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

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Shahbaz, T.
Paice, J. A .
Rajwade, K. M.
Veledina, A.
Gandhi, P.
Dhillon, V. S.
Marsh, T. R.
Littlefair, S.
Kennedy, Mark R.
Breton, R. P.
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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 mag in gs) ‘blue’ flares (i.e. stronger at shorter wavelengths) on time-scales of ∼minutes as well as fast, small amplitude (∼0.1 mag in gs) ‘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.
Accretion , Accretion discs , Stars: neutron , X-rays: binaries , X-rays: individual: Swift J1858.6−0814
Shahbaz, T., Paice, J. A., Rajwade, K. M., Veledina, A., Gandhi, P., Dhillon, V. S., Marsh, T. R., Littlefair, S., Kennedy, M. R., Breton, R. P. and Clark, C. J. (2023) 'A rapid optical and X-ray timing study of the neutron star X-ray binary Swift J1858. 6–0814', Monthly Notices of the Royal Astronomical Society, 520(1), pp. 542–559. doi: 10.1093/mnras/stad163
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© 2023, the Authors. Published by Oxford University Press on behalf of Royal Astronomical Society.