Gamma-Ray Bursts (GRBs)

• Gamma-ray bursts (GRBs) were first detected in 1967 by the Vela nuclear test ban satellites.

  • 16 bursts were detected between 1969 and 1972.

  • Results were reported in 1972 (when de-classified!).

  • Observations had sufficient spatial resolution to show bursts were:
    • Not terrestrial in origin.
    • Not solar in origin.

• The Compton Gamma-ray Observatory (CGRO) was launched in 1991

• The Burst and Transient Source Experiment (BATSE) on CGRO was designed to detect and monitor GRBs
  • It detects ~ 1 burst per day

  • Typical GRBS:
    • are bright (brightest sources in the sky when on)
    • are pure gamma-rays (only ~ 2% also show X-ray emission)
    • are short (5ms -- 500s duration)

• BATSE positions are not good enough to obtain quiescent optical identifications.

• However, the isotropic distribution of the bursts on the sky implies that the souces are not galactic.

• Further, the logN-logf test implies that we are seeing all the way to the edge of the GRB distribution.

• The only plausible "edge" for an extragalactic population is the edge of the universe.

• We must therefore be seeing GRBs at cosmological distances.

• If so, GRBs must be incredibly bright:

  • For typical GRB fluxes F ~ 10^-8 W m^-2 from sources at distances D ~ 1 Gpc, the luminosity will be L = 4 pi D^2 F ~ 10^43 W in pure gamma-rays (c.f. a galaxy, whose total luminosity at all wavelengths is ~ 10^37 W).

  • We could, of course, reduce these energies significantly if the emission is tightly beamed.

• If GRBs are at cosmological distances, there should be observational consequences:
  • Fainter bursts will, on average, be more distant. We would therefore expect them to show stronger cosmological time dilation effects --- the fainter bursts should have longer durations. This phenomenon has been observed.

  • Similarly, fainter bursts will also be more redshifted, so the mean energy of the photons in fainter bursts should be lower. This effect has also been observed.

  Of course, these effects could just reflect intrinsic differences between faint and bright GRBs...

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