Negative and positive superhumps of V503 Cyg as seen in TESS data. I. Seasons 2019 and 2021

Abstract

We report the TESS observations of the SU UMa-type dwarf nova V503 Cyg in 2019 and 2021 during a superoutburst, normal outbursts and quiescence. We identified stage A of the growing superhumps and stage B in positive superhump evolution. We found that the stage A lasted ∼ 0.8 d; during stage B a mean period of 0.081367(93) d decreased with the derivative P dot /P = −4.8 * 10 −5 . We estimated the mass ratio q = 0.195 using a fractional period excess for stage B. The quiescent state was represented by a strong signal at negative superhumps and a weak one at the orbital period. We detected a gradual increase of the 0.076-d period of negative superhumps in quiescence preceding normal outbursts in 2019 and 2021, and an abrupt decrease of the period during normal outbursts. This means that the radius of the accretion disk increases sharply during the outburst and gradually decreases towards the onset of the next outburst. Such behavior is consistent with the thermal-tidal instability model.We found that the light curve folded on the orbital period shows a profile that changes from double-peak to single-peak, respectively, in the tilted and non-tilted state of the accretion disk. We speculate that this difference may be caused by a state of the accretion disk: in the tilted disk state, one spot is the hot spot on the edge of the disk, while another one may be a spot caused by the matter hitting the inner disk. During the non-tilted disk state, there is one spot on its edge.