Astron. Astrophys. 355, L43-L46 (2000)

Letter to the Editor

Rapid fading of optical afterglows as evidence for beaming in gamma-ray bursts

Y.F. Huang ^1,2, Z.G. Dai ^1,3 and T. Lu <sup>1,3

1</sup> Nanjing University, Department of Astronomy, Nanjing 210093, P.R. China
² Nanjing University, Astronomical and Astrophysical Center of East China, Nanjing 210093, P.R. China
³ IHEP, Chinese Academy of Sciences, Beijing 100039, P.R. China

Received 21 January 2000 / Accepted 24 February 2000

Abstract

Based on the refined dynamical model proposed by us earlier for beamed -ray burst ejecta, we carry out detailed numerical procedure to study those -ray bursts with rapidly fading afterglows (i.e., ). It is found that optical afterglows from GRB 970228, 980326, 980519, 990123, 990510 and 991208 can be satisfactorily fitted if the -ray burst ejecta are highly collimated, with a universal initial half opening angle . The obvious light curve break observed in GRB 990123 is due to the relativistic-Newtonian transition of the beamed ejecta, and the rapidly fading optical afterglows come from synchrotron emissions during the mildly relativistic and non-relativistic phases. We strongly suggest that the rapid fading of afterglows currently observed in some -ray bursts is evidence for beaming in these cases.

Key words: Gamma rays: bursts - ISM: jets and outflows - stars: neutron - relativity

Send offprint requests to: T. Lu (tlu@nju.edu.cn)

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Contents

• 1. Introduction
• 2. Model
• 3. Numerical results
  • 3.1. GRB 990123
  • 3.2. GRB 990510
  • 3.3. GRB 980519
  • 3.4. GRB 991208
  • 3.5. GRB 970228 and 980326
• 4. Discussion and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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