FIRST DETECTION OF THE OPTICAL COUNTERPART OF A GAMMA-RAY BURST

WHT+Prime Focus, INT Prime Focus

Since their discovery Gamma-Ray Bursts (GRBs) have been one of astronomy's great mysteries. The distribution and properties of the bursts are explained naturally if they lie at cosmological distances, but there is an opposing view that they are relatively local objects, perhaps distributed in a very large halo around our Galaxy. For a long time it was expected that the detection of a counterpart at other wavelengths would provide the key to understanding the GRB phenomenon. However, such counterparts were not found, in spite of much effort during the last 25 years. The main problem was the lack of fast and accurate GRB positions. With the launch of the Wide Field Cameras (WFCs) on board the Italian-Dutch X-ray satellite BeppoSAX this has changed. For the first time GRB positions can be determined with accuracies of a few arcminutes within a few hours after the burst, unprecedented in GRB astronomy.

Finally the situation changed dramatically on February 28, 1997 when a team of astronomers led by Jan van Paradijs of the University of Amsterdam and the University of Alabama in Huntsville pointed the William Herschel Telescope to the part of the sky where shortly before a new GRB had been detected (GRB 970228) by the Gamma-Ray Burst Monitor onboard BeppoSAX satellite.

On February 28, UT 23 h 48 m, 20.8 hours after the GRB occurred, the astronomers obtained a V-band and an I-band image (exposure times 300 s each) of the WFC error box with the Prime Focus camera of the William Herschel Telescope. The 1024 × 1024 pixel CCD frames cover a 7.2' × 7.2' field, well matched to the size of the GRB error box. The limiting magnitudes of the images were V=23.7, and I=21.4. They obtained a second I-band image on March 8, UT 21 h 12 m with the same instrument on the WHT (exposure time 900 s), and a second V-band image on March 8, UT 20 h 42 m with the Isaac Newton Telescope (exposure time 2500 s).
 

Discovery image of the optical counterpart of a Gamma-Ray Burst (GRB). OT: Optical Transient. The left panel shows the part of the sky where the GRB occurred as seen by the William Herschel Telescope. The right panel shows the same part of the sky only a few days later with the Isaac Newton Telescope, when the source had already become much fainter. After more than two decades of intensive searches for signs in the visible light of these extremely energetic events in the Universe, these two images show that GRBs can be spotted with optical telescopes from the ground. This discovery is helping scientists to unreveal the true nature of the cataclysmic events known as Gamma-Ray Bursts. [ GIF ]

A comparison of the two image pairs immediately revealed one object with a large brightness variation: it was clearly detected in both the V- and I-band images taken on 28 February, but not in the second pair of images taken on 8 March. The position of this object was coincident with all the known error-boxes of GRB 970228. This led the discovery team to conclude that they had identified the first example of optical afterglow of a GRB.

Following this first detection, other optical counterparts of GRBs have been discovered and followed up photometrically and spectroscopically. In most of these subsequent detections the ING telescopes have played an important role.

References
 

• T Galama et al, 1997, "The decay of optical emission from the gamma-ray burst GRB 970228", Nature, 387, 479.
• T Galama et al, 1997, "Radio and optical follow-up observations and improved interplanetary network position of GRB 970111", Astrophys J, 486, L5.
• P J Groot and T J Galama, 1997, "Optical Afterglow of a Gamma-Ray Burst: GRB970228", Spectrum Newsletter, 14, 8.
• P J Groot et al, 1997, "GRB 970228", IAU circular 6584.
• J van Paradijs et al, 1997, "Transient optical emission from the error box of the gamma-ray burst of 28 February 1997", Nature, 386, 686.