GECKO Optical Follow-up Observation of Three Binary Black Hole Merger Events: GW190408_181802, GW190412, and GW190503_185404

Abstract

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.

Gravitational-wave EM Counterpart Korean Observatory (GECKO)

“…The KMTNet telescopes are located in Australia, South Africa, and Chile, so that it can follow up GW events in the southern hemisphere sky any time. The limiting magnitude of the KMTNet telescopes is 23 AB mag for the point source detection at 5-σ in 6 minutes of integration in R-band, making it possible to detect an EM counterpart for a GW170817-like event out to 400 Mpc (Kim et al 2021). Figure 2 shows AT2017gfo (the EM counterpart of GW170817) observed by KMTNet (Troja et al 2017), demonstrating the capability of KMTNet to trace time evolution of the kilonova signal in time and color.…”

“…(3) There are many other signals that could be confused as kilonovae. The expected number of transients can be over 1000 or more over about 100 deg 2 area (Kim et al 2021), and about 7000 variable sources are also expected in the same area (Sesar et al 2007) to the depths of potential kilonovae in future GW run. On top of that, there are numerous artificial signals that look like transient.…”

“…(2) The EM counterparts are faint and fade quickly. BNS mergers produce kilonovae, but they are more than 10 times fainter at peak luminosity than supernovae, with R = 22 − 24 mag at 400 Mpc (Kim et al 2021). They quickly reach their peak luminosity in hours to 0.5 days since the merger event, and fade rapidly in optical (a few days).…”

“…During the LIGO/Virgo O3 run, we followed up 11 GW event with GECKO, but without detecting any kilonovae. However, the observations proved that we can start the follow-up observation within 30 minutes of GW alerts, and can cover the localization area of about 50 deg 2 (expected median 90 % confidence area during the O4 run) within about one hour, to detect kilonovae as far as a few hundred Mpc (Kim et al 2021). To speed up the transient identification process, we are now implementing artificial intelligence techniques in our transient search process.…”

“…To follow up the great success of the GW170817 studies, intensive efforts have been made during the Advanced LIGO and Advanced Virgo (LIGO/Virgo) O3 run in 2019 -2020, to uncover more kilonovae in the GW localization areas. However, the efforts have not yet revealed any promising EM counterparts, although useful constraints have been obtained on kilonovae signals on various GW events (e.g., Kim et al 2021).…”