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 \sim 10 ^ { 33 } ( d / 2.3 { kpc } ) ^ { 2 } erg s ^ { -1 } .
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 \sim 20 hours , with the brightest flares reaching \sim 2 \times 10 ^ { 34 } erg s ^ { -1 } .
The source continuously exhibited flaring activity on time-scales of \sim 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 \sim 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 .