Polarization in GRB 990510

The object of study is the remnant of a mysterious cosmic explosion far out in space, first detected as a gigantic outburst of gamma rays on May 10. Gamma-Ray Bursters (GRBs) are brief flashes of very energetic radiation — they represent by far the most powerful type of explosion known in the Universe and their afterglow in optical light can be 10 million times brighter than the brightest supernovae. The May 10 event ranks among the brightest one hundred of the over 2500 GRBs detected in the last decade. It is a preliminary polarization measurement of the optical image of the afterglow of GRB 990510, as observed with the VLT 8.2-m ANTU telescope and the multi-mode FORS1 instrument. The abscissa represents the measurement angle; the ordinate the corresponding intensity. The sinusoidal curve shows the best fit to the data points (with error bars); the resulting degree of polarization is 1.7 ± 0.2 percent.