Abstract
The afterglow of the Gamma-Ray Burst (GRB) 000301C exhibited achromatic, short time-scale variability that is difficult to reconcile with the standard relativistic shock model. We interpret the observed light curves as a microlensing event superimposed on power-law flux decays typical of afterglows. In general, a relativistic GRB shock appears on the sky as a thin ring expanding at a superluminal speed. Initially the ring is small relative to its angular separation from the lens and so its flux is magnified by a constant factor. As the ring grows and sweeps across the lens its magnification reaches a maximum. Subsequently, the flux gradually recovers its unlensed value. This behavior involves only three free parameters in its simplest formulation and was predicted theoretically by Loeb & Perna (1998). Fitting the available R-band photometric data of GRB 000301C to a simple model of the microlensing event and a broken power-law for the afterglow, we find reasonable values for all the parameters and a reduced chi^2/DOF parameter of 1.48 compared with 2.99 for the broken power-law fit alone. The peak magnification of ~2 occurred 3.8 days after the burst. The entire optical-IR data imply a width of the GRB ring of order 10% of its radius, similar to theoretical expectations. The angular resolution provided by microlensing is better than a micro-arcsecond. We infer a mass of approximately 0.5 M_Sun for a lens located half way to the source at z_s=2.04. A galaxy 2'' from GRB 000301C might be the host of the stellar lens, but current data provides only an upper-limit on its surface brightness at the GRB position.
Highlights
The rapid localization of gamma-ray bursts (GRBs) has brought a new dimension to Gamma-Ray Burst (GRB) research by allowing many events to be followed up at longer wavelengths
A broad-band break in the light curve power-law index was predicted for shocks produced by collimated jets (Rhoads 1997) and such breaks have been seen in GRB 990510 (Stanek et al 1999; Harrison et al 1999), GRB 991216 (Halpern et al 2000), and GRB 000301C (Sagar et al 2000; Masetti et al 2000; Jensen et al 2000; Berger et al 2000)
Deep Hubble Space Telescope (HST) imaging of the GRB and field were obtained by Fruchter, Metzger & Petro (2000) but no host or intervening galaxy was detected to a limit of 28.5 mag
Summary
The rapid localization of gamma-ray bursts (GRBs) has brought a new dimension to GRB research by allowing many events to be followed up at longer wavelengths. The light curves and spectral energy distributions are well fit by power-laws as expected from the shock model. A broad-band break in the light curve power-law index was predicted for shocks produced by collimated jets (Rhoads 1997) and such breaks have been seen in GRB 990510 (Stanek et al 1999; Harrison et al 1999), GRB 991216 (Halpern et al 2000), and GRB 000301C (Sagar et al 2000; Masetti et al 2000; Jensen et al 2000; Berger et al 2000). The synchrotron afterglow model has been very successful in matching most of the observations. It is heavily strained explaining the well-studied afterglow of GRB 000301C which shows a peculiar achromatic fluctuation which deviates significantly from the broken power-law fit to the lightcurve (Sagar et al 2000; Berger et al 2000).
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