Neutron Star Binaries

Neutron Stars are among the most exotic objects in the Universe. With a mass of about 1.5 the mass of our Sun and a radius of about 10 km, a neutron star is the most compact stable configuration in which degeneracy pressure of matter can still balance gravity, further compression leading to gravitational collapse and formation of a black hole. As gravity is extreme, rotation is extreme: NSs are the fastest rotating stars known, with periods as short as a millisecond. When a neutron star is in a binary system with a normal star, its huge gravitational field can capture matter from the companion star that is accreted onto the neutron star. The enormous release of gravitational energy of the accreting matter makes the system to outshine with a luminosity up to a hundred thousand Suns in the X-ray band. Neutron stars also have very strong surface magnetic fields, from 1e7 Gauss up to 1e15 Gauss – note that the strongest magnetic fields realised on Earth are about 8e4 Gauss. This magnetic field which can funnel towards magnetic poles the motion of matter close to their surface. When this happens, their emission appears modulated at the spin period, because of the lighthouse effect, and a neutron star is observed as pulsar. These are the most stable clocks of the Universe, given their huge moment of inertia (~1e45 g cm^2).

In old binary systems, when the neutron star magnetic field is decayed to values around 1e8-1e9 Gauss, the accretion of matter and angular momentum can accelerate the neutron star to very fast spin periods, which can reach the frequency of several hundreds turns in one second. The maximum rotation speed of a neutron star depends on its compactness, that is on its mass-to-radius ratio. In other words, it depends on the equation of state of ultra-dense matter in a neutron star. The search for the fastest spinning neutron star may give important constraints on the equation of state of the most dense configuration of matter in the Universe.

An artist's impression of an accreting X-ray millisecond pulsar. Credit: NASA / Goddard Space Flight Center / Dana Berry

An artist’s impression of an accreting X-ray millisecond pulsar. Credit: NASA / Goddard Space Flight Center / Dana Berry

Author: Tiziana Di Salvo (University of Palermo)

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