Multi-Wavelength Observations of GRB 111228A and Implications for the Fireball and its environment

Abstract

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 {\em Swift} and {\em 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 ${\epsilon }_e=(6.9\pm 0.3)\times 10^{-2}$, ${\epsilon }_B=(7.73\pm 0.62)\times 10^{-6}$, $E_{\mathrm{K}}=(6.32\pm 0.86)\times 10^{53}\mathrm{erg}$, $n=0.100\pm 0.014$ 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 (${\Gamma }_0=476_{-237}^{+225}$) derived from our afterglow model fit, it is found that GRB 111228A satisfies the $L_{\mathrm{iso}}-E_{\mathrm{p},\mathrm{z}}-{\Gamma }_0$ relation and bridges the typical GRBs and low luminosity GRBs in this relation.

33 pages, 18 figures, 2 tables. Accepted by ApJ

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