Two possible optical–X-Ray anti-correlations of PSR J1023+0038

ABSTRACT X-ray emission is generally believed to be one of the major heating sources for the optical modulation in redback pulsar binaries as we have seen similar phenomena in many low-mass X-ray binaries (LMXBs). While, e.g. MeV/GeV $\gamma$-rays from the neutron stars are also possible heating sources, X-ray observations are currently much more sensitive, and therefore, joint optical–X-ray data are observationally unique on the irradiation mechanism investigation. Using 18 X-ray/B-band simultaneous XMM–Newton observations (717 ks in total) of the redback system PSR J1023+0038 taken during the LMXB state, we find a general trend that the amplitude of the B-band orbital modulation was lower when the observed X-ray flux was higher. Depending on the analysis method adopted, the statistical significance of the anticorrelation can be from 1.7$\sigma$ to 3.1$\sigma$. We also extended the analysis to the GeV $\gamma$-ray band using the Fermi-LAT data, but the result is insignificant to claim any relations. Moreover, another X-ray/optical correlation regarding the low modes of the system was found in some of the XMM–Newton observations, and the astrophysical reason behind is currently unclear yet. These anticorrelations likely suggest that the irradiation is generally stronger when the X-ray flux is in a fainter state, indicating that there is a more dominant irradiation source than the X-ray emission.