Probing the Atoll/Z Continuum with Neutron Star Low Mass X-ray Binary 1A 1744-361

Neutron star low mass X-ray binaries (NS LMXBs) have traditionally been broken down into two classes based on their correlated spectral and timing properties, as well as their positions on the X-ray color-color diagram, which implies a dichotomy. In this work, we report on X-ray (NICER/NuSTAR/Swift/MAXI) timing and spectroscopic results and radio (MeerKAT) observations from a three-month monitoring campaign (in 2022) of the NS LMXB 1A 1744-361. The X-ray hardness-intensity and color-color diagrams suggest that 1A 1744-361 spent the majority of the outburst in an atoll state, but we discovered that the source exhibited properties consistent with traditional Z sources at the peak of the outburst. For instance, the source showed normal branch-like quasi-periodic oscillations (NBO-like QPOs) with a fractional rms amplitude of ~5%, which we connect to the NBOs seen from traditional Z sources. Our observations of 1A 1744-361 are fully consistent with the idea of the mass accretion rate being the main distinguishing parameter between atoll and Z sources. MeerKAT monitoring data in the radio showed that the source was at its radio-brightest during the peak of the outburst, and that the source transitioned from the ‘island’ to the ‘banana’ spectral state within ~3 days of the outburst onset, launching transient jet ejecta. The MeerKAT observations present the strongest evidence for radio flaring, including jet ejecta, during the island-to-banana state transition at the lowest accretion rates. We thus show that atoll and Z sources are not two distinct source classes, but instead represent two states on a mass accretion rate continuum; and with 1A 1744-361, we are probing the boundary between the atoll and Z states.