Gamma rays: Highest energy form of light in the universe.
Emitted by extreme cosmic objects.
Difficulty in focusing gamma rays using traditional methods.
Challenges in Focusing Gamma Rays
Gamma rays have the shortest wavelength and highest energy in the electromagnetic spectrum.
Photons of gamma rays pass through materials that make up mirrors, making focusing impractical.
X-ray telescopes can focus X-rays using grazing angles, but this is not possible for gamma rays, which would require even shallower angles and longer tubes.
Earth's atmosphere absorbs most gamma rays, necessitating space-based telescopes.
Gamma-Ray Detection Techniques
Focusing gamma rays is difficult; therefore, gamma-ray telescopes often do not focus light.
Instead, they utilize different methods to create useful images and locate light sources.
Example: Fermi Gamma-Ray Space Telescope
Launched: June 2008.
Size: Detector comparable to a small car.
Instruments:
One optimized for detecting gamma-ray bursts: Sudden, short-lived, but bright.
LAT (Large Area Telescope): Similar to detectors in particle accelerators, but in orbit.
Detection Mechanism
When gamma rays hit metal sheets in the LAT, they convert into electron-positron pairs (charged particles).
These particles travel through layers of silicon microstrip detectors, creating small electric currents at impact points.
Paths of these particles help determine the source of gamma rays, allowing for the creation of images of gamma-ray emitting objects.
Limitations of Gamma-Ray Telescopes
Low resolution: Gamma-ray images are fuzzier than those from optical or infrared telescopes.
Example:
Crab Nebula in visible light: High detail.
Crab Nebula in gamma rays: Fewer pixels, lower detail.
Fermi's LAT has a field of view of 20% of the sky, imaging the entire sky in about three hours.
Resolution comparison:
Fermi: Few arc minutes.
Hubble: 0.04 arc seconds.
Chandra X-ray telescope: Half an arc second.
Discoveries and Importance of Gamma-Ray Astronomy
Fermi has discovered Fermi bubbles: Enormous gamma-ray emissions around the Milky Way; origin still debated.
All-sky gamma-ray map shows various sources:
Blazers: Supermassive black holes emitting jets of radiation.
Cosmic rays, supernovae, pulsars, and neutron star merger afterglows.
Importance in multi-messenger astronomy: Combined with gravitational waves and electromagnetic detection.
Conclusion
Gamma-ray astronomy opens a new window for observing the universe and understanding extreme cosmic events.
Many sources of gamma rays remain mysterious, indicating ongoing research opportunities.