The omnipresent flux-dependent optical dips of the black hole transient Swift J1357.2−0933

¹ Instituto de Astrofísica de Canarias (IAC), Vía Láctea, La Laguna, 38205 Santa Cruz de Tenerife, Spain
e-mail: guayente.panizo@gmail.com
² Departamento de Astrofísica, Universidad de La Laguna, 38206 Santa Cruz de Tenerife, Spain
³ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
⁴ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁵ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁶ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁸ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
⁹ Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA Wales, UK
¹⁰ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 13 September 2023
Accepted: 3 November 2023

Abstract

Swift J1357.2-0933 is a black hole transient that is of particular interest due to the optical recurrent dips found during its first two outbursts (in 2011 and 2017), which lack an obvious X-ray equivalent. We present a study based on fast optical photometry during its two most recent outbursts, in 2019 and 2021. Our observations reveal that the optical dips were present in every observed outburst of the source, although they were shallower and showed longer recurrence periods in the two most recent and fainter events. We performed a global study of the dips properties in the four outbursts and found that they do not follow a common temporal evolution. In addition, we discovered a correlation with the X-ray and optical fluxes, with the dips being more profound and showing shorter recurrence periods for brighter stages. This trend seems to extend even to the faintest, quiescent states of the source. We discuss these results in the context of the possible connection between optical dips and outflows found in previous works.