Neutron Star Cooling: II

It was more than 70 years ago when Baade and Zwicky [3] speculated that an “exotic” star consisting mostly of neutrons, now known as a neutron star, may be formed when a normal star collapses through a supernova explosion. During the subsequent years in the 1930s several theorists, including Oppenheimer and Volkoff [35], discussed the properties of neutron stars. However, it was not until the late 1950s to the early 1960s, when curiosity on such a hypothetical object revived [11,73]. As far as I am aware Cameron [11] is the first author who discussed thermodynamic problems of neutron stars. This article's author chose to explore this problem as one of the projects on neutron stars as her PhD thesis [59]. The research started as a purely theoretical endeavor, but before the calculations were completed we learned of the discovery of the first Galactic X-ray source Sco X-1, which was soon followed by the second such Galactic X-ray source detection, this time in the Crab supernova remnant [15]. It was immediately suggested by several theorists [19, 59, 66] that these strong X-ray sources might be neutron stars, because if these X-rays are blackbody radiation as expected, the radius of the emitting region has to be as small as ˜10 km (because the temperature is so high), just the correct size predicted for a neutron star.¹

The first series of our detailed cooling calculations [59, 66] indeed showed that these stars can be hot enough to emit X-rays for approximately a million years after a supernova explosion. However, subsequent observations indicated that the Crab X-ray source is “extended”, and the spectrum of Sco X-1 radiation is not blackbody [9]. At about the same time Bahcall and Wolf (1965) [4] suggested that if pions are present in a neutron star, the star will cool too fast to be observable. The unexpected discovery of a neutron star, however, was soon reported in 1968, in the form of a radio pulsar [20]. With the subsequent discovery of many more radio pulsars [57], as well as X-ray pulsars by UHURU and other X-ray satellite missions [24], a neutron star is now an established, no longer “exotic” member of the celestial family. On the other hand, the prospect of directly “seeing” a neutron star, in the sense of detecting the radiation directly from the stellar surface, as in the case for ordinary stars, has turned out to be elusive. This is because the circumstellar plasmas, in accretion disks and/or the stellar magnetosphere, can emit X-rays, too, often stronger than the stellar surface radiation, and it was beyond the capability of the earlier pioneering detectors to separate one from the other.