Simultaneous X-ray and radio observations of the transitional millisecond pulsar candidate CXOU J110926.4–650224

The discovery of a variable radio counterpart

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
e-mail: cotizelati@ice.csic.es
² Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità 2–4, 08034 Barcelona, Spain
³ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, LC, Italy
⁴ South African Radio Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, Cape Town 7925, South Africa
⁵ Department of Physics and Electronics, Rhodes University, PO Box 94 Grahamstown 6140, South Africa
⁶ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain
⁷ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy
⁹ South African Astronomical Observatory, PO Box 9 7935 Observatory, Cape Town, South Africa
¹⁰ Department of Physics, University of the Free State, PO Box 339 Bloemfontein, 9300, South Africa
¹¹ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹² INAF–Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153, 90146 Palermo, Italy
¹³ Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
¹⁴ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, No. 96 JinZhai Road Baohe District, Hefei, 230026 Anhui, PR China
¹⁵ School of Astronomy and Space Science, University of Science and Technology of China, No. 96 JinZhai Road Baohe District, Hefei, 230026 Anhui, PR China
¹⁶ Aurora Technology BV for the European Space Agency, ESAC/ESA, Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁷ Institute of Cosmos Sciences, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
¹⁸ Department of Physics and Astronomy, University of the Western Cape, Bellville, Cape Town 7535, South Africa
¹⁹ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, UK

Received: 31 May 2021
Accepted: 23 September 2021

Abstract

We present the results of simultaneous observations of the transitional millisecond pulsar (tMSP) candidate CXOU J110926.4–650224 with the XMM-Newton satellite and the MeerKAT telescope. The source was found at an average X-ray luminosity of L_X ≃ 7 × 10³³ erg s⁻¹ over the 0.3−10 keV band (assuming a distance of 4 kpc) and displayed a peculiar variability pattern in the X-ray emission, switching between high, low and flaring modes on timescales of tens of seconds. A radio counterpart was detected at a significance of 7.9σ with an average flux density of ≃33 μJy at 1.28 GHz. It showed variability over the course of hours and emitted a ≃10-min long flare just a few minutes after a brief sequence of multiple X-ray flares. No clear evidence for a significant correlated or anticorrelated variability pattern was found between the X-ray and radio emissions over timescales of tens of minutes and longer. CXOU J110926.4–650224 was undetected at higher radio frequencies in subsequent observations performed with the Australia Telescope Compact Array, when the source was still in the same X-ray sub-luminous state observed before, down to a flux density upper limit of 15 μJy at 7.25 GHz (at 3σ). We compare the radio emission properties of CXOU J110926.4–650224 with those observed in known and candidate tMSPs and discuss physical scenarios that may account for its persistent and flaring radio emissions.