Constraints on the Bulk Lorentz Factors of GRB X-Ray Flares

Abstract

X-ray flares were discovered in the afterglow phase of gamma-ray bursts (GRBs) by the {\em Swift} satellite a decade ago and known as a canonical component in GRB X-ray afterglows. In this paper, we constrain the Lorentz factors of GRB X-ray flares using two different methods. For the first method, we estimate the lower limit on the bulk Lorentz factor with the flare duration and jet break time. In the second method, the upper limit on the Lorentz factor is derived by assuming that the X-ray flare jet has undergone saturated acceleration. We also re-estimate the initial Lorentz factor with GRB afterglow onsets, and find the coefficient of the theoretical Lorentz factor is 1.67 rather than the commonly used 2 for interstellar medium (ISM) and 1.44 for the wind case. We find that the correlation between the limited Lorentz factor and the isotropic radiation energy of X-ray flares in the ISM case is more consistent with that of prompt emission than the wind case in a statistical sense. For a comparison, the lower limit on Lorentz factor is statistically larger than the extrapolation from prompt bursts in the wind case. Our results indicate that X-ray flares and prompt bursts are produced by the same physical mechanism.

16 pages, 6 figures, 1 table, accepted for publication in ApJ

References in corpus (7)

• The Physics of Gamma-Ray Bursts and Relativistic Jets
• Gamma-Ray Bursts: Progress, Problems & Prospects
• X-Ray Flares from Postmerger Millisecond Pulsars
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
• A new method of determining the initial size and Lorentz factor of gamma-ray burst fireballs using a thermal emission component
• Swift and Fermi observations of X-ray flares: the case of Late Internal Shock

Cited by in corpus (5)

• Statistical Distributions of Optical Flares from Gamma-Ray Bursts
• Central Engine of Late-Time X-ray Flares with Internal Origin
• External Shock in a Multi-Bursting Gamma-ray Burst: Energy Injection Phase induced by the Later Launched Ejecta
• Could an X-ray Flare after GRB 170817A Originate from a Post-merger Slim Accretion Disc?
• Constraining External Reverse Shock Physics of GRBs from ROTSE-III Limits