Gamma‐Ray Bursts as a Probe of the Very High Redshift Universe

Abstract

There is increasingly strong evidence that gamma-ray bursts (GRBs) are associated with star-forming galaxies and occur near or in the star-forming regions of these galaxies. These associations provide indirect evidence that at least the long GRBs detected by BeppoSAX are a result of the collapse of massive stars. The recent evidence that the light curves and the spectra of the afterglows of GRB 970228 and GRB 980326 appear to contain a supernova component, in addition to a relativistic shock-wave component, provides more direct clues that this is the case. We show that, if many GRBs are indeed produced by the collapse of massive stars, GRBs and their afterglows provide a powerful probe of the very high redshift universe. We Ðrst establish that GRBs and their afterglows are both detectable out to (z Z 5) very high redshifts. We then show that one expects GRBs to occur out to at least z B 10, and possibly to z B 15È20, redshifts that are far larger than those expected for the most distant quasars. This implies that there are large numbers of GRBs with peak photon number Ñuxes below the detection thresholds of BATSE and HET E 2, and even below the detection threshold of Swift. The mere detection of very high redshift GRBs would give us our Ðrst information about the earliest generations of stars. We show that GRBs and their afterglows can be used as beacons to locate core-collapse supernovae at redshifts z ? 1 and to study the properties of these supernovae. We describe the expected properties of the absorptionline systems and the Lya forest in the spectra of GRB afterglows and discuss various strategies for determining the redshifts of very high redshift GRBs. We then show how the absorption-line systems and the Lya forest visible in the spectra of GRB afterglows can be used to trace the evolution of metallicity in the universe and to probe the large-scale structure of the universe at very high redshifts. Finally, we show how measurement of the Lya break in the spectra of GRB afterglows can be used to constrain, or possibly measure, the epoch at which reionization of the universe occurred by using the Gunn-Peterson test.

Strategies for prompt searches for GRB afterglows: The discovery of the GRB 001011 optical/near-infrared counterpart using colour-colour selection

“…This effect would enlarge the photometric errors of the high redshift afterglows and hence complicate their discrimination (see Lamb & Reichart 2000 for a detailed discussion on the effect of the monochromatic cosmological dimming factor).…”

“…The effect of Lyman-α blanketing in the R-band has its onset at z = 3.7 and is essentially independent of the intrinsic spectral slope. At a redshift of z ∼ 6.5, the attenuation reaches 5 mag, implying that GRB afterglows are unlikely to be detected in the R-band at redshifts much beyond 6 (see Lamb & Reichart 2000 for a discussion on the detectability of very high redshift GRBs).…”

“…Lattimer & Schramm 1974). The intrinsic brightness of GRBs and their afterglows allows one to probe the nature of their distant host galaxies and potentially the star-formation history of the early universe (Lamb & Reichart 2000).…”

On the nature of X-ray flashes

“…2.1). Nevertheless, even at large redshift a bright GRB is still observable (Lamb & Reichart 2000) but can appear as an XRF so that the XRF/GRB ratio is expected to increase with z. Figure 5 shows that this is indeed the case: the XRF/GRB ratio at z > 5 is more than 2 times larger than at z = 1.…”

GRBs and Fundamental Physics

“…The association of long GRBs with core-collapse supernovae has been confirmed from observations in recent years (Stanek et al 2003;Hjorth et al 2003), which provides a complementary technique for measuring the high-redshift SFR (Totani 1997;Wijers et al 1998;Lamb & Reichart 2000;Porciani & Madau 2001;Bromm et al 2002). The selection effects should be considered (for a review, see Coward 2007).…”