Variable Radio Emission of Neutron Star X-Ray Binary Ser X-1 during Its Persistent Soft State

Ser X-1 is a low-mass neutron star X-ray binary and has been persistently accreting since its discovery in the 1960s. It has always been observed to be in a soft spectral state and has never showed substantial long-term X-ray variability. Ser X-1 has one previous radio observation in the literature in which radio emission was detected during this soft state, which is contrary to the behavior of black hole X-ray binaries. We have recently obtained 10 randomly sampled radio epochs of Ser X-1 in order to further investigate its anomalous soft-state radio emission. Out of 10 epochs, we find 8 nondetections and 2 detections at 10 GHz flux densities of 19.9 +/- 4.2 mu Jy and 32.2 +/- 3.6 mu Jy, respectively. We do not detect polarization in either epoch, ruling out very high polarization levels (less than or similar to 63% and 34%). We compare these Ser X-1 results to other X-ray binaries and consider explanations for its long-term variable radio behavior.