Neutron Stars (lecture 15)

Neutron Stars

Discovery of the Neutron

  • 1932: James Chadwick discovered the neutron, proposing that there exists a neutral particle in the nucleus of the atom with a mass similar to that of a proton.

    • The term "neutron" was subsequently assigned to this particle.

Proposal of Neutron Stars

  • 1934: Baade and Zwicky proposed that a neutron star could emerge from the explosion of a supernova.

    • Key Contributors:

    • Wilhelm Walter Baade (1893-1960)

    • Fritz Zwicky (1898-1974)

Baade & Zwicky Proposal Explanation

  • Their proposal suggested that:

    • The stellar core collapses, leading to a remnant composed of neutrons.

    • Initial elemental composition:

    • Protons + Electrons = Neutrons?

Birth of a Neutron Star and Supernova Remnant

  • Diagrammatic representation:

    • Stages:

    • Red giant → Neutron star → Core implosion → Supernova explosion → Supernova remnant.

Neutron Stars and Magnetic Fields

  • 1967: Franco Pacini published a paper suggesting that neutron stars may maintain a very strong magnetic field.

    • These magnetic fields cause the neutron star to beam radiation from its poles as it rotates.

Radiation Emission from Neutron Stars

Axis of Rotation and Beam Direction

  • Radiation is beamed along the magnetic axis.

    • Direction toward Earth:

    • When a beam is directed at Earth, it results in detectable radiation (pulse).

    • Half a rotation later, if the beam is not aimed at Earth, the radiation detection ceases.

Nobel Prize Recognition

  • Recipients include:

    • Wilhelm Walter Baade

    • Fritz Zwicky

    • Jocelyn Bell

    • Anthony Hewish

    • Franco Pacini

    • James Chadwick

Common Terminology

  • The term "pulsar" was adopted by the wider scientific community for these neutron stars emitting regular beams of radiation.

Characteristics of Neutron Stars

  • Neutron stars are essentially vast nuclei:

    • A volume of just 1 cm³ has a mass of approximately 10^8 tons.

    • They have typical magnetic fields ranging from 10^{12} to 10^{15} Gauss.

    • For comparison, Earth's magnetic field is about 0.5 Gauss.

    • They exhibit fast rotation due to the conservation of angular momentum.

Sources of Radiation Beams

  • Intense magnetic field strength is capable of creating positrons and electrons.

  • These charged particles accelerate from the poles, resulting in synchrotron radiation.

Structure of a Neutron Star

Surface and Interior Composition

  • Core:

    • Characterized by:

    • Homogeneous matter

    • 'Swiss cheese' phase

    • 'Spaghetti' phase

  • Crust:

    • Composed of nuclei and neutron superfluid.

  • Atmosphere/Envelope:

    • Contains various structures including:

    • Neutron superfluid

    • Proton superconductor

    • Magnetic flux tubes

    • Polar caps

    • Cones of open magnetic field lines.

Pulsar Dynamics

  • Pulsar Slowdown Phenomena:

    • Evidence observed with the Vela Pulsar showing periodic slowdowns:

    • Recorded periods: 0.089236s to 0.089233s indicating gradual changes over time.

High Velocity Neutron Stars

  • Observations indicate movement directions of neutron stars relative to supernova remnants.

Formation Scenario of Neutron Stars

  1. An ordinary star evolves into a giant or supergiant, filling its Roche lobe, with some gas escaping into space.

  2. Gas escapes and forms an accretion disk around the resulting neutron star.

  3. The neutron star's magnetic field channels this gas onto its magnetic poles, forming hot spots.

  4. As the neutron star rotates, beams of X-rays emitted from these hot spots traverse through space.

Observations of Neutron Stars' X-ray Emissions

  • Data from the X-Ray Timing Explorer Satellite indicates variability in counts per half-second.

  • Notable events include transformations of radiation via supernova remnants such as SGR 1900+14.

Magnetars

  • Definition: Magnetars possess extraordinarily strong magnetic fields compared to typical neutron stars.

    • They release substantial amounts of energy, leading to X-ray and gamma-ray bursts.

    • Over time, the magnetic fields decay, with activity typically lasting no more than 10,000 years.

Magnetar Life Cycle

  • Phases:

    • Supernova forms a magnetar.

    • Soft Gamma Repeaters (SGR) and Anomalous X-ray Pulsars (AXP) exist until they become "dead" magnetars.

    • Timeframe for evolution: Moderate activity observed on the timescale of 10⁰ to 10⁵ years since the supernova event.

Pulsar Planets

  • 1993 Discovery: Stephen Thorsett reported on PSR B1620-26 that hosts an unusual planet.

    • The Jovian planet, with a mass of 2.5 Jupiter masses, originates from matter expelled by a Sun-like star that later merged with a neutron star.

    • Significant interactions occurred with a red giant that also contributed to the neutron star's acceleration into a pulsar that spins 100 times a second.

Further Discoveries of Pulsar Planets

  • 1992 Notable Findings: Aleksander Wolszczan and Dale Frail identified planets around pulsar PSR B1257+12, which included terrestrial-sized planets possibly formed from supernova remnant debris.

  • 2006 Discovery: Deepto Chakrabarty identified a circumstellar disk around pulsar PSR 4U 0142+61 which may contain heavy elements from fallback debris. This suggests pulsar planets might be more common than previously thought.

Merging Neutron Stars and Their Implications

  • Key Concept: Neutron stars have a maximum mass limit before collapsing, approximately 3 solar masses.

    • When two neutron stars merge, the mass can exceed this limit, resulting in a gamma-ray burst as well as a nucleosynthesis explosion, contributing to the generation of new elements in the universe.

Characteristics of Neutron Star Masses

  • Generally, neutron stars fall within the range of 1 to 1.5 solar masses.

  • The heaviest known neutron star is about 2.3 solar masses.

  • It is computationally estimated that the maximum viable mass for a neutron star is approximately 3 solar masses, beyond which instability or collapse occurs.