A new instrument has seen First Light at the ESO La Silla  Observatory. Equipping the 2.2-m MPI/ESO telescope, GROND takes  images simultaneously in seven colours. It will be mostly used to  determine distances of gamma-ray bursts.

Taking images in different filters simultaneously is important for  the study of many astrophysical sources, and in particular of  variable sources, such as close binaries or active galactic nuclei.  But it is most crucial in the follow-up of gamma-ray bursts. Gamma- ray bursts (GRBs) are short flashes of energetic gamma-rays lasting  from less than a second to several minutes. They release a tremendous  quantity of energy in this short time making them the most powerful  events since the Big Bang.

Gamma-ray bursts, which are invisible to our eyes, are discovered by  telescopes 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. This 'afterglow' fades very  rapidly, making detailed analysis possible for only a few hours after  the gamma-ray detection. This analysis is important in particular in  order to determine the GRB's distance and, hence, intrinsic brightness.

A first determination of the distance can be done by taking images  through different filters, using the so-called photometric redshift  [1]. Because a typical GRB afterglow becomes 15 times fainter after  just 10 minutes, and over 200 times fainter after an hour, it is  important to observe the object in as many filters as possible  simultaneously.

"To make the determination of distance of far-away objects as  accurate as possible, we decided to use four different filters in the  optical and three different filters in the near-infrared," says  Jochen Greiner, who led the development of the GROND instrument.  GROND stands for Gamma-Ray burst Optical/Near-Infrared Detector.

GROND takes thus images of the same region of the sky in 7 different  filters. The field of view in the near-infrared is 10 times 10  arcminutes, or 1/7th the area of the Full Moon. It is smaller in the  visible, slightly above 5 x 5 arcmin.

GROND is presently in its commissioning phase and its first science  demonstration has been achieved, showing that all technical systems  work properly. In particular, GROND observed a quasar located more  than 12 billion light-years away.

As for many instruments specialising in the follow-up of gamma-ray  bursts (see e.g. ESO 17/07 and 26/07), GROND can also be activated  with a Rapid Response Mode (RRM): GRB alerts will be automatically  fed into the system thus minimising the delay between the gamma-ray  burst detection by a satellite and its observation by GROND.

"The implementation of the RRM at the 2.2-m telescope is done in  exactly the same way as for the VLT, and boosts ESO's leadership to  offer observing systems with ultra fast response time towards GRB  follow-up," says Michael Sterzik, Head of Science Operations  Department at ESO La Silla.

A dedicated data analysis pipeline is also being tested which will  provide the distance of the burst a few minutes after the first  observations.

"Ultimately, the goal is to trigger ESO's VLT to perform spectroscopy  of the source with fine-tuned settings, thereby maximising the  scientific return of GRB follow-up observations of the VLT," says  Greiner.

GROND has been built by the Max-Planck Institute for Extraterrestrial  Physics in collaboration with the Thüringer Landessternwarte Tautenburg.

  Note

[1] 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 larger the distance of an object is, the larger is its velocity  and, due to the Doppler effect, the spectral shift of its emission  towards longer (redder) wavelengths. Thus, the measured colour  provides a rough indication of the distance.

Contacts:

Jochen Greiner
MPI f. extraterrestrical Physics, Garching, Germany
Phone: +49-89-30000-3847
Email: jcg@mpe.mpg.de

Michael Sterzik
ESO, Chile
Email: msterzik@eso.org