Soft-state optical spectroscopy of the black hole MAXI J1305-704

¹ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, Via Archirafi 36, 90123 Palermo, Italy
² INAF/IASF Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy
e-mail: carlotta.miceli@inaf.it
³ IRAP, Université de Toulouse, CNRS, UPS, CNES, 9, Avenue du Colonel Roche BP 44346, 31028 Toulouse, Cedex 4, France
⁴ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Univ. de La Laguna, 38206 La Laguna, Tenerife, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità 2-4, 08034 Barcelona, Spain
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁹ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
¹⁰ Department of Astronomy, Ohio State University, 140 W 18th Avenue, Columbus, OH 43210, USA
¹¹ Department of Physics, University of Trento, Via Sommarive 14, 38122 Povo (TN), Italy

Received: 3 November 2023
Accepted: 25 January 2024

Abstract

The X-ray dipper MAXI J1305-704 is a dynamically confirmed black hole (BH) X-ray binary discovered a decade ago. While its only outburst has been studied in detail in X-rays, follow-up at other wavelengths has been scarce. We report here the results from an optical spectroscopy campaign across the outburst of MAXI J1305-704. We analysed two epochs of data obtained by the Magellan Clay Telescope during two consecutive nights, when the source was in a soft X-ray spectral state. We identified typical emission lines from outbursting low-mass X-ray binaries, such as the hydrogen Balmer series, He II 4686 Å and the Bowen blend. We focused our analysis on the prominent Hα line, which exhibits asymmetric emission and variable absorption components. We applied both traditional analytical methods and machine-learning techniques in order to explore the association of the absorption features with outflowing phenomena, and we conclude that they are best explained by broad absorption. This result is consistent with reports from other outbursting BHs, where optical outflows have predominantly been observed in the hard state. Further observations at different X-ray states are key to properly test whether this behaviour is universal and to determine the implications for the disc wind physics.