Spiders are millisecond pulsars in compact binary systems. The variable optical emission from their companion stars reveals how the relativistic pulsar wind irradiates the companion and interacts with its outflowing matter. By modelling this optical emission, we can precisely constrain the fundamental parameters of spiders and find super-massive neutron stars, which are usually hosted in these systems. In this paper, we present the discovery of the variable optical counterpart to PSR J2055+1545, a spider millisecond pulsar. The optical light curves show a 0.4−0.6 mag amplitude modulation, indicating that the companion star is only mildly irradiated by the pulsar wind—unlike the strong irradiation seen in many redbacks with amplitudes >1 mag. Our best-fit model is consistent with a super-massive neutron star of mass 1.7 M_sun. The peculiar optical light curve of this system closely resembles those of the two transitional millisecond pulsars PSR J1023+0038 and PSR J1227−4853, rare systems that rapidly switch (on timescales of weeks) between the rotation-powered state and the low mass X-ray binary state powered by the mass accretion from the companion star. This suggests that PSR J2055+1545 may develop an accretion disc in the future and join the exclusive club of transitional millisecond pulsars.
LOVE-NEST
Looking for Super-Massive Neutron Stars
