Gamma-ray Burst Optical/Near-infrared Detector
Attached to the MPG/ESO 2.2-metre telescope at La Silla Observatory, located on the outskirts of the Chilean Atacama Desert, GROND or the Gamma-ray Burst Optical/Near-infrared Detector, is investigating some of the most mysterious and energetic explosions in the Universe called Gamma-ray Bursts (GRB).
GRBs, which are invisible to our eyes, are first detected by gamma-ray observatories in space. After releasing their intense burst of high-energy radiation, they become detectable for a fleeting moment in the optical and in the near-infrared, which can be observed from the ground. This “afterglow” fades very rapidly, making detailed analysis possible for only a few hours after the initial detection. GROND’s observations are critical in determining the GRB's distance and, hence, intrinsic brightness.
GRBs are thought to be beacons announcing the formation of a black hole through the collapse of an extremely massive star. All of the GRBs observed so far have been found to come from outside the Milky Way.
As time is of the essence, GROND observes these afterglows in seven different colours simultaneously. These can be used to quickly measure an approximate distance to the GRB, using the photometric redshift. The photometric redshift method makes it possible to judge the distance to a remote celestial object (a galaxy, a quasar, a gamma-ray burst afterglow) from its measured colours. It is based on the proportionality between the distance and the velocity along the line of sight (Hubble's law) that reflects the expansion of the Universe. The greater the distance to an object, the larger its velocity and, due to the Doppler effect, the greater the spectral shift of its emission towards longer (redder) wavelengths. Thus, the measured colour provides a rough indication of the distance.
Rapid follow-up spectroscopic observations can then be triggered at other telescopes, such as ESO’s Very Large Telescope (VLT) at Paranal Observatory. In fact, GROND was the first instrument at La Silla to implement the Rapid Response Mode (RRM) originally developed for the VLT.
Through collaboration with NASA’s Swift gamma-ray burst telescope, GROND has observed some of the most distant GRBs ever detected. In 2008, GROND observed the afterglow of a GRB (080913) that was discovered to have occurred only 900 million years after the Big Bang. The precise nature of these violent events is still a mystery.
GROND’s unique capabilities also mean that it can address a variety of additional scientific topics, including the study of X-ray, optical, or radio transients, observing transiting exoplanets in order to characterise their properties and the search for very red objects such as very distant quasars and brown dwarfs.
The authoritative technical specifications as offered for astronomical observations are available from the Science Operation page.