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

A neutron star in a binary system causes X-ray binaries through mass accretion, and X-ray bursts via explosive fusion on its surface.

Step-by-step solution

Background Information

Before understanding why a neutron star in a binary system can be an X-ray binary, we need to know that neutron stars are dense remnants of supernovas with strong gravitational fields. When they are part of a binary system, their gravitational pull can influence the companion star.

Understanding Mass Transfer

In a close binary system, the powerful gravity of the neutron star can strip gas from the companion star, a process known as mass transfer. The material forms an accretion disk around the neutron star due to the transfer of angular momentum.

X-ray Production

As the matter in the accretion disk spirals inward, it accelerates and heats up to millions of degrees due to the immense gravitational energy converted into heat. This superheated material emits X-rays, making the system appear as an X-ray binary.

Cause of X-ray Bursts

X-ray bursts occur in some X-ray binaries when accumulated hydrogen or helium on the neutron star's surface undergoes explosive nuclear fusion. This explosive fusion releases a significant burst of X-rays, which we observe as X-ray bursts.

Summary Analysis

Therefore, the presence of a neutron star in a close binary system can lead to the phenomena of X-ray binaries and X-ray bursters, due to the processes of mass accretion and explosive nuclear fusion.

Key concepts

Neutron Star

Neutron stars are fascinating celestial bodies that form when a massive star undergoes a supernova explosion. After the explosion, the core that remains is incredibly dense. In fact, a neutron star is so dense that it can have a mass about 1.4 times that of our Sun, squeezed into a sphere just about 10 kilometers in diameter. This results in an extremely high gravitational pull.

Their gravitational fields are so strong that they can significantly affect nearby stars, especially if they are part of a binary star system. Neutron stars do not shine like regular stars; instead, they can be detected by the behaviors they induce in their surroundings, such as the emission of X-rays.

Binary Star System

A binary star system consists of two stars orbiting a common center of mass. These systems are quite common in our universe. They range from being two stars of similar size to systems where one star might be a neutron star, which greatly influences the companion star with its strong gravitational force.

When a neutron star and a normal star form a close binary system, the gravitational interaction between them can lead to the stripping of matter from the companion star. This matter then forms an accretion disk around the neutron star, which is crucial for the formation of X-ray binaries.

Accretion Disk

An accretion disk is a rotating disk of gas and dust that forms around a massive object, such as a neutron star, in a binary star system. When the neutron star's gravitational pull strips material from its companion star, this material doesn’t fall directly onto the neutron star. Instead, it spirals inward, forming an accretion disk.

The disk is crucial as the material compresses and heats up due to friction and gravitational forces. This process converts gravitational energy into heat, causing the disk to emit X-rays. This is why such systems appear to us as X-ray binaries, where the intense X-ray radiation comes from the hot gas in the accretion disk.

X-ray Bursts

X-ray bursts are a spectacular phenomenon observed in some X-ray binaries. Over time, hydrogen or helium from the companion star accumulates on the surface of the neutron star. The intense gravitational pressure causes this layer to heat up until thermonuclear reactions are ignited explosively.

These thermonuclear explosions result in sudden and intense bursts of X-rays, which can be detected by telescopes here on Earth. These bursts are short-lived but very powerful, marking them as some of the brightest events in the X-ray spectrum. Therefore, an X-ray burst is a hallmark of strong gravitational interactions and fusion reactions at play in these systems.