Abstract
At peak, long-duration gamma-ray bursts are the most luminous sources of electromagnetic radiation known. Since their progenitors are massive stars, they provide a tracer of star formation and star-forming galaxies over the whole of cosmic history. Their bright power-law afterglows provide ideal backlights for absorption studies of the interstellar and intergalactic medium back to the reionization era. The proposed THESEUS mission is designed to detect large samples of GRBs at z > 6 in the 2030s, at a time when supporting observations with major next generation facilities will be possible, thus enabling a range of transformative science. THESEUS will allow us to explore the faint end of the luminosity function of galaxies and the star formation rate density to high redshifts; constrain the progress of re-ionisation beyond zgtrsim 6; study in detail early chemical enrichment from stellar explosions, including signatures of Population III stars; and potentially characterize the dark energy equation of state at the highest redshifts.
Highlights
A major goal of contemporary astrophysics and cosmology is to achieve a detailed understanding of the formation of the first collapsed objects (Pop-III and earlyExtended author information available on the last page of the article.Experimental AstronomyPop-II stars, black holes and the primordial galaxies in which they were born) during the first billion years in the life of the Universe
Accounting for the observational selection function, we will obtain the evolution of the global star formation rate density at redshifts where uncertainties from traditional galaxy observations begin to rise. This will firmly establish whether the tension between GRB counts and extrapolated galaxy counts remains, demanding a more radical reappraisal of high-z star formation (Fig. 2). As another way to demonstrate the power of the THESEUS sample, in Fig. 3 we show the accuracy at which will measure the decline slope of the star formation rate density with redshift (−dlog(SFRD)/d log(1 + z)) for the cosmic star formation at z > 6 after 1 and 3.45 years of operations
The identification of any discrepancy between GRB-based measurements and those obtained by different methods could provide important insights about role of faint galaxies, missed even in future deep surveys, in shaping the cosmic star formation history, about the possible evolution of the stellar initial mass function (IMF) in the very high-z Universe, and about the existence of an important population of bright X-ray transients, such as those expected from Pop-III stars (Section 9)
Summary
A major goal of contemporary astrophysics and cosmology is to achieve a detailed understanding of the formation of the first collapsed objects GRBs offer a unique route to detecting the death-throes of individual massive stars to very high redshifts (Fig. 1; [33,34,35,36,37]), which in turn provides multiple powerful probes of early star formation, metal enrichment and galaxy evolution, potentially even before the main phase of reionization They are detectable independently of the luminosity of their underlying hosts and so can pinpoint the presence of massive star formation in distant galaxies below the sensitivity limit of even the most powerful facilities foreseen in the long-term future. Their afterglow counterparts can be used as bright background lighthouses probing in
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