Simultaneous X-ray and radio observations of the transitional millisecond pulsar candidate CXOU J110926.4-650224. The discovery of a variable radio counterpart

Abstract

We present the results of simultaneous observations of the transitional millisecond pulsar (tMSP) candidate <ASTROBJ>CXOU J110926.4-650224</ASTROBJ> with the XMM-Newton satellite and the MeerKAT telescope. The source was found at an average X-ray luminosity of L<SUB>X</SUB> ≃ 7 × 10<SUP>33</SUP> erg s<SUP>−1</SUP> 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. <ASTROBJ>CXOU J110926.4-650224</ASTROBJ> 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 <ASTROBJ>CXOU J110926.4-650224</ASTROBJ> with those observed in known and candidate tMSPs and discuss physical scenarios that may account for its persistent and flaring radio emissions.