Classifying things is the starting point for almost all scientific fields – from flowers to fundamental particles. Once one has classes the next step is to find subclasses, and then sub-subclasses. Finding correlations between different classification schemes, then, often leads to significant understandings.
Neutron stars are not stars in the normal sense. They are remnants composed entirely of neutrons left after a star larger than the Sun, but not too large, explodes as a supernova at the end of its life as a star. There are different types of both neutron stars and supernovae.
Consider neutron stars first. When a star whose inner core is more massive than the Chandrasekhar limit – about 1.4 M⊙ (M⊙ denoting the mass of the Sun) – exhausts its nuclear fuel it collapses as a supernova because it is too heavy to support itself through degeneracy pressure. If the material remaining after the explosion is less than about 3 M⊙ (the Tolman–Oppenheimer–Volkoff limit) the remnant is a neutron star. Otherwise the result is a black hole. Typically, the total mass of the progenitor of a neutron star is in the range of 5 to 15 M⊙.
Since a neutron star can no longer release energy from thermonuclear reactions, it may radiate very little electromagnetic energy. Consequently it may be rather difficult to detect, like a black hole, unless it’s a member of a binary system, so that there are visible gravitational effects on the companion.
However, some neutron stars may have energy sources that allow them to emit electromagnetic radiation at wavelengths all the way from radio to X-rays or even gamma rays. If the neutron star also has a strong magnetic field, such emissions may be observable in periodic pulses occurring at frequencies from a thousandth of a second on up. In this case the neutron star is a pulsar.
There are different types of pulsars too, depending on the energy source.
A pulsar’s possible energy sources are rotational kinetic energy, the energy released by matter accreting onto the neutron star (where the matter is usually drawn from a companion), or the decay of very strong magnetic fields (where the object is termed a “magnetar“). (There’s been some interesting recently reported research on a gamma-ray pulsar.)
Research just published has revealed a different distinction among certain types of pulsars. This distinction occurs when an X-ray pulsar is in a binary system with a massive young star. It shows up as differences in spin period of the pulsar, orbital period, and orbital eccentricity.
The astronomers analysed data on a large sample of high-mass X-ray binaries, which are double star systems in which a fast-spinning neutron star orbits a massive young companion. The neutron star in these systems also periodically siphons off material from its partner. During such phases, the neutron star becomes an X-ray pulsar: its brightness increases tremendously, but the resulting X-ray radiation is pulsed on the neutron star spin period. Such systems are very useful, because by timing their pulses, astronomers can accurately measure the neutron star spin periods.
The astronomers detected two distinct groupings in a large set of spin periods measured in this way, with one group of neutron stars typically spinning once every 10 seconds, and the other once every 5 minutes. This finding has led them to conclude that the two distinct neutron star populations formed via two different supernova channels.
The binary systems studied in this research are fairly common and known as Be/X binaries. They consist of relatively low-mass neutron stars that are strong X-ray emitters, paired with fast-rotating, massive (8 to 18 M⊙) main-sequence stars of stellar type Be. Although the different types of neutron stars that result from different types of supernovae might occur outside of binary systems, it’s only in such systems that the differences may be readily apparent.
The connection with different types of supernovae that may lead to these distinct types of pulsars is hypothetical but intriguing. Theorists have proposed that core collapse supernovae may differ depending on the composition of the progenitor star’s core. In the most common case, the core consists mainly of iron. Thermonuclear reactions between nuclei heavier than iron absorb energy instead of producing it. Consequently, reactions stop as soon as most of the available matter is in the form of iron. If the mass of the core exceeds the Chandrasekhar limit an explosion ensues, as there is no longer an energy source to support the inward pressure of the stellar matter.
But there is another possibility, which should be much less common because of the restrictive conditions it requires. Electron capture is a process that can occur in a very dense core consisting of proton-rich nuclei satisfying additional conditions. This can happen, for example, when a very massive core contains mainly oxygen, neon, and magnesium. When the density passes a threshold, electrons are captured by the nuclei, where they combine with protons to form neutrons and release copious amounts of energy and neutrinos. A supernova explosion ensues.
The differences in the characteristics of the two types of explosion are calculated to show up in properties of a resulting Be/X binary systems. According to the research paper:
The outcome of electron-capture supernovae differs from that of iron-core-collapse supernovae in two fundamental ways. First, electron-capture supernovae should produce somewhat less massive neutron stars (≲ 1.3 M⊙) than iron-core-collapse supernovae (1.4 M⊙). Second, electron-capture supernovae are expected to impart much smaller kicks to the neutron stars they produce (average kick velocity of ≲50 km s−1) than are iron-core-collapse supernovae (≳200 km s−1). In binary systems, where kicks induce orbital eccentricity, these differences could naturally give rise to two distinct subpopulations. The more conventional iron-core-collapse channel would produce high-eccentricity binaries containing high-mass neutron stars, and the electron-capture channel would produce low-eccentricity binaries containing low-mass neutron stars.
In observations of about 100 Be/X systems studied, the researchers have found a statistical distribution that roughly matches these characteristics. However, a considerable amount of additional statistical data needs to be analyzed, especially involving orbital eccentricities, in order to really nail down the correlation.
|Knigge, C., Coe, M., & Podsiadlowski, P. (2011). Two populations of X-ray pulsars produced by two types of supernova Nature, 479 (7373), 372-375 DOI: 10.1038/nature10529|