Unveiling the origin of the optical/UV emission from the Galactic ULX Swift J0243.6+6124 during its 2017-2018 giant outburst

J. Alfonso-Garzón; J. van den Eijnden; N. P. M. Kuin; F. Fürst; A. Rouco-Escorial; J. Fabregat; P. Reig; J. M. Mas-Hesse; P. A. Jenke; C. Malacaria; C. Wilson-Hodge

doi:10.1051/0004-6361/202347712

arXiv · 2312.08080

Unveiling the origin of the optical/UV emission from the Galactic ULX Swift J0243.6+6124 during its 2017-2018 giant outburst

J. Alfonso-Garzón·J. van den Eijnden·N. P. M. Kuin·F. Fürst·A. Rouco-Escorial·J. Fabregat·P. Reig·J. M. Mas-Hesse·P. A. Jenke·C. Malacaria·C. Wilson-Hodge

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Abstract

From late September 2017 to February 2018, the Be X-ray binary (BeXB) Swift J0243.6+6124 underwent an unprecedently bright giant outburst. The reported X-ray luminosities were so high that the system was classified as an Ultraluminous X-ray source (ULX). It was also the first BeXB pulsar showing radio jet emission. The source was not only bright in X-rays and radio, but also in optical and UV wavelenghts. In this paper we aim to understand the origin of the observed optical/UV fluxes simultaneous to the X-ray emission. We studied the optical/UV light curves in comparison with the X-ray fluxes along the outburst, considering the main mechanisms that can explain the optical/UV emission in X-ray binaries. Due to the tight correlation observed between the optical/UV and X-ray light curves, reprocessing of X-rays seems to be the most plausible explanation. We calculated the timescales of the light curves decays and studied the correlation indexes between the optical and X-ray emission. Finally, we built a physical model considering X-ray heating of the surface of the donor star, irradiation of the accretion disk, and emission from a viscously heated accretion disk, in order to reproduce the observed optical/UV SEDs along the outburst. We considered in our model that the Be circumstellar disk was co-planar to the orbit, and then we neglected its irradiation in the current model. As an input of the model, we used as incident X-ray luminosities those calculated from the bolometric X-ray fluxes obtained from the spectral fit of the Swift/XRT and BAT observations. We conclude that reprocessing of X-rays as X-ray heating of the Be star surface and irradiation of the accretion disk are the two main mechanisms that can reproduce the observed optical/UV emission during the 2017-2018 giant outburst of Swift J0243.6+6124.

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J. Alfonso-Garzón, J. van den Eijnden, N. P. M. Kuin, F. Fürst, A. Rouco-Escorial, J. Fabregat, P. Reig, J. M. Mas-Hesse, P. A. Jenke, C. Malacaria, C. Wilson-Hodge. 2023-12-13. Unveiling the origin of the optical/UV emission from the Galactic ULX Swift J0243.6+6124 during its 2017-2018 giant outburst. https://doi.org/10.1051/0004-6361/202347712

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