TITLE: GCN CIRCULAR NUMBER: 24250 SUBJECT: LIGO/Virgo S190426c: a possible lensed NS-NS merger DATE: 19/04/26 21:27:30 GMT FROM: Graham P Smith at U of Birmingham G. P. Smith (Birmingham), M. Bianconi (Birmingham), R. Massey (Durham), and A. Robertson (Durham) report on behalf of the Gravitationally Lensed Gravitational Wave Hunters The non-zero value of Prob_MassGap identifies S190426c (GCN24237) as a possible strongly-lensed NS-NS merger - i.e. its true redshift and luminosity distance may be larger, and true mass may be smaller than inferred by LIGO/Virgo. The rate of such detections is predicted to be ~0.01 per Earth year during O3. The putative lens could be an individual galaxy, or a group/cluster of galaxies. None of the 130 known strong-lensing clusters in the sample discussed by Smith et al. (2018) are located in the updated 90% credible sky localization released an hour after detection and stated as the currently preferred skymap in GCN24237. At the estimated luminosity distance to the source (D_L~375Mpc; GCN24237) an AT2017gfo-like counterpart would have an apparent B/V-band magnitude of AB<~24.5 within ~2 days of the LIGO/Virgo detection. This estimate (albeit redshifted in to the i-band, and time-dilated to ~4 days post-detection) is also valid if the source is strongly-lensed and actually at a redshift of z~1. We encourage colleagues to observe the sky localization of this source down to AB~25, to search for a kilonova-like counterpart, and thus explore the possibility that this source is strongly-lensed, for example by a massive galaxy or group of galaxies. Background information on the details of this circular can be found in these publications: Smith, Jauzac, Veitch, et al., 2018, MNRAS, 475, 3823 Smith, Robertson, Bianconi, Jauzac, arXiv:1902.05140 Smith, Bianconi, Jauzac, et al., 2019, MNRAS, 485, 5180