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

Neutron stars in binary systems emit X-rays due to intense accretion heating, and bursts occur from sudden ignition of accumulated material on the neutron star surface.

Step-by-step solution

Understanding Neutron Stars in Binary Systems

In a close binary star system, one of the stars can be a neutron star. A neutron star is a dense remnant of a massive star that has exploded as a supernova. They are made almost entirely of neutrons and have incredibly strong gravitational fields. These characteristics are critical for their role in X-ray binaries.

Role of Accretion in X-ray Emission

As the two stars orbit each other, the neutron star can pull matter from its companion star due to its strong gravitational field. This matter forms an accretion disk as it spirals towards the neutron star. As the matter falls into the neutron star, it is heated to millions of degrees, emitting X-rays in the process. This is why the system is called an X-ray binary.

Understanding X-ray Bursters

In some neutron star binary systems, the accreting matter builds up on the surface of the neutron star. When enough material accumulates, nuclear fusion can suddenly ignite in a burst-like manner. This process releases an intense burst of X-rays lasting for a short duration, causing the system to be classified as an X-ray burster.

Key concepts

Neutron Stars

Neutron stars are fascinating objects in the cosmos. They form when massive stars, much larger than our sun, end their life cycles in a spectacular supernova explosion. The core that remains is incredibly dense, as it is composed almost entirely of neutrons. This density gives neutron stars some unique characteristics. For example, they have a huge gravitational pull despite being only about 20 kilometers in diameter.
Because of their strong gravity, neutron stars can affect nearby objects significantly, especially in binary star systems. Their gravitational fields are strong enough to pull in material from a companion star, playing a crucial role in creating X-ray binaries.

Accretion Disks

Accretion disks are fascinating structures that form in X-ray binary systems due to the gravitational interaction between a neutron star and its companion star. Matter from the companion star is drawn towards the neutron star, but instead of falling directly, it forms a disk around it.
As this matter spirals inward, its gravitational potential energy is converted into heat through friction and compression, heating the disk to extremely high temperatures. This heating process causes the disk to emit X-rays. Thus, the presence of an accretion disk is essential for a binary system to be observed as an X-ray binary.

• Accretion disks allow for significant energy release as matter slowly inches toward the neutron star.
• They can vary in brightness and X-ray output based on the rate of mass transfer.
• The presence of these disks is a key indicator of the dynamic interactions within these systems.

Binary Star Systems

Binary star systems consist of two stars orbiting around a common center of mass. They are quite common in the universe and come in various forms, with different types of stars and orbits. These systems can significantly impact each participating star.
In some binary systems, particularly those involving a neutron star, the interaction between the two stars is intense. The neutron star's strong gravity can pull in material from its companion, forming an accretion disk. This interaction is pivotal for forming X-ray binaries.

• The nature of a binary star system influences the type of radiation we detect, such as X-rays.
• Dynamics in these systems can lead to various phenomena, including pulsations and potential transformation into different types of celestial objects.

X-ray Bursters

Within certain X-ray binary systems, something remarkable can occur: the phenomenon of X-ray bursts. These bursts happen when matter accreted onto the surface of a neutron star builds up over time. Essentially, layers of gas accumulate until they reach a critical mass.
When this accumulated fuel ignites, it does so explosively through nuclear fusion, releasing a quick burst of energy in the form of X-rays. This is why certain X-ray binary systems are known as X-ray bursters. These bursts can last for only a few seconds but are immensely powerful, showcasing the volatile nature of these systems.

• X-ray bursters provide crucial insights into the conditions and processes on neutron star surfaces.
• The repetitive nature of these bursts reveals the ongoing cycle of matter accumulation and fusion ignition.
• X-ray bursters pose intriguing questions about the detailed physics of neutron stars and nuclear processes.