Abstract
The gamma-ray burst (GRB) central engine intrinsic activity time {{T}ce} is usually described through either the γ-ray duration T90 or through a generalized burst duration {{t}burst} that includes both the γ-ray emission and (when present) an extended flaring X-ray plateau. Here, we define a more specific operational description of {{T}ce}, and within the framework of the internal-external shock model, we develop a numerical code to study the relationship between T90 and {{T}ce}, as well as between {{t}burst} and {{T}ce}, for different initial conditions. We find that when {{T}ce}≲ {{10}4} s, late internal collisions or refreshed external collisions result in values of {{T}90} and {{t}burst} larger than {{T}ce}, usually by factors of 2-3. For {{T}ce}≳ {{10}4} s, the {{t}burst} is always a good estimator for {{T}ce}, while T90 can underpredict {{T}ce} when the late central engine activity is moderate. We find a clear bimodal distribution for {{T}ce}, based on our simulations as well as on the observational data for T90 and {{t}burst}. We suggest that {{t}burst} is a reliable measure for defining “ultra-long” GRBs. Bursts with T90 of order 103 s need not belong to a special population, while bursts with {{t}burst}\gt {{10}4} s, where the late central engine activity is more moderate and shows up in X-rays, may represent a new population. These conclusions are insensitive to the initial conditions assumed in the models.
- Publication:
- The Astrophysical Journal, Volume 802, Issue 2, article id. 90,
10 pp. (2015). - Pub Date:
- April 2015
- DOI:
- 10.1088/0004-637X/802/2/90
- arXiv:
- arXiv:1411.2650
- Bibcode:
- 2015ApJ...802...90G
- Keywords:
- gamma-ray burst: general;Astrophysics - High Energy Astrophysical Phenomena