Environment of The Gamma-Ray Burst GRB971214 : A Giant H II Region surrounded by A Galactic Supershell

Abstract

Among a number of gamma ray bursts whose host galaxies are known, GRB971214 stands out for its high redshift $z\ge 3$ and the Ly$α$ emission line having a P-Cygni type profile, which is interpreted to be a direct consequence of the expanding supershell. From a profile fitting analysis we estimate the expansion velocity of the supershell $v_{exp} = 1500\kms$ and the neutral column density $N_{HI}=10^{20}\cm^{-2}$. The redshift $z=3.418$ of the host galaxy proposed by Kulkarni et al. (1998) has been revised to be $z=3.425$ from our profile analysis. The observed Ly$α$ profile is fitted well by a Gaussian curve, which yields the Ly$α$ luminosity $L_{Lyα}=(1.8\pm0.8)\times10^{42}\ergs \s^{-1}$. Assuming that the photon source is a giant H II region, we deduce the electron number density in the H II region $n_e=(40\pm10) ({R \over {100 \pc}})^{-1.5}\cm^{-3}$, which corresponds to the illumination by about $10^4$ O5 stars. We estimate the star-formation rate to be $R_{SF} = (7\pm3){\rm M}_\odot\yr^{-1}$ with the internal and the Galactic extinction corrected. The theory on the evolution of supernova remnants is used to propose that the supershell is at the adiabatic phase, with its radius $R = 18 E_{53}^{1/2} \pc$, its age $t = 4.7\times10^3\ E_{53}^{1/2} \yrs$, and the density of the ambient medium $n_1 = 5.4\ E_{53}^{-1/2}\cm^{-3}$, where $E_{53}= E/10^{53}\ergs$.And we estimate the kinetic energy of the supershell to be $E_k=7.3\times10^{52} E_{53} \ergs$. These values are consistent with the hypothesis that the supershell is the remnant of a gamma ray burst.