Long GRBs are believed to be powered by the core collapses of massive stars, but the progenitors of short GRBs are mergers of binary compact stars. Because of the spectroscopy of their optical afterglows and their extreme brightness, GRBs can be detected up to very high redshifts. Being bright beacons in the deep Universe, high-z GRBs have been deemed as an ideal tool to explore the properties of the early universe.
Using the empirical correlations between GRB energetics and spectral properties, GRBs can be treated as “relative standard candles” to constrain cosmological parameters and dark energy, complementary to Type Ia supernova surveys. The collapsar model suggests that long GRBs should trace to some extent the star formation activity through cosmic times, and they therefore provide an independent way for measuring the high-z star formation history. They also offer the exciting opportunity to indirect search for Population III (Pop III) stars. The first generation of stars, the so-called Pop III stars, are formed with pristine gas containing no metals. Although such events are expected to be rare, occur at high redshifts, and have no detectable hosts, they had probably played an important role in early Universe evolution, including reionization, metal enrichment history. Some studies proposed that a few Pop III stars will end as GRBs, called Pop III GRBs which will be more energetic than any GRB populations. Direct observations of Pop III stars have been out of reach of the modern observational techniques, and the only viable way to reveal the properties of Pop III stars is through the detection of Pop III GRBs. In addition, through the observations of the smooth continuum spectra of GRB afterglows, one can obtain abundant information on the circumburst environment, on the gas and dust in their host galaxies, on the intergalactic medium (IGM) and the intervening systems. The metal absorption lines in the GRB spectra also make them as an effective approach to probe the metal enrichment history. GRBs can also be used to survey the neutral hydrogen fraction in the IGM by fitting the damping wing of the Ly and thereby study the cosmic reionization. Figure 1 shows the expected number of high-z GRBs detected with EP as a function of the mission lifetime. Over the nominal 3-year lifetime of the mission.
In the EP era, prompt follow-up and identification of candidate high-z GRBs detected are essential. A fast-responding optical and near-infrared follow-up network is required comprising of small and large telescopes (the next generation telescopes) with imaging and spectroscopic capability. That is, fast distribution of the EP alerts to the world-wide community will trigger coordinated follow-up observations, which would significantly improve the detection of high-z GRBs. The combined abundant observational data might reveal some of the mysteries of the early universe.