Gamma-ray bursts are cosmic explosions that release intense bursts of high-energy radiation. Discovered in the 1960s, these events have captivated astronomers with their immense power and mysterious origins. They come in short and long varieties, each with distinct causes and characteristics.

Scientists now believe gamma-ray bursts stem from the collapse of massive stars or the merger of dense stellar remnants. By studying these fleeting flashes, researchers gain insights into the early universe, star formation, and the deaths of the most massive stars.

Discovery and Characteristics of Gamma-Ray Bursts

Discovery of gamma-ray bursts

• Gamma-ray bursts (GRBs) first discovered in late 1960s by Vela satellites designed to monitor compliance with Nuclear Test Ban Treaty
• Compton Gamma-Ray Observatory (CGRO) launched in 1991 with Burst and Transient Source Experiment (BATSE) onboard detected over 2,700 GRBs
• BeppoSAX satellite launched in 1996 provided accurate positions of GRBs enabling follow-up observations at other wavelengths (optical, radio)
• Swift Gamma-Ray Burst Mission launched in 2004 detects and provides rapid localization of GRBs for follow-up observations
• Fermi Gamma-ray Space Telescope launched in 2008 provides high-energy gamma-ray observations of GRBs

Beamed energy in bursts

• GRBs emit energy in form of collimated jets which are narrow beams of energy focused in specific direction (relativistic jets)
• Beaming reduces total energy output required to produce observed luminosity as actual energy released is less than if emission were spherical
• Beaming angle affects observed properties of GRBs
  • Observers located within beam observe more luminous burst
  • Observers outside beam may not detect GRB at all

Physical Processes and Types of Gamma-Ray Bursts

Radiation process and afterglows

• GRBs thought to originate from collapse of massive stars (progenitor stars) or merger of compact objects (neutron stars, black holes)
• Initial burst of gamma-rays produced by internal shocks within relativistic jet caused by collisions between shells of material moving at different velocities
• Afterglow emission occurs as jet interacts with surrounding medium
  • External shocks between jet and interstellar medium produce longer-wavelength emission observed in X-ray, optical, and radio wavelengths
• Synchrotron radiation is primary emission mechanism in GRB afterglows where electrons spiraling in magnetic fields produce synchrotron emission

Short vs long-duration bursts

• GRBs classified into two main categories based on duration
  1. Short-duration GRBs last less than 2 seconds
  2. Long-duration GRBs last more than 2 seconds
• Short-duration GRBs thought to originate from merger of compact objects like binary neutron stars or neutron star and black hole (compact object mergers)
• Long-duration GRBs associated with collapse of massive stars more than 25 times mass of Sun in core-collapse supernovae
• Short and long-duration GRBs differ in host galaxies and locations within galaxies
  • Short GRBs found in both old and young stellar populations, often in galaxy outskirts
  • Long GRBs typically found in star-forming regions of young, low-metallicity galaxies

Bursts and early universe insights

• GRBs are most luminous events known in universe and can be detected from very high redshifts corresponding to early universe
• Long-duration GRBs associated with deaths of massive stars provide insight into star formation rates and metallicity in early universe
• GRBs can be used as probes of intergalactic medium as absorption features in GRB afterglow spectra reveal composition and evolution of intergalactic medium
• GRBs may contribute to reionization of universe as high-energy radiation from GRBs could have helped ionize neutral hydrogen in early universe

Multi-messenger astronomy and GRBs

• GRBs play a crucial role in multi-messenger astronomy, combining electromagnetic observations with other forms of information
• Gravitational waves from compact object mergers can be detected in conjunction with short GRBs, providing new insights into these events
• GRBs serve as standard candles for measuring cosmic distances, contributing to our understanding of the cosmological distance scale