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 ${ϵ}_e=(6.9\pm 0.3)\times {10}^{-2}$, ${ϵ}_B=(7.73\pm 0.62)\times {10}^{-6},E_K=(6.32\pm 0.86)\times {10}^{53}erg$, $n=0.100\pm 0.014$ 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 E_p of its time-integrated prompt gamma-ray spectrum is ∼26 KeV. Using the initial Lorentz factor (${\Gamma }_0={476}_{-237}^{+225}$) derived from our afterglow model fit, it is found that GRB 111228A satisfies the $L_{iso}-E_{p,z}-{\Gamma }_0$ relation and bridges the typical GRBs and low luminosity GRBs in this relation.