Short-term variability of the transitional pulsar candidate CXOU J110926.4−650224 from X-rays to infrared

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
² Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
³ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, (LC), Italy
⁴ INAF–Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
⁵ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
⁶ Instituto de Astrofísica de Canarias (IAC), Vía Láctea, E-38205 La Laguna, Santa Cruz de Tenerife, Spain
⁷ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁸ Departamento de Astrofísica, Universidad de La Laguna, 38206 Santa Cruz de Tenerife, Spain
⁹ Institució Catalana de Recerca i Estudis Avanşats (ICREA), E-08010 Barcelona, Spain
¹⁰ INAF–Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monte Porzio Catone, (RM), Italy
¹¹ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
¹² South African Astronomical Observatory, P.O Box 9 Observatory, 7935 Cape Town, South Africa
¹³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁴ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, Italy
¹⁵ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, I-00185 Roma, Italy
¹⁶ Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu km 0.7, I-09042 Monserrato, Italy
¹⁷ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy

Received: 29 May 2024
Accepted: 7 August 2024

Abstract

CXOU J110926.4−650224 is a candidate transitional millisecond pulsar (tMSP) with X-ray and radio emission properties reminiscent of those observed in confirmed tMSPs in their X-ray ‘subluminous’ disc state. We present the results of observing campaigns that, for the first time, characterise the optical and near-infrared variability of this source and establish a connection with the mode-switching phenomenon observed in X-rays. The optical emission exhibited flickering activity, frequent dipping episodes where it appeared redder, and a multi-peaked flare where it was bluer. The variability pattern was strongly correlated with that of the X-ray emission. Each dip matched an X-ray low-mode episode, indicating that a significant portion of the optical emission originates from nearly the same region as the X-ray emission. The near-infrared emission also displayed remarkable variability, including a dip of 20 min in length during which it nearly vanished. Time-resolved optical spectroscopic observations reveal significant changes in the properties of emission lines from the disc and help infer the spectral type of the companion star to be between K0 and K5. We compare the properties of CXOU J110926.4−650224 with those of other tMSPs in the X-ray subluminous disc state and discuss our findings within the context of a recently proposed scenario that explains the phenomenology exhibited by the prototypical tMSP PSR J1023+0038.