A rapid optical and X-ray timing study of the neutron star X-ray binary Swift J1858.6−0814

ABSTRACT We present a rapid timing analysis of optical (HiPERCAM and ULTRACAM) X-ray (NICER) observations the transient Swift J1858.6−0814 during 2018 2019. The light curves show relatively slow, large amplitude (∼1 mag in gs) ‘blue’ flares (i.e. stronger at shorter wavelengths) on time-scales ∼minutes as well fast, small (∼0.1 ‘red’ longer ∼seconds. are consistent with reprocessing optically thin synchrotron emission, respectively, similar to what is observed other binaries. simultaneous versus soft- hard-band time- energy-dependent correlations. 2019 March 4 parts June data nearly symmetric positive cross-correlations (CCFs) lags simple disc reprocessing. CCFs can be reproduced if dominates one component (disc or Comptonization) both soft hard X-rays. A part shows very different CCFs. correlation negative lag band emission correlated hot flow emission. properties qualitative agreement hybrid inner accretion model, where relative role components that vary course outburst, time-scales, govern shape optical/X-ray