Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these neutron stars must have had its angular momentum changed by a binary companion? (a) a pulsar that pulses 30 times per second (b) a pulsar that pulses 600 times per second (c) a neutron star that does not pulse at all.

Short answer

Option (b), the pulsar that pulses 600 times per second, is likely spun up by a binary companion.

Step-by-step solution

Analyze Pulsars

Consider how pulsars work. Pulsars are rotating neutron stars that emit beams of electromagnetic radiation. When this radiation is directed towards Earth, we can detect the pulses. The rotational speed of a pulsar, indicating how frequently it pulses per second, can be affected by external factors.

Identify Effects of Angular Momentum Change

An external force, such as the gravitational influence of a binary companion, can change a pulsar's angular momentum. This can make the pulsar spin up, causing it to rotate and pulse faster.

Evaluate Pulsar Candidates

Let's consider each option: - (a) A pulsar that pulses 30 times per second. - (b) A pulsar that pulses 600 times per second. - (c) A neutron star that does not pulse. A pulsar that rotates exceptionally fast, like the 600 times per second one, is often the result of a process known as spin-up due to mass transfer from a binary companion.

Match Analysis with Options

Option (b), a pulsar pulsing 600 times per second, is more likely due to accretion-induced spin-up caused by a binary companion, as such high rotation rates are typically achieved through angular momentum transfer from another object.

Key concepts

Angular Momentum

Angular momentum is a fundamental concept related to the rotation of objects. Think of it as a measure of how much "spin" something has. In physics, angular momentum is conserved, meaning it stays the same unless acted upon by an external force. This principle is crucial when considering neutron stars.

Neutron stars, remnants of massive stars that have exploded in supernovae, can conserve much of the original star's angular momentum. However, if a neutron star is part of a binary system, the presence of a companion star can significantly alter its angular momentum. This happens through a process called accretion, where matter from the companion star is transferred to the neutron star.

When this happens, the added mass increases the neutron star's angular momentum, effectively "spinning it up." This results in faster rotation, manifesting as rapid pulses if the neutron star is a pulsar. Angular momentum transfer is a powerful force that can dramatically change a neutron star's spin, often leading to millisecond pulsars that rotate hundreds of times per second.

Pulsars

Pulsars are fascinating celestial objects, often called the lighthouses of the universe. These are a type of neutron star known for their precise and regular pulses of radiation. As a pulsar spins, it emits beams of electromagnetic radiation along its magnetic poles. If Earth lies in the path of these beams, we detect regular pulses, much like seeing a flashlight beam each time it sweeps past.

The rate at which a pulsar pulses can tell us a lot about its history and possible interactions with other celestial bodies. Some pulsars spin incredibly fast, with some rotating hundreds of times per second. This rapid rotation is usually a sign that the pulsar has been "spun up" by a binary companion in a process called accretion. The material falling onto the neutron star from the companion can increase its spin speed significantly.

Pulsars are reliable timekeepers thanks to the precision of their pulses. This consistency allows scientists to study gravity, test general relativity, and even search for gravitational waves. Despite being distant cosmic objects, pulsars play an essential role in understanding the universe.

Binary Companions

Binary companions are stars that orbit each other, bound by gravitational forces. When one of these objects is a neutron star, its interaction with the other star can lead to significant changes, particularly in its angular momentum.

If the companion star orbits closely enough, it can transfer matter to the neutron star. This phenomenon is called accretion. The added mass results in an increased spinning speed for the neutron star, leading to what is known as "spin-up." The faster rotation of the neutron star often leads it to become a millisecond pulsar, exhibiting extremely rapid pulsing rates.

Binary systems are also crucial for understanding processes like supernova explosions and the creation of exotic objects like black holes. Moreover, these systems provide opportunities to study stellar evolution and the transfer of mass and angular momentum between stars.

In summary, binary companions are not just interesting as individual celestial entities. Their interactions with neutron stars make them significant players in the cosmic dance, influencing the behavior and characteristics of neutron stars in profound ways.