A Spectral Feature of High-redshift Gamma-ray Bursts: Probing the Earliest Starlight Background Radiation

Gamma-ray bursts (GRBs) and their afterglows at high redshifts have been widely believed to be detectable. Here we analyze a new feature of the MeV spectra of high-redshift GRBs, which is unlikely to appear in low-redshift GRBs. We generally discuss high-energy emission above a few decades of GeV due to synchrotron self-Compton scattering in the internal shock model. Our discussion seems to be supported by the high-energy spectra of several low-redshift GRBs. However, if GRBs originate at high redshifts (e.g., z ∼> 6), such photons cannot be detected because they may collide with cosmic optical and ultraviolet background photons, leading to electron/positron pair production. We show that inverseCompton scattering of the resulting electron/positron pairs off cosmic microwave background photons will produce an additional multi-MeV component, resulting thus in a spectral “bump”. We also derive the scattered photon spectrum of such a bump, ν, where p is the index of the electron energy distribution behind internal shocks. This is slightly harder than the synchrotron photon spectrum in the internal shock model, ν. The lower energy emission property of this bump is dependent on the spectral energy distribution of the starlight background radiation. Therefore, future observations on the MeV spectral bump of high-redshift GRBs would provide a probe of the earliest starlight background radiation as well as the first star formation. Subject headings: diffuse radiation — gamma rays: bursts — gamma rays: theory