A new instrument has seen First Light at the ESO La Sillaย Observatory. Equipping the 2.2-m MPI/ESO telescope, GROND takesย images simultaneously in seven colours. It will be mostly used toย determine distances of gamma-ray bursts.
Taking images in different filters simultaneously is important forย the study of many astrophysical sources, and in particular ofย variable sources, such as close binaries or active galactic nuclei.ย But it is most crucial in the follow-up of gamma-ray bursts. Gamma- ray bursts (GRBs) are short flashes of energetic gamma-rays lastingย from less than a second to several minutes. They release a tremendousย quantity of energy in this short time making them the most powerfulย events since the Big Bang.
Gamma-ray bursts, which are invisible to our eyes, are discovered byย telescopes in space. After releasing their intense burst of high- energy radiation, they become detectable for a fleeting moment in theย optical and in the near-infrared. This ‘afterglow’ fades veryย rapidly, making detailed analysis possible for only a few hours afterย the gamma-ray detection. This analysis is important in particular inย order to determine the GRB’s distance and, hence, intrinsic brightness.
A first determination of the distance can be done by taking imagesย through different filters, using the so-called photometric redshiftย [1]. Because a typical GRB afterglow becomes 15 times fainter afterย just 10 minutes, and over 200 times fainter after an hour, it isย important to observe the object in as many filters as possibleย simultaneously.
“To make the determination of distance of far-away objects asย accurate as possible, we decided to use four different filters in theย optical and three different filters in the near-infrared,” saysย Jochen Greiner, who led the development of the GROND instrument.ย GROND stands for Gamma-Ray burst Optical/Near-Infrared Detector.
GROND takes thus images of the same region of the sky in 7 differentย filters. The field of view in the near-infrared is 10 times 10ย arcminutes, or 1/7th the area of the Full Moon. It is smaller in theย visible, slightly above 5 x 5 arcmin.
GROND is presently in its commissioning phase and its first scienceย demonstration has been achieved, showing that all technical systemsย work properly. In particular, GROND observed a quasar located moreย than 12 billion light-years away.
As for many instruments specialising in the follow-up of gamma-rayย bursts (see e.g. ESO 17/07 and 26/07), GROND can also be activatedย with a Rapid Response Mode (RRM): GRB alerts will be automaticallyย fed into the system thus minimising the delay between the gamma-rayย burst detection by a satellite and its observation by GROND.
“The implementation of the RRM at the 2.2-m telescope is done inย exactly the same way as for the VLT, and boosts ESO’s leadership toย offer observing systems with ultra fast response time towards GRBย follow-up,” says Michael Sterzik, Head of Science Operationsย Department at ESO La Silla.
A dedicated data analysis pipeline is also being tested which willย provide the distance of the burst a few minutes after the firstย observations.
“Ultimately, the goal is to trigger ESO’s VLT to perform spectroscopyย of the source with fine-tuned settings, thereby maximising theย scientific return of GRB follow-up observations of the VLT,” saysย Greiner.
GROND has been built by the Max-Planck Institute for Extraterrestrialย Physics in collaboration with the Thรผringer Landessternwarte Tautenburg.
ย Note
[1] The photometric redshift method makes it possible to judge theย distance to a remote celestial object (a galaxy, a quasar, a gamma- ray burst afterglow) from its measured colours. It is based on theย proportionality between the distance and the velocity along the lineย of sight (Hubble’s law) that reflects the expansion of the Universe.ย The larger the distance of an object is, the larger is its velocityย and, due to the Doppler effect, the spectral shift of its emissionย towards longer (redder) wavelengths. Thus, the measured colourย 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