Unveiling the enigma of ATLAS17aeu^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
e-mail: andrea.melandri@brera.inaf.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ ASI – Science Data Centre, Via del Politecnico snc, 00133 Roma, Italy
⁵ INAF – Osservatorio Astronomico di Roma, Via di Frascati, 33, 00040 Monteporzio Catone, Italy
⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁷ Gran Sasso Science Institute, Viale F. Crispi 7, 67100 L’Aquila, Italy
⁸ INFN – Laboratori Nazionali del Gran Sasso, 67100 L’Aquila, Italy
⁹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía, s/n, 18008 Granada, Spain
¹⁰ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini, 64100 Teramo, Italy
¹¹ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹³ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
¹⁴ Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
¹⁵ Urbino University, Via Santa Chiara 27, 61027 Urbino, Italy
¹⁶ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁷ Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile
¹⁸ INFN – Sezione Milano Bicocca, Gruppo Collegato di Parma, Parco Area delle Scienze 7/A, 43124 Parma, Italy
¹⁹ Dipartimento di Fisica, Universitá degli Studi di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
²⁰ INAF – IASF Milano, Via E. Bassini 15, 20133 Milano, Italy
²¹ Astronomical Institute, Academy of Sciences of the Czech Republic, Bocni II 1401, 141 00 Prague, Czech Republic
²² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²³ Department of Physics, University of Trieste, and INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste, Italy
²⁴ INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
²⁵ Special Astrophysical Observatory, Nizhnij Arkhyz, Karachai-Cherkessian Republic 369167, Russia

Received: 10 July 2018
Accepted: 9 November 2018

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.

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.

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_p–E_iso 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.