Unveiling the enigma of ATLAS17aeu

TL;DR

ATLAS17aeu was a supernova associated with a gamma-ray burst, not linked to the gravitational wave GW170104, based on detailed photometric, spectroscopic, and host galaxy analysis.

Contribution

This study provides a detailed multi-epoch analysis of ATLAS17aeu, clarifying its nature as a supernova connected to a gamma-ray burst and ruling out its association with a gravitational wave event.

Findings

ATLAS17aeu is likely a supernova at z ≈ 0.5.

It is associated with GRB170105A, not GW170104.

Spectroscopic features resemble a broad-lined supernova.

Abstract

Aim. The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW170104. 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 GW170104. Methods. With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between $\sim 3$ and $\sim 230$ days after the first detection. Results. We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. We detected spectral features similar to those of a broad lined supernova superposed to an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum and host galaxy SED, we conclude that the redshift of the source is probably $z \simeq 0.5 \pm 0.2$. 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$_{\rm p}$-E$_{\rm iso}$ correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB170105A. This rules out the association of the GRB170105A/ATLAS17aeu transient with the gravitational wave event GW170104, which was due to a binary black hole merger.