Accessing the population of high redshift Gamma Ray Bursts

TL;DR

This paper presents a tool to estimate high-redshift Gamma Ray Burst detection rates, emphasizing the importance of soft X-ray sensitivity for discovering the most distant GRBs and assessing follow-up strategies.

Contribution

It introduces a population model for high-z GRBs, evaluates detection strategies across energy bands, and estimates the effectiveness of NIR follow-up for redshift confirmation.

Findings

Soft X-ray detectors with high sensitivity can detect approximately 40 high-z GRBs per year per steradian.

High-energy detectors mainly detect the most luminous high-z GRBs due to spectral energy correlations.

Early NIR afterglow flux estimates improve redshift measurement prospects for high-z GRBs.

Abstract

Gamma Ray Bursts (GRBs) are a powerful probe of the high redshift Universe. We present a tool to estimate the detection rate of high-z GRBs by a generic detector with defined energy band and sensitivity. We base this on a population model that reproduces the observed properties of GRBs detected by Swift, Fermi and CGRO in the hard X-ray and gamma-ray bands. We provide the expected cumulative distributions of the flux and fluence of simulated GRBs in different energy bands. We show that scintillator detectors, operating at relatively high energies (e.g. tens of keV to the MeV), can detect only the most luminous GRBs at high redshifts due to the link between the peak spectral energy and the luminosity (Ep-Liso) of GRBs. We show that the best strategy for catching the largest number of high-z bursts is to go softer (e.g. in the soft X-ray band) but with a very high sensitivity. For instance, an imaging soft X-ray detector operating in the 0.2-5 keV energy band reaching a sensitivity, corresponding to a fluence of ~10^-8 erg cm^-2, is expected to detect ~40 GRBs yr^-1 sr^-1 at z>5 (~3 GRBs yr^-1 sr^-1 at z>10). Once high-z GRBs are detected the principal issue is to secure their redshift. To this aim we estimate their NIR afterglow flux at relatively early times and evaluate the effectiveness of following them up and construct usable samples of events with any forthcoming GRB mission dedicated to explore the high-z Universe.