PAPITTO, ALESSANDROALESSANDROPAPITTORea, N.N.ReaCoti Zelati, F.F.Coti ZelatiDE MARTINO, DomitillaDomitillaDE MARTINOScaringi, S.S.ScaringiCAMPANA, SergioSergioCAMPANAde Ońa Wilhelmi, E.E.de Ońa WilhelmiKnigge, C.C.KniggeSerenelli, A.A.SerenelliSTELLA, LuigiLuigiSTELLATorres, D. F.D. F.TorresD'AVANZO, PaoloPaoloD'AVANZOISRAEL, Gian LucaGian LucaISRAEL2020-12-112020-12-1120182041-8205http://hdl.handle.net/20.500.12386/28807We report on the first continuous, 80-day optical monitoring of the transitional millisecond pulsar PSR J1023+0038 carried out in mid 2017 with Kepler in the K2 configuration, when an X-ray subluminous accretion disk was present in the binary. Flares lasting from minutes to 14 hr were observed for 15.6% of the time, which is a larger fraction than previously reported on the basis of X-ray and past optical observations, and more frequently when the companion was at superior conjunction of the orbit. A sinusoidal modulation at the binary orbital period was also present with an amplitude of ≃16%, which varied by a few percent over timescales of days, and with a maximum that took place 890 ± 85 s earlier than the superior conjunction of the donor. We interpret this phenomena in terms of reprocessing of the X-ray emission by an asymmetrically heated companion star surface and/or a non-axisymmetric outflow possibly launched close to the inner Lagrangian point. Furthermore, the non-flaring average emission varied by up to ≈40% over a timescale of days in the absence of correspondingly large variations of the irradiating X-ray flux. The latter suggests that the observed changes in the average optical luminosity might be due to variations of the geometry, size, and/or mass accretion rate in the outer regions of the accretion disk.STAMPAenArticle10.3847/2041-8213/aabee92-s2.0-85047421987000431647400003https://doi.org/10.3847/2041-8213/aabee92018ApJ...858L..12PFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering