MULTI-WAVELENGTH OBSERVATIONS OF GRB 111228A AND IMPLICATIONS FOR THE FIREBALL AND ITS ENVIRONMENT

Observations of very early multi-wavelength afterglows are critical to reveal the properties of the radiating fireball and its environment as well as the central engine of gamma-ray bursts (GRBs). We report our optical observations of GRB 111228A from 95 s to about 50 hr after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data from the Swift and Fermi missions. Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features a low dust-to-gas ratio. Incorporating the energy injection effect, our best fit to the afterglow light curves with the standard afterglow model via the Markov Chain Monte Carlo technique shows that ϵ e = ( 6.9 ± 0.3 ) × 10 − 2 ?> , ϵ B = ( 7.73 ± 0.62 ) × 10 − 6 , E K = ( 6.32 ± 0.86 ) × 10 53 erg ?> , n = 0.100 ± 0.014 ?> cm−3. The low medium density likely implies that the afterglow jet may be in a halo or in a hot ISM. A chromatic shallow decay segment observed in the optical and X-ray bands is well explained with the long-lasting energy injection from the central engine, which would be a magnetar with a period of about 1.92 ms inferred from the data. The Ep of its time-integrated prompt gamma-ray spectrum is ∼26 KeV. Using the initial Lorentz factor ( Γ 0 = 476 − 237 + 225 ?> ) derived from our afterglow model fit, it is found that GRB 111228A satisfies the L iso − E p , z − Γ 0 ?> relation and bridges the typical GRBs and low luminosity GRBs in this relation.