GRBs as Probes of the Early Universe with TSO

TL;DR

This paper discusses how gamma-ray bursts (GRBs) can be used as powerful probes to study the early universe, including star formation and reionization, with the proposed TSO telescope enabling rapid, detailed observations of these distant phenomena.

Contribution

It introduces the TSO telescope concept, capable of prompt, multi-band spectroscopy of high-redshift GRBs, to directly investigate early universe processes.

Findings

GRBs are the most luminous sources, ideal for probing the early universe.

Current telescopes cannot promptly observe optically dark, high-z GRBs.

TSO will enable rapid, multi-band spectroscopy of GRBs at z > 6.

Abstract

Long gamma-ray bursts (GRBs) are the most luminous known electromagnetic radiation sources in the Universe for the 3 to 300 sec of their prompt flashes (isotropic X/ gamma-ray luminosities up to 10^53 ergs/sec). Their afterglows have first day rest-frame UV/optical absolute magnitudes AB = -30 to -23. This luminous continuum nUV-nIR back-light provides the ultimate probe of the SFR(z) back to the first Pop III to II.5 stars, expected to be massive and GRB progenitors. GRB afterglow spectra in the first 1 to 3 hours will directly measure their host galaxy ionization fraction x_i vs. z in the Epoch of Reionization (EOR), tracing the growth of structure. Only 28% of Swift GRBs have measured redshifts due to limited followup at R, J >21. Some ~25% of GRBs are optically dark due to dust absorption in their host galaxies, but those with low NH in their X-ray spectra are likely at z >7. Current 8-10m telescopes and coming ELTs cannot pursue optically dark GRBs promptly, nor can JWST or WFIRST slew within 0.5 to 1 days of a GRB. The Time-domain Spectroscopic Observatory (TSO) is a proposed Probe-class 1.3m telescope at L2, with imaging and spectroscopy (R = 200, 1800) in 4 bands (0.3 to 5um) and rapid slew capability to 90% of sky. TSO would finally utilize z > 6 to 12 GRBs as the most direct probe of the SFR(z), EOR(z), and possibly the first direct detection of the core collapse of the very first (Pop III) stars.