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