Binary Stars and Gamma Rays Module 10

Binary Stars

  • Definition: Binary stars are two stars that form in each other’s gravitational field and orbit a common center of mass.
  • Prevalence: As many as half of the stars in the galaxy are classified as binary.
  • Material Exchange:
    • Stars can exchange material especially when one star swells into a giant or supergiant phase.
    • Material flow impacts mass: decreases mass of the donor star and increases mass of the recipient star.

White Dwarf Explosions

  • Types of Explosions:
    • Mild Explosions:
    • Occur when a white dwarf is in a binary system and mass is transferred slowly below the Chandrasekhar limit, which is approximately 1.4 M_{ ext{sun}}.
    • This leads to a nova event, where the white dwarf is not destroyed.
    • Violent Explosions:
    • Happen when the white dwarf accumulates mass rapidly, approaching the Chandrasekhar mass limit.
    • Results in contraction and new nuclear reactions in the core, leading to a large explosion that completely destroys the white dwarf.

Neutron Star Binary Systems

  • Components: In a binary system, one star is a main sequence star and the other is a neutron star.
  • Process:
    • The strong gravitational field of the neutron star pulls material from the companion star.
    • Infalling gas becomes compressed and heated, resulting in high amounts of X-ray radiation.
  • Outcome: Mass transfer can lead to an increased spin rate in neutron stars, which can become millisecond pulsars.

Gamma-Ray Bursts (GRBs)

  • Discovery: Accidentally discovered in the 1960s during nuclear bomb detection.
  • Characteristics:
    • Irregular and bright flashes of gamma rays originating from outside the Milky Way Galaxy.
    • Associated phenomena include GRB afterglows, created from interaction of jets with surrounding gas.
  • Distance Measurement:
    • Difficult due to low resolution of gamma-ray telescopes and the need to survey wide areas of the sky.
    • First measurements occurred in 1997, revealing GRB distances of around 2 billion parsecs (over 6 billion light-years).
    • Example: GRB 221009A showed infrared afterglow (highlighted in circular diagram).

Causes of Gamma-Ray Bursts

  • Characteristics: GRBs are extremely small (roughly 100 km across) yet very energetic.
  • Model Explanation: A relativistic fireball theory is used to explain GRBs.
    • Mechanism: Superhot gases are expelled in jets that emit gamma-ray radiation as they move near the speed of light.
    • The burst and afterglow phenomena are attributed to the fireball's expansion, cooling, and its interaction with the environment.

Short vs. Ultra-long Gamma-Ray Bursts

  • Short Gamma Ray Bursts:
    • Duration lasts from 0.2 to 2 seconds.
    • Typically caused by the collision of neutron stars, or combinations of neutron stars and black holes, or the collision of two black holes.
  • Ultra-long Gamma Ray Bursts:
    • Durations greater than 16 minutes.
    • Generally linked to the formation of a black hole from a collapsing star.