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 sec to about 50 hours after the burst trigger and investigate its properties of the prompt gamma-rays and the ambient medium using our data and the data observed with Swift and Fermi missions .
Our joint optical and X-ray spectral fits to the afterglow data show that the ambient medium features as low dust-to-gas ratio .
Incorporating the energy injection effect , our best fit to the afterglow lightcurves with the standard afterglow model via the Markov Chain Monte Carlo ( MCMC ) technique shows that \epsilon _ { e } = ( 6.9 \pm 0.3 ) \times 10 ^ { -2 } , \epsilon _ { 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 .
Achromatic 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 \sim 26 KeV .
Using the initial Lorentz factor ( \Gamma _ { 0 } = 476 ^ { +225 } _ { -237 } ) 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 .