Photometric variability of TW~Hya from seconds to years as seen from space and the ground in 2013-2017

M. Siwak, W. Ogloza, A. F. J. Moffat, J. M. Matthews, S. M. Rucinski, T. Kallinger, R. Kuschnig, C. Cameron, W. W. Weiss, J. F. Rowe, D. B. Guenther, D. Sasselov

Published 2018-05-11Version 1

This is the final photometric study of TW Hya based on new MOST satellite observations. During 2014 and 2017 the light curves showed stable 3.75 and 3.69 d quasi-periodic oscillations, respectively. Both values appear to be closely related with the stellar rotation period, as they might be created by changing visibility of a hot-spot formed near the magnetic pole directed towards the observer. These major light variations were superimposed on a chaotic, flaring-type activity caused by hot-spots resulting from unstable accretion - a situation reminiscent of that in 2011, when TW Hya showed signs of a moderately stable accretion state. In 2015 only drifting quasi-periods were observed, similar to those present in 2008-2009 data and typical for magnetised stars accreting in a strongly unstable regime. A rich set of multi-colour data was obtained during 2013-2017 with the primary aim to characterize the basic spectral properties of the mysterious occultations in TW Hya. Although several possible occultation-like events were identified, they are not as well defined as in the 2011 MOST data. The new ground-based and MOST data show a dozen previously unnoticed flares, as well as small-amplitude, 11 min - 3 hr brightness variations, associated with 'accretion bursts'. It is not excluded that the shortest 11-15 min variations could also be caused by thermal instability oscillations in an accretion shock.