A soft and transient ultraluminous X-ray source with six-hour modulation in the NGC 300 galaxy^⋆

¹ Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, Piazza della Vittoria 15, 27100 Pavia, Italy
e-mail: paolo.esposito@iusspavia.it
² Center for Astrophysics|Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
e-mail: andrea.sacchi@cfa.harvard.edu
³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
⁴ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ INAF – Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
⁶ College of Astronomy and Space Sciences, University of the Chinese Academy of Sciences, Beijing 100049, PR China
⁷ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁹ Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹⁰ South African Astronomical Observatory, PO Box 9 7935 Observatory, South Africa
¹¹ Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
¹² Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
¹³ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, 67100 L’Aquila, Italy
¹⁴ Physics and Astronomy Department Galileo Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
¹⁶ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁷ Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno and INFN Sezione di Napoli (Gruppo Collegato di Salerno), Via Giovanni Paolo II 132, 84084 Fisciano, Italy
¹⁸ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
¹⁹ Dipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Via G. Celoria 16, 20133 Milano, Italy

Received: 18 March 2023
Accepted: 23 November 2023

Abstract

We investigate the nature of CXOU J005440.5−374320 (J0054), a peculiar bright (∼4 × 10³⁹ erg s⁻¹) and soft X-ray transient in the spiral galaxy NGC 300 with a six-hour periodic flux modulation that was detected in a 2014 Chandra observation. Subsequent observations with Chandra and XMM-Newton, as well as a large observational campaign of NGC 300 and its sources performed with the Swift Neil Gehrels Observatory, showed that this source exhibits recurrent flaring activity: four other outbursts were detected across ∼8 years of monitoring. Using data from the Swift/UVOT archive and from the XMM-Newton/OM and Gaia catalogues, we determined that the source is likely associated with a bright blue optical/ultraviolet counterpart. This prompted us to perform follow-up observations with the Southern African Large Telescope in December 2019. With the multi-wavelength information at hand, we discuss several possibilities for the nature of J0054. Although none is able to account for the full range of the observed peculiar features, we found that the two most promising scenarios are a stellar-mass compact object in a binary system with a Wolf–Rayet star companion, or the recurrent tidal stripping of a stellar object trapped in a system with an intermediate-mass (∼1000 M_⊙) black hole.