Gamma Ray Bursters
The Biggest Explosions Since the Big Bang*

1967: A Mystery Unfolds

Vela 5B
Vela 5B

In 1967, the Vela satellites were launched as nuclear monitors, to verify adherence to the Nuclear Test Ban Treaty. The Vela 5A and 5B satellites orbited about half way out to the Moon, where they had a view of a full terrestrial hemisphere. Their instruments were sensitive to X-rays and gamma-rays.

Almost immediately, they started detecting bursts of gamma-rays (GRBs). Most did not have the double-peak characteristic of nuclear explosions.
Timing the burst arrival times between different satellites suggested that they came from all over the sky.
They appeared to be extra-terrestrial.

By 1992, over 100 hypotheses had been suggested (R. Nemiroff 1994), including

The isotropic distribution on the sky suggested that:


1990s: The Mystery Deepens

CGRO

The Compton Gamma Ray Observatory (CGRO; 1991-2000) was designed in part to solve the mystery of the gamma ray bursts.

GRB 960924
GRB 960924
Burst animation (NASA)

CGRO/BATSE detected about 1 burst per day.

BATSE GRBs

The GRB distribution is isotropic. They cannot originate in the Milky Way. They must be cosmological in origin. That means they have luminosities > 1051 ergs/second (the energy the Sun puts out in its lifetime).

To determine the distance and nature of the GRBs requires finding an optical counterpart, so we can determine the morphology (galaxy, star, etc) and the distance (redshift). The BATSE positions were only accurate to about 5-10 degrees on the sky.


1999: Mystery Solved

Beppo-SAX
Beppo-SAX: Satellite per Astronomia X (1996-2002)

Beppo-SAX combined a wide-field X-ray imager with a GRB detector. About every 3 months a GRB and a simultaneous X-ray burst appeared within the view of the X-ray imager. This permitted localization to a few arc minutes, which in turn permitted optical followups.

GRB 970508
GRB 970508 seen with absorption line indicating z=0.8

GRB 971214
GRB 971214, z=3.4 (12 GLy distant)

GRB 980425
GRB 980425 coincident with supernova 1998 bw
galaxy is ESO 184-G82, distance = 100 MLy

GRB 990123
GRB 990123. ROTSE detection
GRB 990123
GRB 990123. Optical
GRB 990123
GRB 990123. Host Galaxy, seen 16, 59 and 380 days after the GRB

GRB 011211
GRB 011211 Beppo-SAX X-rays
GRB 021211
GRB 021211 RAPTOR: t=65 seconds (left) and 9 minutes (top right)
GRB 011121
GRB 011121 optical decay

The GRBs appear to be associated with peculiar type Ic supernovae: the collapse of a massive hydrogen-deficient Wolf-Rayet star into a black hole.

GRB 031203
GRB 031203 X-ray rings

Why so bright? GRBs are not isotropic emitters. We are looking along a beam of radiation, perhaps emitted along the rotation axis of the star. Most supernovae are not GRBs because we see them at the wrong angle. The jets are ejected at relativistic velocities (V approximately c), which boosts their apparent brightness in the direction of travel.


Importance of GRBs

There is about 1 detectable gamma ray burst per day in the universe. That means there is about one per galaxy every 200 million years, on average.
At present, the SWIFT satellite is studying gamma ray bursts. SWIFT should detect about 2 GRBs per week, and observe them at gamma ray, X-ray, ultraviolet, and optical wavelengths.
The SWIFT song by the Chromatics.
Further information:
  • non-technical NASA web site
  • Hubble Space Telescope GRB Observations Page
  • GCN: the Gamma Ray Burst Cordinates Network (technical)
  • Coming soon: SWIFT and the SWIFT Education and Public Outreach page

    *The Big Bang was not an explosion.