Supermassive black holes lurk at the hearts of galaxies, powering the brightest objects in the universe: quasars. These cosmic beacons shine with the intensity of billions of stars, fueled by matter spiraling into the black hole's immense gravitational pull.

Quasars reveal the early universe, their light traveling billions of years to reach us. Their extreme brightness, rapid variability, and high-energy emissions stem from the incredible forces at play near the black hole's event horizon, where physics takes a wild ride.

Supermassive Black Holes and Quasars

Key features of quasars

• Extremely bright, point-like sources of light
  • Luminosities can exceed that of an entire galaxy (Milky Way)
  • Appear star-like in telescopic images despite vast distances
• High redshifts indicate they are very distant objects
  • Implies quasars are some of the most distant objects in the universe (billions of light-years away)
  • Light from quasars originates from the early universe
• Broad emission lines in their spectra suggest presence of hot, fast-moving gas
  • Gas moves at speeds up to 10% the speed of light
  • Indicative of high-energy processes occurring near the quasar
• Rapid variability in brightness on timescales of days to weeks
  • Suggests the energy-producing region is very compact (solar system-sized)
  • Rapid changes not possible if emission region were galactic scales

Supermassive black holes in quasars

• Supermassive black holes reside at the centers of galaxies
  • Can have masses ranging from millions to billions of solar masses (Sagittarius A in Milky Way)
  • Surrounded by an event horizon, beyond which nothing can escape
• Matter falling towards the black hole forms an accretion disk
  • Gravitational energy converted to heat and radiation as matter spirals inward
  • Accretion disk can reach temperatures of millions of degrees, glowing brightly
• Jets of high-energy particles may be launched perpendicular to accretion disk
  • Powered by twisting of magnetic fields near the black hole
  • Can extend for thousands of light-years beyond the galaxy (M87 jet)
• Combination of luminous accretion disk and jets produces enormous energy output of quasars and active galactic nuclei

Energy generation in quasars

1. Matter is drawn towards the supermassive black hole

   • Can come from nearby stars, gas clouds, or even entire galaxies

2. As matter falls towards black hole, it forms an accretion disk

   • Gravitational potential energy converted to kinetic energy and heat
   • Friction and viscosity in disk cause it to heat up to millions of ℃

3. Hot accretion disk radiates energy across the electromagnetic spectrum

   • Produces thermal radiation from infrared to X-rays
   • Also generates non-thermal radiation through synchrotron and inverse Compton processes

4. Some infalling matter may be ejected in the form of jets

   • Powered by twisting of magnetic fields near black hole
   • Jets can accelerate particles to near the speed of light ($c$)
   • Interaction between jet particles and surrounding matter produces additional radiation

• Total energy output of a quasar can be $10^{40}$ watts or more
  • Equivalent to luminosity of hundreds of average galaxies (Andromeda)
  • Most of this energy released in a very small region around the supermassive black hole (Schwarzschild radius)
  • Limited by the Eddington luminosity, balancing radiation pressure and gravity

Observational effects

• Gravitational lensing can distort and magnify light from distant quasars
• When a quasar's jet is aimed directly at Earth, it appears as a blazar, exhibiting extreme variability and polarization