Question

Explain how the presence of a neutron star can make a close binary star system appear to us as an X-ray binary. Why do some of these systems appear to us as \(X\) -ray bursters?

Short answer

Accretion from a companion star to a neutron star forms a hot disk emitting X-rays. X-ray bursts occur from sudden nuclear fusion on the neutron star's surface.

Step-by-step solution

Understanding Binary Systems Involving a Neutron Star

In a binary star system with a neutron star, the powerful gravitational pull of the neutron star can pull matter from its companion star. This is often due to the close proximity of the stars to each other.

Accretion Disk Formation

As matter from the companion star falls towards the neutron star, it forms an "accretion disk" around the neutron star due to its angular momentum. The matter in the disk spirals inwards as it loses energy, heating up to extremely high temperatures.

X-ray Emission

As the material in the accretion disk gets compressed and heated, it reaches temperatures at which X-rays are emitted. This emission is why the system is observed as an X-ray binary.

Explanation of X-ray Burst

In some X-ray binaries, material accumulates on the neutron star's surface until it reaches a critical temperature and density, igniting nuclear fusion. The sudden release of energy results in an X-ray burst, observed as a sudden increase in X-ray luminosity.

Key concepts

Neutron Stars

Neutron stars are fascinating remnants of massive stars that have exploded in a supernova. After a star exhausts its nuclear fuel, its core collapses under gravity, leading to the formation of a neutron star. These stars are incredibly dense, with masses comparable to that of our sun but packed into a sphere with a radius of only about 10 kilometers. Due to their immense density and gravitational pull, neutron stars can affect their surroundings significantly.

In a binary system, the neutron star's gravity is strong enough to draw material from its companion star. This process can lead to the creation of phenomena like accretion disks and X-ray emissions, which we will explore further. The gravitational field of a neutron star can distort space-time itself, making them intriguing objects for astronomers and physicists alike.

Accretion Disks

When material from a companion star in a binary system is drawn towards a neutron star, it doesn't fall directly onto it. Instead, because of the angular momentum, the material spirals around the neutron star, forming what is known as an accretion disk.

An accretion disk is a rotating disk of gas and dust. As the material spirals closer to the neutron star, it loses energy, resulting in the disk getting increasingly hot. The high temperatures cause the gas in the disk to glow brightly, often in the form of X-rays, which can be detected by telescopes from Earth.

These disks are vital in displaying the otherwise invisible processes happening in such binary systems. They provide valuable data, helping scientists study the intense gravitational and magnetic fields of neutron stars.

Nuclear Fusion

Nuclear fusion is a process that powers stars, including our sun. However, its role in X-ray binaries is unique. When matter accumulates on the neutron star's surface from the accretion disk, it compresses under immense pressure due to gravity.

This pressure and heat eventually reach a point where nuclear fusion occurs. In the context of neutron stars, this fusion doesn't sustain like in a regular star but instead happens in bursts. These bursts are due to the explosive fusion reactions when enough material builds up on the star's surface, suddenly releasing a massive amount of energy.

The resulting burst is so powerful that it emits a flash of X-rays, observable even from vast distances, making these systems appear as X-ray bursters.

Binary Star Systems

Binary star systems consist of two stars orbiting a common center of mass. These systems are quite common in our galaxy. When one of these stars is a neutron star, the system can become incredibly dynamic and complex.

In many binary star systems, the proximity of the two stars allows the neutron star to pull material from its companion, forming accretion disks and emitting X-rays. These emissions turn the said binary system into an X-ray binary, making it visible to X-ray telescopes.

Binary systems provide an excellent opportunity for astronomers to study the life cycles of stars, especially when one involves a neutron star, giving insights into the processes that take place when dense stellar remnants interact closely with a living star.