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.