Simultaneous radio and X-ray observations of the transitional millisecond pulsar candidate 3FGL J1544.6-1125

Transitional millisecond pulsars (tMSPs) are neutron star systems that alternate between a rotation-powered radio millisecond pulsar state and an accretion disc-dominated low-mass X-ray binary (LMXB)-like state on multi-year time-scales. During the LMXB-like state, the X-ray emission from tMSPs switches between 'low' and 'high' X-ray brightness modes on a time-scale of seconds to minutes (or longer), while the radio emission shows variability on time-scales of roughly minutes. Coordinated Very Large Array (VLA) and Chandra observations of the nearby tMSP PSR J1023+0038 uncovered a clear anticorrelation between radio and X-ray luminosities such that the radio emission consistently peaks during the X-ray low modes. In addition, there are sometimes also radio/X-ray flares that show no obvious correlation. In this paper, we present simultaneous radio and X-ray observations of a promising tMSP candidate system, 3FGL J1544.6$-$1125, which shows optical, $\gamma$-ray, and X-ray phenomena similar to PSR J1023+0038, but which is challenging to study because of its greater distance. Using simultaneous VLA and Chandra observations, we find that the radio and X-ray emission are consistent with being anticorrelated in a manner similar to PSR J1023+0038. We discuss how our results help in understanding the origin of bright radio emission from tMSPs. The greater sensitivity of upcoming telescopes such as the Square Kilometre Array will be crucial for studying the correlated radio/X-ray phenomena of tMSP systems.