Coasting External Shock in Wind Medium: An Origin for the X-Ray Plateau Decay Component in Swift Gamma-Ray Burst Afterglows
Abstract
The plateaus observed in about one half of the early X-ray afterglows are the most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By analyzing the temporal and spectral indices of a large X-ray plateau sample, we find that 55% can be explained by external, forward shock synchrotron emission produced by a relativistic ejecta coasting in a ρvpropr -2, wind-like medium; no energy injection into the shock is needed. After the ejecta collects enough medium and transitions to the adiabatic, decelerating blast wave phase, it produces the post-plateau decay. For those bursts consistent with this model, we find an upper limit for the initial Lorentz factor of the ejecta, Γ0 <= 46(epsilon e /0.1)-0.24(epsilon B /0.01)0.17; the isotropic equivalent total ejecta energy is E iso ~ 1053(epsilon e /0.1)-1.3(epsilon B /0.01)-0.09(tb /104 s) erg, where epsilon e and epsilon B are the fractions of the total energy at the shock downstream that are carried by electrons and the magnetic field, respectively, and tb is the end of the plateau. Our finding supports Wolf-Rayet stars as the progenitor stars of some GRBs. It raises intriguing questions about the origin of an intermediate-Γ0 ejecta, which we speculate is connected to the GRB jet emergence from its host star. For the remaining 45% of the sample, the post-plateau decline is too rapid to be explained in the coasting-in-wind model, and energy injection appears to be required.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2012
- DOI:
- 10.1088/0004-637X/744/1/36
- arXiv:
- arXiv:1109.3453
- Bibcode:
- 2012ApJ...744...36S
- Keywords:
-
- gamma-ray burst: general;
- radiation mechanisms: non-thermal;
- relativistic processes;
- shock waves;
- X-rays: bursts;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 11 pages, 5 figures, to appear in ApJ, proof-corrected version, added more references