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

• Published in: arXiv.org 2015-12
• Main Authors: Li-Ping, Xin, Yuan-Zhu, Wang, Ting-Ting, Lin, En-Wei, Liang, Hou-Jun Lü, Shu-Qing Zhong, Urata, Yuji, Xiao-Hong, Zhao, Wu, Chao, Jian-Yan, Wei, Kui-Yun Huang, Yu-Lei, Qiu, Jin-Song, Deng
• Format: Article
• Language: English
• Subjects: Afterglows; Computer simulation; Gamma ray bursts; Gamma rays; Lorentz factor; Luminosity; Magnetars; Markov chains; Missions; Monte Carlo simulation; X ray spectra
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1512.07697

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 \(\epsilon_e=(6.9\pm 0.3)\times 10^{-2}\), \(\epsilon_B=(7.73\pm 0.62)\times 10^{-6}\), \(E_{\rm K}=(6.32\pm 0.86)\times 10^{53}\rm erg\), \(n=0.100\pm 0.014\) 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 \(L_{\rm iso}-E_{\rm p,z}-\Gamma_0\) relation and bridges the typical GRBs and low luminosity GRBs in this relation.