AT 2019abn: multi-wavelength observations over the first 200 days^⋆

¹ Physics Department, Lancaster University, Lancaster LA1 4YB, UK
² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Vesilinnantie 5, Quantum 20014 Turku, Finland
e-mail: steven.williams@utu.fi
³ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Gran Telescopio Canarias (GRANTECAN), Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
⁷ Astrophysics Research Institute, Liverpool John Moore s University, IC2 Liverpool Science Park, Liverpool L3 5RF, UK
⁸ Institute of Theoretical Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
⁹ University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
¹⁰ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain

Received: 9 December 2019
Accepted: 18 January 2020

Abstract

Context. AT 2019abn was discovered in the nearby M51 galaxy by the Zwicky Transient Facility at more than two magnitudes and around three weeks prior to its optical peak.

Aims. We aim to conduct a detailed photometric and spectroscopic follow-up campaign for AT 2019abn, with early discovery allowing for significant pre-maximum observations of an intermediate luminosity red transient (ILRT) for the first time.

Methods. This work is based on the analysis of u′BVr′i′z′H photometry and low-resolution spectroscopy using the Liverpool Telescope, medium-resolution spectroscopy with the Gran Telescopio Canarias (GTC), and near-infrared imaging with the GTC and the Nordic Optical Telescope.

Results. We present the most detailed optical light curve of an ILRT to date, with multi-band photometry starting around three weeks before peak brightness. The transient peaked at an observed absolute magnitude of M_r′ = −13.1, although it is subject to significant reddening from dust in M51, implying an intrinsic M_r′ ∼ −15.2. The initial light curve showed a linear, achromatic rise in magnitude before becoming bluer at peak. After peak brightness, the transient gradually cooled. This is reflected in our spectra, which at later times show absorption from such species as Fe I, Ni I and Li I. A spectrum taken around peak brightness shows narrow, low-velocity absorption lines, which we interpret as likely to originate from pre-existing circumstellar material.

Conclusions. We conclude that while there are some peculiarities, such as the radius evolution, AT 2019abn fits in well overall with the ILRT class of objects and is the most luminous member of the class seen to date.