The Flare-Dominated Accretion Mode of a Radio-Bright Candidate Transitional Millisecond Pulsar

Abstract

We report new simultaneous X-ray and radio continuum observations of 3FGL J0427.9-6704, a candidate member of the enigmatic class of transitional millisecond pulsars. These XMM-Newton and Australia Telescope Compact Array observations of this nearly edge-on, eclipsing low-mass X-ray binary were taken in the sub-luminous disk state at an X-ray luminosity of ~10^33(d/2.3kpc)^2 erg/s. Unlike the few well-studied transitional millisecond pulsars, which spend most of their disk state in a characteristic high or low accretion mode with occasional flares, 3FGL J0427.9-6704 stayed in the flare mode for the entire X-ray observation of ~20 hours, with the brightest flares reaching ~2*10^34 erg/s. The source continuously exhibited flaring activity on time-scales of ~10-100 sec in both the X-ray and optical/UV. No measurable time delay between the X-ray and optical/UV flares is observed, but the optical/UV flares last longer, and the relative amplitudes of the X-ray and optical/UV flares show a large scatter. The X-ray spectrum can be well-fit with a partially-absorbed power-law (Gamma~1.4-1.5), perhaps due to the edge-on viewing angle. Modestly variable radio continuum emission is present at all epochs, and is not eclipsed by the secondary, consistent with the presence of a steady radio outflow or jet. The simultaneous radio/X-ray luminosity ratio of 3FGL J0427.9-6704 is higher than any known transitional millisecond pulsars and comparable to that of stellar-mass black holes of the same X-ray luminosity, providing additional evidence that some neutron stars can be as radio-loud as black holes.

17 pages, including 10 figures and 3 tables. Accepted for publication in ApJ

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• The MAVERIC Survey: Simultaneous Chandra and VLA observations of the transitional millisecond pulsar candidate NGC 6652B
• Simultaneous X-ray and radio observations of the transitional millisecond pulsar candidate CXOU J110926.4-650224. The discovery of a variable radio counterpart
• Multiwavelength evidence for a new flare-mode transitional millisecond pulsar
• The MAVERIC survey: A catalogue of radio sources in southern globular clusters from the Australia Telescope Compact Array
• MeerKAT radio observations of the neutron star low-mass X-ray binary Cen X-4 at low accretion rates
• Multiwavelength Evidence for Two New Candidate Transitional Millisecond Pulsars in the Sub-luminous Disk State: 4FGL J0639.1--8009 and 4FGL J1824.2+1231
• Probing multi-band variability and mode switching in the candidate transitional millisecond pulsar 3FGL J1544.6-1125
• There and back again: Mysterious optical pulse profile behavior of the transitional millisecond pulsar PSR J1023+0038