The broken light curves of gamma-ray bursts GRB 990123 and GRB 990510

Citation
S. Holland et al., The broken light curves of gamma-ray bursts GRB 990123 and GRB 990510, ASTRON ASTR, 364(2), 2000, pp. 467-478
Citations number
47
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY AND ASTROPHYSICS
ISSN journal
0004-6361 → ACNP
Volume
364
Issue
2
Year of publication
2000
Pages
467 - 478
Database
ISI
SICI code
0004-6361(200012)364:2<467:TBLCOG>2.0.ZU;2-3
Abstract
We have collected all of the published photometry for GRB 990123 and GRB 99 0510, the first two gamma-ray bursts where breaks were seen in the light cu rves of their optical afterglows, and determined the shapes of their light curves and the break times. These parameters were used to investigate the p hysical mechanisms responsible for the breaks and the nature of the ambient medium that the bursts occurred in. The light curve for GRB 990123 is best fit by a broken power law with a break 1.68 +/- 0.19 days after the burst, a slope of alpha (1) = -1.12 +/- 0.08 before the break, and a slope of alp ha (2) = -1.69 +/- 0.06 after the break. This is consistent with a collimat ed outflow with a fixed opening angle of theta (0) approximate to 5 degrees . In this case the break in the light curve is due to the relativistic fire ball slowing to Gamma approximate to 1/theta (0). The light curve for. GRB 990510 is best fit by a continuous function with an early-time slope of alp ha (1) = -0.54+/-0.14, a late-time slope of alpha (2) = -1.98 +/- 0.19, and a slow transition between the two regimes approximately one day after the burst. This is consistent with a collimated outflow with theta (0) approxim ate to 5 degrees that is initially radiative, but undergoes a sideways expa nsion that begins approximately one day after the burst. This sideways expa nsion is responsible for the slow break in the light curve.