Unveiling the enigma of ATLAS17aeu

Abstract

Aims: The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW 170104. The importance of a possible association with gravitational waves coming from a binary black hole merger led to an extensive follow-up campaign, with the aim of assessing a possible connection with GW 170104. Methods: With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between ∼3 and ∼230 days after the first detection. <BR /> Results: We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. Although at low significance and not conclusive, we found similarities to the spectral features of a broad-line supernova superposed onto an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum, and host galaxy spectral energy distribution, we conclude that the redshift of the source is probably z ≃ 0.5 ± 0.2. <BR /> Conclusions: While the redshift range we have determined is marginally compatible with that of the gravitational wave event, the presence of a supernova component and the consistency of this transient with the E<SUB>p</SUB>-E<SUB>iso</SUB> correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB 170105A. This rules out the association of the GRB 170105A/ATLAS17aeu transient with the gravitational wave event GW 170104, which was due to a binary black hole merger. <P />Based on observations made with the following telescopes: Copernico, TNG (under programme A34TAC_24), GTC (under programmes GTCMULTIPLE2D-16B and GTCMULTIPLE2G-17A), LBT (under programme 2016_2017_19), and HST (under programme GO14270).Spectral data for this source shown on this paper are available on the Weizmann Interactive Supernova Data Repository (WISeREP, <A href="https://wiserep.weizmann.ac.il/">http://https://wiserep.weizmann.ac.il/</A>).