Ly{beta} dark gaps in z<~6 QSOs spectra

With its lower optical depth, Ly{beta} offers a potentially more sensitive probe to any remaining neutral gas compared to the commonly used Ly{alpha} line. We identify dark gaps in the Ly{beta} forest using spectra of 42 QSOs at z_em_>5.5, including new data from the XQR-30 VLT Large Programme.

Approximately 40% of these QSO spectra exhibit dark gaps longer than 10h^-1^Mpc at z~5.8. By comparing the results to predictions from simulations, we find that the data are broadly consistent both with models where fluctuations in the Ly{alpha} forest are caused solely by ionizing ultraviolet background fluctuations and with models that include large neutral hydrogen patches at z<6 due to a late end to reionization. Of particular interest is a very long (L=28h^-1^Mpc) and dark ({tau}eff>~6) gap persisting down to z~5.5 in the Ly{beta} forest of the z=5.85 QSO PSO J025-11.