the system will have become completely circularized by tidal and frictional forces. The only way to then subsequently obtain the large observed orbital eccentricity of the system is: if a second supernova explosion took place. This then implies that the companion of PSRB 1913+16 must itself also be a neutron star: the younger one of the two. As the last-born neutron star did not undergo any accretion, and after the second explosion the system was free of gas, the second neutron star is expected to be a normal newborn “garden variety” radio pulsar with a normal strong magnetic field of order 1012 G [142]. Such pulsars rapidly spin down on a timescale of order a few million years, after which they become unobservable. On the other hand, due to its weak magnetic field, the spin-down timescale of PSRB1913+16 is longer than 108 years.