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

Abstract

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 μJy and 32.2 ± 3.6 μJy, respectively. We do not detect polarization in either epoch, ruling out very high polarization levels (≲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.