Detection of highly correlated optical and X-ray variations in SS Cygni with Tomo-e Gozen and NICER

Abstract We report on simultaneous optical and X-ray observations of the dwarf nova SS Cyg with Tomo-e Gozen/1.05 m Kiso Schmidt and Neutron star Interior Composition ExploreR (NICER)/International Space Station (ISS). A total of four observations were carried out in the quiescent state and highly correlated light variations between the two wavelengths were detected. We have extracted local brightness peaks in the light curves with a binning interval of 1 s, called “shots,” and have evaluated time lags between the optical and X-ray variations by using a cross-correlation function. Some shots exhibit significant optical lags to X-ray variations and most of them are positive, ranging from +0.26 to +3.11 s, which have never been detected. They may be ascribable to X-ray reprocessing in the accretion disk and/or the secondary star. This analysis is possible thanks to the high timing accuracy and the high throughput of NICER and the matching capability of Tomo-e Gozen. Also, it is confirmed that the correlation between the optical and X-ray variations changed in the middle of one of our observation windows and that the X-ray spectrum softer than 2 keV varied in accordance with the correlation.