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., $z\gtrsim 6$), 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 inverse-Compton 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, $\nu^{-(p+6)/4}$, 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, $\nu^{-(p+2)/2}$. 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.