[en] We present optical follow-up observation results of three binary black hole merger (BBH) events, GW190408_181802, GW190412, and GW190503_185404, which were detected by the Advanced LIGO and Virgo gravitational wave (GW) detectors. Electromagnetic (EM) counterparts are generally not expected for BBH merger events. However, some theoretical models suggest that EM counterparts of BBH can possibly arise in special environments, prompting motivation to search for EM counterparts for such events. We observed high-credibility regions of the sky for the three BBH merger events with telescopes of the Gravitational-wave EM Counterpart Korean Observatory (GECKO), including the KMTNet. Our observation started as soon as 100 minutes after the GW event alerts and covered 29–63 deg² for each event with a depth of ∼22.5 mag in the R band within hours of observation. No plausible EM counterparts were found for these events, but based on there being no detection of the GW190503_185404 event, for which we covered the 69% credibility region, we place the BBH merger EM counterpart signal to be M _g > − 18.0 AB mag within about one day of the GW event. The comparison of our detection limits with light curves of several kilonova models suggests that a kilonova event could have been identified within hours of the GW alert with GECKO observations if the compact merger happened at <400 Mpc and the localization accuracy was on the order of 50 deg². Our result shows great promise for the GECKO facilities to find EM counterparts within a few hours from GW detection in future GW observation runs.

[en] Faint z ∼ 5 quasars with M ₁₄₅₀ ∼ −23 mag are known to be potentially important contributors to the ultraviolet ionizing background in the postreionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our z ∼ 5 quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg². From our spectroscopic observations with the Gemini Multi-Object Spectrograph on the Gemini-South 8 m telescope, we discovered eight new quasars at z ∼ 5 with  −26.1 ≤ M ₁₄₅₀ ≤ −23.3. Combining our IMS faint quasars (M ₁₄₅₀ > −27 mag) with the brighter Sloan Digital Sky Survey quasars (M ₁₄₅₀ < −27 mag), we derive the z ∼ 5 quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of M ₁₄₅₀ = −23 mag. We find that the faint-end slope of the QLF is very flat ($\mathrm{\alpha <!--\; ??\; -->}=-<!--\; ???\; -->{1.2}_{-<!--\; ???\; -->0.6}^{+1.4}$), with a characteristic luminosity of $M_{1450}^{\ast <!--\; ???\; -->}=-<!--\; ???\; -->{25.8}_{-<!--\; ???\; -->1.1}^{+1.4}$ mag. The number density of z ∼ 5 quasars from the QLF gives an ionizing emissivity at 912 Å of ϵ ₉₁₂ = (3.7–7.1) × 10²³ erg s⁻¹ Hz⁻¹ Mpc⁻³ and an ionizing photon density of ${\stackrel{\dot{}<!--\; ??\; -->}{n}}_{\mathrm{i}\mathrm{o}\mathrm{n}}=(3.0\text{--}5.7)\times <!--\; ??\; -->{10}^{49}$ Mpc⁻³ s⁻¹. These results imply that quasars are responsible for only 10%–20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at z ∼ 5, disfavoring the idea that quasars alone could have ionized the IGM at z ∼ 5.