CFHT, Current Image of the Week


March 6th, 2000

A very recent (!) observation of a candidate
for the optical counterpart of a Gamma Ray Burst

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The observation of a candidate for the optical counterpart of a GRB000301c
2000 March 5.6 (UTC)

Credit:  Image courtesy of C. Veillet (CFHT)

Gamma ray bursts (GRB) were first seen by accident in the 1960's at the time of the Cold War, when the U.S.A. was monitoringnuclear weapon tests. Since then, GRBs have been the target of ground based and satellite observations in an attempt to answer questions as basic as "where are they in the Universe?" or "what is really happening there?"... They appear as a burst of gamma rays (the highest energy waves (or photons) of the electromagnetic spectrum).

A satellite, the Compton Gamma Ray Observatory carrying an instrument called the Burst and Transient Source Experiment (BATSE), is continuously watching the sky. When a gamma ray burst is detected, a network of observatories around the world, as well as another satellite (BeppoSAX) point the available telescopes to the position determined by BATSE (a large area of the sky which BeppoSAX can often refine) in an attempt to find an optical counterpart to the GRB.

In February 1999, the "explosion" of a GRB (GRB990123) was witnessed in the optical domain, in which a very bright "star" was seen, which rapidly faded to the point where it could be seen only with large telescopes. Through their spectrum, some GRBs observed in the optical or near infrared have been associated with galaxies detected very close to their position. The question "where are they?" has thus been answered: they are located in galaxies far away from us.
The February 1999 event has been called "the largest explosion since the Big Bang", at least the largest we saw! As for what they really are,  one hypothesis, among many, is that they could be very hot matter falling onto a massive black hole in the center of the host galaxy.

The picture of this week shows CFHT images of a candidate for GRB000301c, a gamma ray burst detected on March 1st 2000 only a few days ago. An optical source not found on reference images has been detected by a European team using the 2.5m Nordic Optical Telescope two days after the alert, and since then various observatories have been trying to gather as much information as possible on this object. It has been seen by radio telescopes, and in the visible and infrared. The source is fading in the visible and near infrared at a rate which suggests that it is indeed the counterpart of the GRB.

This new source is labeled as OT (optical transient) on the images. A faint source has also been detected very close to the OT. It is better seen on the image on the right, which is a processed version of the left hand one. Another telescope on Mauna Kea (NASA IRTF) also detected this faint source in the near infrared. It could be the GRB host galaxy... As you can imagine, many more observations using the largest telescopes on the ground and the HST will be conducted  to unveil some of the mysteries of these cataclysmic events.

Here are more details on these CFHT observations.

Technical description:

The image on the left is a raw image obtained with the MultiObject Spectrograph (MOS) at the Cassegrain focus of the Canada-France-Hawaii Telescope, used in imaging mode. The detector is a STIS2 CCD, with 21 micrometers pixels and a scale of 0.4"/pixel.  On the left, the raw image is shown. On the right, the image has been processed (pixel spreading and constrast enhancement) to better show the possible host galaxy. The OT R magnitude is close to 21, while the faint diffuse source is fainter than 24.

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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.