Deep long asymmetric occultation in EPIC 204376071

We have discovered a young M star of mass 0.16 M_⊙ and radius 0.63 R_⊙, likely in the Upper Sco Association, that exhibits only a single 80 per cent deep occultation of 1-d duration. The star has frequent flares and a low-amplitude rotational modulation, but is otherwise quiet over 160 d of cumulative observation during K2 campaigns C2 and C15. We discuss how such a deep eclipse is not possible by one star crossing another in any binary or higher order stellar system in which no mass transfer has occurred. The two possible explanations we are left with are (1) orbiting dust or small particles (e.g. a disc bound to a smaller orbiting body, or unbound dust that emanates from such a body); or (2) a transient accretion event of dusty material near the corotation radius of the star. In either case, the time between such occultation events must be longer than ∼80 d. We model a possible orbiting occulter both as a uniform elliptically shaped surface (e.g. an inclined circular disc) and as a `dust sheet' with a gradient of optical depth behind its leading edge. The required masses in such dust features are then ≳3 × 10¹⁹ g and ≳10¹⁹ g, for the two cases, respectively.