TITLE: GCN CIRCULAR NUMBER: 24433 SUBJECT: LIGO/Virgo S190426c: INT Observations of ZTF19aassfws DATE: 19/05/09 15:13:58 GMT FROM: Mansi M. Kasliwal at Caltech/Carnegie Christoffer Fremling (Caltech), Andrew Levan (Radboud), Mansi Kasliwal (Caltech) and Nial Tanvir (Leicester) report on behalf of a larger collaboration We imaged ZTF19aassfws (Perley et al. GCN 24331, Coughlin et al. GCN 24283) with the INT on UT 2019-04-08. Difference imaging relative to PanSTARRS1 (Chambers et al. 2016) finds no detection in the r-band to r > 22.6 mag and a bright detection in z-band at z ~ 20.8,corresponding to M_Z ~ -17.2 at z=0.093 (Cenko et al. GCN24430). The z-band luminosity is higher than GW170817 at this phase. The red color at this phase is plausible for a kilonova but we cannot definitively rule out unrelated, nuclear activity. We further caution that the INT and PS1 have different filter transmission curves in z-band. We encourage follow-up to confirm or refute the association of this source with the GW event S190426c (LVC et al. GCN 24237, GCN 24411). We thank the INT observers, Lord Dover and Tarik Zegmott for facilitating these observations.